Black-Face Vibro-Champ help

[1969mako]

So I am a Newbee when it comes to tube amps but I do have some knowledge of circuits in a former life so excuse me if I am slow or I am using wrong terminology.
I have a 66 Vibro-Champ blackface amp (AA764) that I purchase knowing it needed a little TLC. Although the amp is near perfect, I have a problem with the amp producing low volume.
Meaning, when I fire up the amp I have to turn the volume up to 10 to hear the guitar a little.

I put a volt meter on pin 8 of the 6V6 and noticed I have about 80 volts when I believe I am supposed to have 21 volts. I also put a bias probe on the 6V6 and I am reading about 80 amp, and not milliamps.
I tested all tubes and they are good, I replaced the resistor and cap on pin 8 of that tube (I believe this determines Bias current) I also replaced the power Capacitor (20/20/20) and lastly, I replaced a cap that goes to ping 5 on that same 6V6 because I believe ping 5 on that tube should read 0 but it doesn’t, the meter just keeps fluctuating. Does this sound like my problem with low volume? Am I barking up the wrong tree?
Starting to get a little frustrated because I am missing something and this circuit is not all that hard.

Any help would be very much appreciated!!!
1969mako

[tubeswell]

Can you please detail all your idle voltages at B+, screen and preamp supply nodes, as well as at plates, screens, cathodes, and signal grids.

[1969mako]

OK, so this is where my Newbee comes in. I am not sure what or were B+ is... Sorry.
Is the screen and preamp supply nodes on the 12AX7??????
as well as at plates, screens, cathodes, and signal grids, I am assuming those are on the 6V6?.

[PRR]

> I am reading about 80 amp, and not milliamps.

That's impossible.

> I believe ping 5 on that tube should read 0

Yes.

> but it doesn’t

Remove the 6V6. What is the 6V6 grid voltage?

[1969mako]

Screen grid (pin 4) is 441 vdc
Control Grid (pin 5), meter bounces all over the place.
You say That's impossible. but with the 6V6 in there from pin 8 to ground, it slowly goes up over 60 vdc and then I shut it off.

[tubeswell]

Control Grid (pin 5), meter bounces all over the place.

The control grid in that amp should be at 0VDC. Test the (.02uF) coupling cap that is connected to that grid for possible leakage (which could be throwing off the bias). To do this, you need to unsolder the coupling cap lead*(at the 6V6 grid end) and hook at VDC meter up to the (now unconnected) end of the coupling cap; switch the amp back on and measure that cap to see if there is any VDC on it. There shouldn't be any.

* the alternative I guess, is to measure pin 5 of the socket without the 6V6 plugged in, but I always disconnecct the coupling cap when I do this test just because I'm an anal nitwit ;-)

[1969mako]

Sorry, I mis-spoke on the Bias voltage. I meant to say 80 miliamps.

I replaced that .02uf cap.

I just looked at that cap and it says f233k f    630mpp1

My sales slip description says ".022uF/630V - Xicon polypropylene coupling capacitor" for that part.

[TIMBO]

Yes ,that is correct

[1969mako]

Found the issue.
The 220k resistor that was in series with that .02 cap  is supposed to be tied to ground (on the control panel) came off as well as the 25uf cap that goes to Vibro 12ax7 tube.
After re-soldering them, Walla, loud music.
Lesson to be learned, check all solder joints for issues before looking at components…
Thanks everyone for all your help!!!

[HotBluePlates]

Further post-mortem:

The grid voltage kept climbing when you had a 6V6 in the amp.

There's 2 main ways that could happen.
1. Coupling cap is leaking, making the grid positive, which makes tube current too high. But leaky-caps usually result in a steady positive grid voltage.
2. No ground reference for the grid. With the grid not tied to ground, its voltage could drift up or down, depending on circumstances. That either makes the tubes run too hot, or clamp off all signal.

Glad you found it.