I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Sent from my iPhone
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Tony, Thanks for the advice. The 5U4 was swapped and further tested in another V amplifier and all was good. I did change the filter capacitors after this happened but made no difference. I’m having difficulty with your method of feeding voltage in reverse to the output transformer. Not sure I’m doing it correctly as I get 0 voltage. I’m putting in 10vac at the speaker plug where the 8 ohm load would connect. I’m not sure where to measure the 400ac? I tried with one lead grounded and the other lead on pin 3 of a 6L6 socket and also on both pin 3’s but no voltage to measure. Can you please explain the method? I must not be doing the test properly. Thank you, Mauro
Sent from my iPhone
On May 18, 2026, at 3:11 PM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote: I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always, Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Tony Miklos Tony's Jukebox Repair
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I have that wrong. Try measuring from the 6L6 to the center tap of the transformer. And from the 6L6 to the other. Don't get zapped.
Tony
On 5/18/2026 4:51 PM, M De Simone via Jukebox-list wrote:
Tony, Thanks for the advice. The 5U4 was swapped and further tested in another V amplifier and all was good. I did change the filter capacitors after this happened but made no difference. I’m having difficulty with your method of feeding voltage in reverse to the output transformer. Not sure I’m doing it correctly as I get 0 voltage. I’m putting in 10vac at the speaker plug where the 8 ohm load would connect. I’m not sure where to measure the 400ac? I tried with one lead grounded and the other lead on pin 3 of a 6L6 socket and also on both pin 3’s but no voltage to measure. Can you please explain the method? I must not be doing the test properly. Thank you, Mauro
Sent from my iPhone
On May 18, 2026, at 3:11 PM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote: I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always, Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Tony Miklos Tony's Jukebox Repair
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Thanks for the method and education! I get 125vac 0n one leg of the primary and 165vac on the other primary of the 6L6’s, at 12v input. On another working amp I get 175v on either side of the primary with 12v input. I used an analogue meter to measure the primaries but didn’t see any bouncing of the meter.
Is it safe to say the problem is in the output transformer?
Sent from my iPhone
On May 19, 2026, at 8:53 AM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
I have that wrong. Try measuring from the 6L6 to the center tap of the transformer. And from the 6L6 to the other. Don't get zapped.
Tony
On 5/18/2026 4:51 PM, M De Simone via Jukebox-list wrote: Tony, Thanks for the advice. The 5U4 was swapped and further tested in another V amplifier and all was good. I did change the filter capacitors after this happened but made no difference. I’m having difficulty with your method of feeding voltage in reverse to the output transformer. Not sure I’m doing it correctly as I get 0 voltage. I’m putting in 10vac at the speaker plug where the 8 ohm load would connect. I’m not sure where to measure the 400ac? I tried with one lead grounded and the other lead on pin 3 of a 6L6 socket and also on both pin 3’s but no voltage to measure. Can you please explain the method? I must not be doing the test properly. Thank you, Mauro Sent from my iPhone
On May 18, 2026, at 3:11 PM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote: I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always, Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Tony Miklos Tony's Jukebox Repair
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On 5/19/2026 1:01 PM, M De Simone via Jukebox-list wrote:
Thanks for the method and education! I get 125vac 0n one leg of the primary and 165vac on the other primary of the 6L6’s, at 12v input. On another working amp I get 175v on either side of the primary with 12v input. I used an analogue meter to measure the primaries but didn’t see any bouncing of the meter.
Is it safe to say the problem is in the output transformer?
That would be my guess. Also sometimes the windings can short to the core. Measure resistance from the center tap to the chassis. It should read infinite.
Sent from my iPhone
On May 19, 2026, at 8:53 AM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
I have that wrong. Try measuring from the 6L6 to the center tap of the transformer. And from the 6L6 to the other. Don't get zapped.
Tony
On 5/18/2026 4:51 PM, M De Simone via Jukebox-list wrote: Tony, Thanks for the advice. The 5U4 was swapped and further tested in another V amplifier and all was good. I did change the filter capacitors after this happened but made no difference. I’m having difficulty with your method of feeding voltage in reverse to the output transformer. Not sure I’m doing it correctly as I get 0 voltage. I’m putting in 10vac at the speaker plug where the 8 ohm load would connect. I’m not sure where to measure the 400ac? I tried with one lead grounded and the other lead on pin 3 of a 6L6 socket and also on both pin 3’s but no voltage to measure. Can you please explain the method? I must not be doing the test properly. Thank you, Mauro Sent from my iPhone
On May 18, 2026, at 3:11 PM, Tony Miklos via Jukebox-list jukebox-list@lists.netlojix.com wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote: I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always, Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Tony Miklos Tony's Jukebox Repair
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On 5/18/2026 4:51 PM, M De Simone via Jukebox-list wrote:
Tony, Thanks for the advice. The 5U4 was swapped and further tested in another V amplifier and all was good. I did change the filter capacitors after this happened but made no difference.
This could be a great example of why we never plug in an amp with the old filter caps, or with any old caps unless you are familiar with trouble shooting and know how to test stages as you go.
The old filter caps very well *could* have fried a transformer.
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
John,
Thanks for chiming in. I agree Tony’s test is neat? I love it. Now to answer your questions, the amp does not draw excessive current without the rectifier. I tried a different 5U4 with no difference, also put the first rectifier along with all the tubes on another amplifier and there was no problem. Rechecked capacitors and they are installed correctly. Now during my latest trouble shooting I found that if I disconnected the choke with rectifier installed the symptoms disappeared so I changed the choke for a new one but it made no difference? I have to believe it is the output transformer failing as with the readings from Tony’s test it is, at least, partially shorted. I’d love to get your perspective on this. Thank you,
Mauro
Sent from my iPhone
On May 19, 2026, at 2:09 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
Jukebox-list mailing list -- jukebox-list@lists.netlojix.com To unsubscribe send an email to jukebox-list-leave@lists.netlojix.com %(web_page_url)slistinfo%(cgiext)s/%(_internal_name)s Searchable Archives: http://jukebox.markmail.org/
Hi, Mauro:
Actually it was me that typed that post. Sounds like you are onto the problem, though. To be sure, unhook the audio transformer leads and just ohm from one of the primary leads to one of the secondary leads.
Best
Mike
On 2026-05-19 17:14, M De Simone via Jukebox-list wrote:
John,
Thanks for chiming in. I agree Tony’s test is neat? I love it. Now to answer your questions, the amp does not draw excessive current without the rectifier. I tried a different 5U4 with no difference, also put the first rectifier along with all the tubes on another amplifier and there was no problem. Rechecked capacitors and they are installed correctly. Now during my latest trouble shooting I found that if I disconnected the choke with rectifier installed the symptoms disappeared so I changed the choke for a new one but it made no difference? I have to believe it is the output transformer failing as with the readings from Tony’s test it is, at least, partially shorted. I’d love to get your perspective on this. Thank you,
Mauro
Sent from my iPhone
On May 19, 2026, at 2:09 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
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Mike,
My apologies as Johns name was displayed on my iPhone for some reason. Thank you for helping out! I disconnected the center tap from the OT and measured from all primaries to the secondaries and get about 10 meg ohm resistance between them. Not sure what it should be but I assume this is ok?
Mauro
Sent from my iPhone
On May 19, 2026, at 6:22 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
Hi, Mauro:
Actually it was me that typed that post. Sounds like you are onto the problem, though. To be sure, unhook the audio transformer leads and just ohm from one of the primary leads to one of the secondary leads.
Best
Mike
On 2026-05-19 17:14, M De Simone via Jukebox-list wrote: John,
Thanks for chiming in. I agree Tony’s test is neat? I love it. Now to answer your questions, the amp does not draw excessive current without the rectifier. I tried a different 5U4 with no difference, also put the first rectifier along with all the tubes on another amplifier and there was no problem. Rechecked capacitors and they are installed correctly. Now during my latest trouble shooting I found that if I disconnected the choke with rectifier installed the symptoms disappeared so I changed the choke for a new one but it made no difference? I have to believe it is the output transformer failing as with the readings from Tony’s test it is, at least, partially shorted. I’d love to get your perspective on this. Thank you,
Mauro
Sent from my iPhone
On May 19, 2026, at 2:09 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
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Hi, Mauro:
I don't have a schematic for your machine, but that is kind of an iffy number. With the kind of current you are pulling on the power transformer, I would expect it to be lower than that, but with all of the audio transformer leads disconnected, it should be an open circuit. If you still have other leads connected, maybe there is a path there that is giving you the 10 meg reading. I don't think 10 meg would make it pull that much current. At this point, I would suggest this:
I'm presuming you have push-pull outputs? Disconnect the secondarys, install your tube and try it. If the problem is gone, there may be a problem in your speaker wiring. If you still have the short, it pretty much has to be the audio transformer, but, again, I don't have a schematic.
Maybe some of the more experienced guys here can be of more assistance.
Best
Mike
On 2026-05-19 17:59, M De Simone via Jukebox-list wrote:
Mike,
My apologies as Johns name was displayed on my iPhone for some reason. Thank you for helping out! I disconnected the center tap from the OT and measured from all primaries to the secondaries and get about 10 meg ohm resistance between them. Not sure what it should be but I assume this is ok?
Mauro
Sent from my iPhone
On May 19, 2026, at 6:22 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
Hi, Mauro:
Actually it was me that typed that post. Sounds like you are onto the problem, though. To be sure, unhook the audio transformer leads and just ohm from one of the primary leads to one of the secondary leads.
Best
Mike
On 2026-05-19 17:14, M De Simone via Jukebox-list wrote: John,
Thanks for chiming in. I agree Tony’s test is neat? I love it. Now to answer your questions, the amp does not draw excessive current without the rectifier. I tried a different 5U4 with no difference, also put the first rectifier along with all the tubes on another amplifier and there was no problem. Rechecked capacitors and they are installed correctly. Now during my latest trouble shooting I found that if I disconnected the choke with rectifier installed the symptoms disappeared so I changed the choke for a new one but it made no difference? I have to believe it is the output transformer failing as with the readings from Tony’s test it is, at least, partially shorted. I’d love to get your perspective on this. Thank you,
Mauro
Sent from my iPhone
On May 19, 2026, at 2:09 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote: > I have an HFMA1 amplifier that has just been completely > rebuilt, all caps changed and resistors checked. Tubes are all > verified good. > Problem: > As I slowly bring up voltage I start to hear the signal but if > I bring it up any more than about 70-80 volts on the mains it > begins to make a crackling sound and the current runs away. I > have it connected to an amp meter and shut it down when it starts > quickly climbing past 2 amps and crackling. > Now, through troubleshooting I have found that it still happens > with only the 5U4 installed in the amp. > I’m thinking it has to be one of the transformers breaking down > at higher voltages BUT I am getting proper voltages from the > power transformer and preliminary primary resistance measurements > indicated the output transformer is not shorted. > Need some opinions from the wisdom in this group. > Any thoughts on which it might be or perhaps some other > opinions? > Thanks as always, > > Mauro > Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
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That doesn't sound good. It's really high but it should be infinite.
On 5/19/2026 6:59 PM, M De Simone via Jukebox-list wrote:
Mike,
My apologies as Johns name was displayed on my iPhone for some reason. Thank you for helping out! I disconnected the center tap from the OT and measured from all primaries to the secondaries and get about 10 meg ohm resistance between them. Not sure what it should be but I assume this is ok?
Mauro
Sent from my iPhone
On May 19, 2026, at 6:22 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
Hi, Mauro:
Actually it was me that typed that post. Sounds like you are onto the problem, though. To be sure, unhook the audio transformer leads and just ohm from one of the primary leads to one of the secondary leads.
Best
Mike
On 2026-05-19 17:14, M De Simone via Jukebox-list wrote: John,
Thanks for chiming in. I agree Tony’s test is neat? I love it. Now to answer your questions, the amp does not draw excessive current without the rectifier. I tried a different 5U4 with no difference, also put the first rectifier along with all the tubes on another amplifier and there was no problem. Rechecked capacitors and they are installed correctly. Now during my latest trouble shooting I found that if I disconnected the choke with rectifier installed the symptoms disappeared so I changed the choke for a new one but it made no difference? I have to believe it is the output transformer failing as with the readings from Tony’s test it is, at least, partially shorted. I’d love to get your perspective on this. Thank you,
Mauro
Sent from my iPhone
On May 19, 2026, at 2:09 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote: > I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. > Problem: > As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. > Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. > I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. > Need some opinions from the wisdom in this group. > Any thoughts on which it might be or perhaps some other opinions? > Thanks as always, > > Mauro > Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
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You could save some time following the path of the output transformer and disconnect each *stage* at a time. It doesn't matter if the filter caps are new, add them to the list of things to disconnect. Also, I mentioned that windings can short to the core, the same goes for the choke. Which end did you disconnect?
On 5/19/2026 6:14 PM, M De Simone via Jukebox-list wrote:
John,
Thanks for chiming in. I agree Tony’s test is neat? I love it. Now to answer your questions, the amp does not draw excessive current without the rectifier. I tried a different 5U4 with no difference, also put the first rectifier along with all the tubes on another amplifier and there was no problem. Rechecked capacitors and they are installed correctly. Now during my latest trouble shooting I found that if I disconnected the choke with rectifier installed the symptoms disappeared so I changed the choke for a new one but it made no difference? I have to believe it is the output transformer failing as with the readings from Tony’s test it is, at least, partially shorted. I’d love to get your perspective on this. Thank you,
Mauro
Sent from my iPhone
On May 19, 2026, at 2:09 PM, mboessen--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:
My first question would be "does it pull too much current without the rectifier in there"? If it does, then you almost surely have a bad power transformer. If it is fine without the rectifier, double check that tube. If it isn't shorted, look for a backwards installed filter cap. That will for sure give you a "crackling" noise while the guts are frying inside that. Least likely is an audio output transformer shorted from primary to secondary. I just had one in a Braun radio, and I spent a lot of time checking everything else, as that is not a common failure.
On 2026-05-19 12:29, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
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On 5/19/2026 1:29 PM, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
Thanks, but I believe on alt.rec.juke-box.com, (or what ever that group is named) mentioned it 25 or more years ago, and I thought the same as you! Wow, so easy! :)
On 2026-05-20 3:51 a.m., Tony Miklos via Jukebox-list wrote:
On 5/19/2026 1:29 PM, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
Thanks, but I believe on alt.rec.juke-box.com, (or what ever that group is named) mentioned it 25 or more years ago, and I thought the same as you! Wow, so easy! :)
alt.collecting.juke-boxes was the Usenet group...last posting there was 2023.
John :-#)#
On 5/20/2026 2:59 PM, John Robertson via Jukebox-list wrote:
On 2026-05-20 3:51 a.m., Tony Miklos via Jukebox-list wrote:
On 5/19/2026 1:29 PM, John Robertson via Jukebox-list wrote:
On 2026-05-18 12:10 p.m., Tony Miklos via Jukebox-list wrote:
On 5/18/2026 2:19 PM, M De Simone via Jukebox-list wrote:
I have an HFMA1 amplifier that has just been completely rebuilt, all caps changed and resistors checked. Tubes are all verified good. Problem: As I slowly bring up voltage I start to hear the signal but if I bring it up any more than about 70-80 volts on the mains it begins to make a crackling sound and the current runs away. I have it connected to an amp meter and shut it down when it starts quickly climbing past 2 amps and crackling. Now, through troubleshooting I have found that it still happens with only the 5U4 installed in the amp. I’m thinking it has to be one of the transformers breaking down at higher voltages BUT I am getting proper voltages from the power transformer and preliminary primary resistance measurements indicated the output transformer is not shorted. Need some opinions from the wisdom in this group. Any thoughts on which it might be or perhaps some other opinions? Thanks as always,
Mauro Mississauga, Ontario, Canada
Although it tested good, try replacing the 5U4, they can short out. Also disconnect the B+ coming out of the 5U4 from the filter caps, if it stops, one of the electrolytics could be shorting, test one by one. If using a capacitor tester, use one that will actually put 450 vdc into the cap when checking for leakage. Take a look at it with the lights dimmed, when something is shorted there, the 5u4 usually gives off a little fireworks display inside the tube.
For the output transformer, hook up 10-12vac at the speaker connections (with no speakers or other power hooked up) then measure the input at the 6L6 sockets. I think it goes up to about 450 vac or so to ground. Put a meter and/or a scope on it to look for fluctuations. You could probably hear it *ticking* away if it's shorting, and watch for smoke of course. :)
Hi Tony,
I LOVE this test of yours! What a simple way to test the output transformers!!
I was looking for LQ-Meters, etc, in other words over-thinking the problem. I expect this is an old tube amp trick used in radios and amps.
John :-#)#
Thanks, but I believe on alt.rec.juke-box.com, (or what ever that group is named) mentioned it 25 or more years ago, and I thought the same as you! Wow, so easy! :)
alt.collecting.juke-boxes was the Usenet group...last posting there was 2023.
John :-#)#
So they are in a little slump huh?
jukebox-list@lists.netlojix.com
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John Robertson -
M De Simone -
mboessen＠ktis.net -
Tony Miklos