On 8/20/2025 2:34 AM, John Robertson via Jukebox-list wrote:

On 2025-08-19 10:04 p.m., organlists1--- via Jukebox-list wrote:

...

I'm a bit late to the party here....

I believe when you disconnect the loudspeaker plug, which is a EM (Electro-Magnetic), it will disconnect the B+ as the field coil is connected in series.

Don Resor

-----Original Message----- From: ascott822--- via Jukebox-list jukebox-list@lists.netlojix.com Sent: Tuesday, August 19, 2025 8:11 PM To: jukebox-list@lists.netlojix.com Cc: ascott822@aol.com Subject: [Jukebox-list] 780 fuse blowing issue

Still trying to chase down the reason the amp fuse blows.  I replaced the 5Z3 tube for the chance that it might have a short as was suggested. That wasn’t the cause. I removed all plugs into the amp and made selections  and as I replaced one at a time as a process of elimination.  The fuse only blows when the speaker is plugged in. Any help please. Thanks, Allen

If the fuse only blows with the speaker is plugged in AND the 5Z3 tube is unplugged, but doesn't when the speaker not plugged in, then something about the speaker field coil is most likely the problem.

John :-#)#

This all is not quite in order, it's all jumbled up, but I just woke up.

I didn't quite follow everything you tried because you started a new thread, but how about a shorted filter cap in the amp? (or one accidentally installed backwards.) That is if it only blows when the speaker AND the 5z3 are plugged in. If it blows with the speaker plug in and the 5z3 out, that does sound like a speaker plug wire mix up.

Did you re cap the amp? Did you happen to power it up before it was capped and did it make sound then, even if it was just a hum? Are you using a slow or fast blow fuse? How fast does the fuse blow, immediately, or does it take a few seconds. Are you using the correct value fuse? Try clipping your meter onto the first filter cap and to ground to see if it shows any voltage before the fuse blows. If you have an auto ranging meter, pre select your voltage range for over 500 volts so you might get a reading before the fuse blows. If it's blowing immediately, I'd go with John and check out the speaker plug or something else miss wired, or again, did it power up before being capped?

Clip the B+ wire off at the 5z3 or at the input to the first filter cap and check the DC voltage from the 5Z3.

By removing the speaker plug you are disabling the B+ in the amplifier. No high voltage present.

Sent from my iPhone

On Aug 20, 2025, at 9:09 AM, ascott822--- via Jukebox-list jukebox-list@lists.netlojix.com wrote:

Thanks for your reply. Are you saying there is a short in the B+ section of the amp? The amp works as long as speaker is disconnected. Blows fuse when plugged in. Box makes selections and amp comes on as long as speaker is disconnected.

Sent from the all new AOL app for iOS

On Wednesday, August 20, 2025, 1:04 AM, organlists1@sonic.net organlists1@sonic.net wrote:

I'm a bit late to the party here....

I believe when you disconnect the loudspeaker plug, which is a EM (Electro-Magnetic), it will disconnect the B+ as the field coil is connected in series.

Don Resor

-----Original Message----- From: ascott822--- via Jukebox-list jukebox-list@lists.netlojix.com Sent: Tuesday, August 19, 2025 8:11 PM To: jukebox-list@lists.netlojix.com Cc: ascott822@aol.com Subject: [Jukebox-list] 780 fuse blowing issue

Still trying to chase down the reason the amp fuse blows. I replaced the 5Z3 tube for the chance that it might have a short as was suggested. That wasn’t the cause. I removed all plugs into the amp and made selections and as I replaced one at a time as a process of elimination. The fuse only blows when the speaker is plugged in. Any help please. Thanks, Allen

Sent from the all new AOL app for iOS _______________________________________________ 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/

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