Mystery 70s - 80s Tube Amp Troubleshooting (Terrible Feedback)

[ScoopMyMidsDaddy]

Alright so I got a weird one here. I bought this for dirt cheap from a guy on fb marketplace that didn't know too much about tube amps. He said it worked when he last played it and had a video to prove it. He was told it was a marshall clone kit but with less gain than marshall. It has two inputs (one is a boost), a weird 1/4 "bias" jack, a mystery knob on the rear, and a unlisted 1/4 jack on the rear. The amp has a brilliance knob on the front of the amp so the mystery rear knob shouldn't be a presence

I turned the amp on and the power tubes warmed up properly, once I put the amp to on it gave me some loud and terrible feedback through the cabinet which kept going even after I swapped it back to standby. This happened at any volume and I was using the "spkr" output. I had the rear knob dialed to 9 oclock (exactly where it was when I bought it). The volume pot feels beyond trashed so I'm wondering if that pot is part of the problem.

I'm just posting this here to see if anybody recognizes this general build or has any insight on this.

3x 5751 preamp tubes, 2x 6CA7 power tubes, 25BQ6GA mystery tube, GZ34 rectifier tube

The guy told me that when he got it the tube between the transformers wasn't installed (25BQ6GA). He bought the tube to install, but he couldn't give me any additional info on why or how he chose that tube. He seemed very inexperienced with tube amps and seemed to be "oh look its missing a tube, let me throw one in there real quick". Apparently he posted this on forums at one point but I couldn't find any posts with this amp anywhere.

Any guidance would be greatly appreciated and additional photos could be provided if anybody needs to see something specific. My buddy is coming with his tube tester next week so we are going to make sure all the tubes are in good shape. Some of the tubes seem ancient, for example the rectifier tube is a 1965 Mullard GZ34.









poop smileys

[Helmholtz]

Try reversing the OT primary leads.

[g1]

The part labelled 'bias' is not a jack, but a pot. It should have a slotted recessed shaft like a classic Fender bias pot. Between that pot and the fuse holder is a test point jack for a meter probe.
I think it's extremely unlikely that a 25BQ6 (or any 25 prefix tube) is used in this amp. If he was able to find such info he would also know make & model of the amp, or who built it and what design it was based on.

You would need to draw out a schematic for the other pot and jack on the back. Wild guess would be a line out jack with volume control.

[Helmholtz]

I think it's extremely unlikely that a 25BQ6 (or any 25 prefix tube) is used in this amp.

That.

And without a cap contact the tube couldn't work, as the cap is the plate connection.

[ScoopMyMidsDaddy]

The part labelled 'bias' is not a jack, but a pot. It should have a slotted recessed shaft like a classic Fender bias pot. Between that pot and the fuse holder is a test point jack for a meter probe.
I think it's extremely unlikely that a 25BQ6 (or any 25 prefix tube) is used in this amp. If he was able to find such info he would also know make & model of the amp, or who built it and what design it was based on.

You would need to draw out a schematic for the other pot and jack on the back. Wild guess would be a line out jack with volume control.

So after interrogating the guy I bought this from he told me he saw the empty tube socket and "threw in anything he had laying around that fit". The guy admitted he didn't know anything about tube amps.......

That 25bq6 tube also has a burn mark on one side about halfway down from the top. I haven't tried the amp with that tube pulled yet, I was told it worked and tried it out just as I bought it.

Only thing I've done to this is deoxit the pots and make sure nothing was no bulging or burnt parts in the amp.

[glebert]

So it worked the last time he played it, and then just threw a tube into an empty socket? Can't tell if people are more stupid or more awful. At least it was cheap.

[Helmholtz]

I haven't tried the amp with that tube pulled yet, I was told it worked and tried it out just as I bought it.

Is the socket with the mystery tube even wired?
I don't think there is a risk with pulling the tube.

[ScoopMyMidsDaddy]

Is the socket with the mystery tube even wired?
I don't think there is a risk with pulling the tube.

I can't tell without pulling the entire board, which involves pulling other crap cause the enclosure barely fits the build. Guy I bought it from said he got it with no tube installed there and just threw a random tube that fit the socket in there cause he had some noise issues with the amp........

I'm at work atm but gonna test without that tube for the hell of it. He also mentioned replacing a fuse at one point a while back and I found this in the amp. Literally no markings on the fuse anywhere to tell me what its.

[g1]

It's a slow-blow fuse so that's a good start. Get a magnifier and check right on the flat ends for markings, then all the way around the metal at both ends.