Tube Amp Archive

You're right about the importance of the phasing wrt nfb. A multi-tap secondary must have the common lead going to ground so that the 4-8-16 taps will have the correct impedances. I've never seen anything like the dot markings on the diagram attached to the box, but white is clearly the CT since it...

No sweat... Glad you got it rockin'. That is a lesson that you won't forget very soon. ;) Also, those dots indicate the polarization of the windings. It really doesn't affect your OT situation much but helps when connecting PT dual primaries in series or parallel. Pay attention to those dots then. ...

If that is the OT you have in there now (Hammond)... the white wire is the CT. Blue and red are plates. Swap the white and blue wires. That should restore full volume and get rid of the crackle. If it squeals, swap the red and blue to opposite plates. ...sigh... Really? Oh boy... Just so you all ca...

Ok that all sounds good. This issue started happening after changing the transformers, are the OT primaries definitely hooked up ok? Wondering if the centre tap has been connected to one of the EL34s by mistake. And have you checked continuity between the EL34 cathodes and ground? This was somethin...

Now this is a puzzle! Just to clarify, do you use two bias probes? One per socket? If so, have you tried swapping the probes? And can you please measure screen voltages. It sure is, isn't it? I actually only have one bias tester, I just power down and alternate when checking different sockets or tu...

... the bias current doesn't seem to stop rising. It's very slow, but I haven't found the top point and I'm not sure that it's related to this distortion. Only thing I can think of at the moment. How about if the amp is operated with the valve in V3 is removed? ie does the output valve anode or cat...

Littlewyan wrote: ↑Fri Apr 22, 2022 7:57 pm Can you measure the plate voltage on pin 3 of both EL34s please? And check that the cathodes are grounded ok.
Do you have an oscilloscope?

The plate voltages are 476 and 477

Littlewyan wrote: ↑Fri Apr 22, 2022 7:57 pm Can you measure the plate voltage on pin 3 of both EL34s please? And check that the cathodes are grounded ok.
Do you have an oscilloscope?

I forgot to write those voltages down... I'll post them in a little while.

And yes, I have an oscilloscope. It's a cheap hantek USB scope, but it works

... moving on... The reading at the grid leak resistors is -45.6 and at both pin 5s it's about -44.6. That's with one of the tubes current reading around 36.8mA and the other reading 24ish. If I adjust the bias trim down, I can't get it to -50V. It only gets to around -48. But this brings the bias c...

Thank you. I wish that didn't need to be said. I mean, I did mention at the very beginning of this post that this amp was something that somebody had brought in. I thought for sure that would have explained the situation. I don't run some big shop or anything, this just helps me cover a little groce...

Actually, that's 490 unloaded. More like 475 with the tubes in. Still, this issue seemed to be happening regardless of where I set the bias or what tubes I used

The transformers both failed a shorting test. I've built a tester that utilizes a 30V neon bulb lit by the voltage kickback from "charging" a transformer with a <5V DC supply. The primary on the OT failed, as did the heater winding on the PT I did leave some things out of my initial post. In additio...

Hey, all. I've been working on this Marshall that somebody brought in, and after replacing both the power and output transformers, it began to have this weird distortion. First half of the clip is with presence maxed out, second half is with it turned below 4 on the dial mrsh_1987x.mp3 Which I reall...