A general question:   JM said to add another diode in series with the existing one, then a resistor/trimmer in parallel with those.  So the question is, what does adding an extra diode in series do?   

As I said in the first post, I'm way more familiar with older tube circuits.  As such, I'm not so familiar with diodes (or transistors for that matter).

Thanks so much for looking in to this.  This is definitely over my head, but it's so interesting.

I know you guys are the experts here, so I don't want to sound like I'm second guessing.  But I do want to understand this problem a little better.  So if what your'e saying is right, and this is just a design flaw, then wouldn't this problem have been present since the amp's birth?  We're talking mid-70s.  And while there isn't a lot of info about this amp on the internet, this particular model was in production for probably 8 or 9 years.  Don't you think Ampeg would have had to address this problem at some point?

Or, on the other hand, is this a problem that, given the design flaw, wouldn't necessarily be present from the beginning?  As in, the poor design caused this distortion to only appear after a certain amount of use/time?

I will try the trim pot idea - that sounds safer than the possible overheating.  I'll have to order the part, so it'll be a few days before I can report back on anything.  In the meantime, if there's any alternate theories here, let me know!

Thanks again.  You guys are amazing.

Sorry for the confusion.  We're back to the original question about the Ampeg GT10.

Across diode D1:  -.7v

Across R44+R45:  0v

Across E10 to E11 (which is really the same as R44+R45):  0v

(To be exact, those 0v readings did read at .1mv for a second, and finally rested at 0.0v.)

The diode seems right, compared to the schematic.  But what about emitter to emitter?  Can that be right?

Thanks for the idea Enzo.  I would have never thought about plugging into the reverb switch jack.  Plugging in from there, the problem persists.  So that means it's somewhere in the power section, right?

I checked voltages where they were notated on the schematic.  I'm attaching here the schematic with my readings in red.  In general, my voltages were a touch higher, but as far as I know, that's normal considering this schematic was written in 1971.

The only place I thought was a little too high is right around Q7 and Q8 - the schematic has it at 6.5v and I'm reading 7.5v.  Is that still within the margin of error for 1971 voltages vs 2016 voltages?

I hope the attached schematics/readings are legible enough.  Please let me know if I can post this in a more clear or understandable way.

Thanks for the replies here.

I don't see any bias pot...  although it's entirely possible that I'm missing something. 

I thought I did post the schematic, but who knows.  I'm new here.  I'm going to try to attach it to this post as well.  Here's hoping.

Hello.

I'm a relative beginner to amp repair, but I've gotten pretty comfortable inside of tube amps.  I don't know the first thing about solid state, though.  What better way to learn than just to dive in?

I bought an old Ampeg GT10 (schematic attached) that has this really faint distortion that sort of sits on top of a note.  It's tough to describe, but it's not like the note itself is distorted, it just has this "fizziness" that follows it, and is most apparent when I let a note ring out.  It occurs at any volume.  It almost sounds like a problem with the speaker, but I've tested the amp with a known-good speaker in a different cab, and the problem persists.

Just as a blind guess, I went ahead and replaced the filter caps - C30 and C31 on the schematic - as well as C29.  No change.  Can anyone suggest a potential next step?