Fender Vibro-Champ AA764 Distortion

[fenderguy81]

I am working on an early 70s Fender Vibro-Champ that is giving me trouble. The output is distorted at every level--almost like a distorted tone running underneath the clean signal. I've replaced all the electrolytic capacitors including the multi can cap (upped the first stage to 40uf like the later versions), checked all the resistor values, tried new preamp tubes and power tube (haven't touched the rectifier yet), bias set to ~91% with Eurotube bias meter(352 PV; 37 PC) tried different speakers, but haven't been able to track down the issue.

I read in another post someone was having a similar problem and noted a fix by lowering the screen voltage on the 6V6 because it was higher than the plate voltage. I believe they upped the 1K-1W resistor to a 4.7k. My screen voltage is 373 and plate voltage is 370 so 3v higher, which I read is normal at idle. There is also a video on Youtube that apparently a bit controversial where the guy swaps the 1k and 10k power resistors. Would it be advisable to up the 1k power resistor, swap the 1k and 10k or look elsewhere? Plate resistors? NFB? Output transformer? etc. Any help would be greatly appreciated!

Video mentioned in post for reference:

[sluckey]

There is also a video on Youtube that apparently a bit controversial where the guy swaps the 1k and 10k power resistors.

Easy to try. Tell us what you think.

[stratomaster]

It's scope time, hoss.  If no scope then listening amp. 

[HotBluePlates]

... a fix by lowering the screen voltage on the 6V6 because it was higher than the plate voltage. ...

Idle "screen voltage being higher than the plate" doesn't matter, unless the screen is something like 200v higher than the plate voltage.

When you actually play the amp, the plate voltage is lower than the screen voltage half the time anyway.

I am working on an early 70s Fender Vibro-Champ that is giving me trouble. The output is distorted at every level--almost like a distorted tone running underneath the clean signal. ...

Try plugging into a different speaker (yes, you'll probably have to rig up a 1/4" to RCA converter).

[dude]

Check the plate preamp 100ks, or just replace, do the swap. l did 5k first drop, 3watt (2w fine) and 1k, to the preamp. Leave the 470k bias resistor, you can run class at 100 % dissipation. Your voltages are fine. Remember that tiny OT is 5 watts and can get over saturated easily, anything pass 7, gets distorted with that tiny OT. I upgraded with an old vintage SE 10 watt OT, very satisfied.
You don’t want to butcher a vintage amp but a 10” Alnico with impedance matching bigger OT and that little amp sings, on 10.
Might also try another 6V6GT if all else fails.

[tubeswell]

If it’s ‘distorted at every level’ and you’ve changed the caps and the tubes, it makes me wonder whether the filter cap stage for the preamp power supply node is not properly decoupling. Measure the HT voltages along the power rail. Look to see if the preamp supply node HT is the same as the Screen supply node HT. (It shouldn’t be).

[AlNewman]

So was the distortion a symptom before you did the work or after?

You cleaned all the pots and jacks, and tightened all the screws?

I agree with using a listening amp.  Gotta narrow down where it starts.

[jjasilli]

I had a similar issue by being too cavalier with NFB.  Note the schematic: 


1.  There is i) global NFB; and ii) local NFB at the power tube: signal grid to cathode.


2.  There must be the split cathode resistance at the global NFB insertion point, or the global NFB will drain to ground & be defeated.

[fenderguy81]

I tried a few more things, but still no luck. Here's a picture of the chassis and tube pin voltages. The problem was present BEFORE any work was done. This is everything that has been tried:
-different speaker
-different tubes except for rectifier (also tried pulling v2 out)
-replaced all electrolytic caps (had even tempted in F&T caps before committing to the new can cap)
-checked all resistors
-replaced 100k plate resistors (metal oxide)
-replaced power resistors and swapped places (1k and 10k; metal oxide)
-new 3-prong grounded power cable
-problem present on both hi and lo inputs
-probing/listening amp: distortion begins at pin 2 of V1 so unfortunately not a lot of help

I'm a bit stumped. The only thing left I can think of to try is removing the 2.7k NFB resistor and/or a new 5y3 rectifier tube?

[sluckey]

-probing/listening amp: distortion begins at pin 2 of V1 so unfortunately not a lot of help

That says the distortion is coming from outside the amp. What signal level are you plugging into the input jack?

[AlNewman]

Try jigglin' your guitar cord.

[tubeswell]

Here's a picture of the chassis and tube pin voltages

As I said previously, Measure the HT voltages along the power rail. Look to see if the preamp supply node HT is the same as the Screen supply node HT. (It shouldn’t be).

[AlNewman]

It is kinda strange your screens are sitting 50v below the plate of your 6v6...?

[sluckey]

It is kinda strange your screens are sitting 50v below the plate of your 6v6...?

Not really. He swapped positions of the 10K and 1K droppers. So the 10K now sits between the plate node and screen node.

[fenderguy81]

Well it ended up being the replacement WGS speaker. I had tried another speaker early on, but didn't notice any difference at that point so the issue must have been resolved with the parts change and reverting to the stock speaker. Thanks for the input and I'm glad it's working great again!

[sluckey]

You may want to put the 10K and 1K resistors back in the proper position.

[HotBluePlates]

I tried a few more things ...
...
-replaced power resistors and swapped places (1k and 10k; metal oxide)
...

You may want to put the 10K and 1K resistors back in the proper position.

As the amp sits, the 6V6 is idling a little under 13w based on the numbers he provided.  Seems reasonable to leave it be.