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 :-#)#