Gibson Ranger GA20-T Rebuild

[TomCarlos]

Hi Jeff...

Yes, I applied signal so that I reach an output just before clipping. Even at lower signal levels, I still see the imbalance.

The 220Ks (everything you see in that diagram) and brand new, metal oxide, all within 5% of each other. The 6V6 are brand new JJs, matched. I put those in to see if there was any difference compared to the old RCAs- no difference at all.

Tom

[guitician]

I wouldn't worry too much about that imbalance. That output waveform is not a pure sinusoidal wave, and the power from it is complex, but the power equation simple. You may already be putting out 12 watts. Have you played it into a speaker?

[TomCarlos]

Yes... I am playing through a Celestion Seventy 80 for testing- Channel 1 sounds ok. I want to get this working properly before I test out Channel 2.

Eventually, my friend has a new Celestion G12-M, 25 Watt that he will use.

BTW- In another thread, I am dealing with a "squeal" issue.

[Jazz P Bass]

That PI 'imbalance' is to be expected of that type of inverter circuit.
My 2 cents.

[TomCarlos]

Thanks John!

[TomCarlos]

Ok...

If the PI is ok, let me attack what can be the last issue... the "high pitch squeal."

In reading other posts, the place to start is the Power Supply.

With no input and with the output tubes removed, I am seeing .258 volts of ripple, measured coming right off the transformer, between the 10K and 20uf cap on the right, the 350v point.

If I insert the output tubes, I am measuring 4.8 volts of ripple at that same point. That seems excessively high.

When I measure between the 10K resistors (at the 275v point), the ripple is negligible, maybe 30mv.

The last 10K and 10uf cap have maybe 3mv ripple.

So, my question is.. the first point off the transformer.. 4.8vac ripple, does that seem too high?

[guitician]

No, and a well made Push-Pull transformer balanced winding cancels the hum because its 180 degrees out of phase on each half of the primary.

[TomCarlos]

Ok.... Let's call it good then for the ripple.

So, more testing and here is what I observed. The "Squeal" kicks in the minute I put the Tone Control at 12 O'Clock high (or higher). It seems like the amp does not like the high ends coming through. The .01uf feeding the 1 Meg Volume pot is new. The .01uf that goes to the 250K Tone pot is new. So this is telling me the problem is "frequency" related.

The research continues.

[g1]

Can you give a rundown of which electrolytics have been replaced?

[TomCarlos]

As part of this rebuild, I replaced ALL electrolytic caps.

In the Power Supply, I used F&T caps - the two 22uf, 500v and one 10uf, 500 volt.
For the 6V6 cathodes, I have a 22uf, 100v, Non-Polarized cap.
In the 5879 circuit, I have a 25uf, 63v cap.

I think I may have discovered my culprit... the 5879 tube. Even though I was running a guitar into Channel 1, I guess Channel 2 can affect what is going on.

When testing Channel 2, I noticed the Tremolo had an excessive "breathing" sound (sorry, not sure how to describe it). I replaced the 6SQ7, no difference. But when I replaced the 5879, the Tremolo became more quiet. In fact, that Channel cranked up and the Tone cranked up sounded great.

So I am back testing again, on Channel 1. So far, seems ok. Maybe it was the 5879 from the other Channel that was causing the issue?

[guitician]

Is this an acoustic, or electrical "Squeal"? Does it do it without anything plugged into the input? Can we rule out tube microphonics and pickup squeal?

[TomCarlos]

Shoot... swapping the 5879 did nothing... Squeal is still there.

I disconnected the guitar. With controls at high noon, I can still hear the high pitched squeal - definitely electric. Gonna look closely at the input jacks.

And still trying to figure out why this amp gets quiet (hum disappears) when I put a finger on the chassis?!?!?!?

[guitician]

How is that 1st tube wired? Like the schematic? I noticed that both halfs use the same plate resistor, that could be an issue. And that the grid resistors are 1M ohm for grid leak bias those are small, but may need to be that way for this circuit.

[TomCarlos]

Yes, the wiring matches the schematic.

There is a new 100K metal oxide that feeds both plates. I am measuring 45vdc on the plates.

The 1 Megs actually measure to 923K and 928K - close enough.

The input caps are new - .02uf (I measured and verified the values).

I did find another schematic for a Gibson 20T. There are some differences in how the Tone control is wired up. I guess there is more than 1 way to skin a cat, eh?

[TomCarlos]

Something else I just noticed... and going back to using a signal generator and speaker...

If I run a signal into Channel 2, Volume 2 turned up, the Voicing (Tone Control) half way, I get a nice signal. If I then turn up the Channel 1 volume (with nothing plugged into Channel 1), say to 7, 8 , or 9, the volume drops from Channel 2 (significantly) and the wave begins to oscillate.

I am going to test for a couple days, jot down results, and report back.

Grrrrrrr!!