Circuit function questions on 6v6 amp

[Tonewood]

Hi all-
I just got an "amplified speaker" at the flea market that's not working. It shouldn't be hard to get working but there are some things that I'm not familiar with & would like to understand. My experience is mostly with basic Champ & Princeton circuits. If anyone could comment on the questions/comments below, that would be great. Thanks.

1: There is no diode in the bias circuit. Is that normal in old audio?
2: The cathode of 6v6 looks closer to + output than ground. That seems odd to me. Why not just connect it directly to ground?
3: There are resistors before the anode/plates of the 6x4. Any practical reason to drop voltage this way or is there some other reason for this?
4: Can anyone explain how the bass circuit works? Is signal from the output getting through the 97k of resistance and becoming part of the local feedback of the first section of v1?

General question: Is the + output signal from the transformer in phase or out of phase with the signal from the 6V6?
Thanks again!
Scott

[Dave H]

1: It's back biassed by the 300R 5W in series with the PT CT.
2: I'm not sure. It looks like negative feedback from output to cathode.
3: I think they are just to reduce the B+ voltage.
4: It's (global) negative feedback from the output to the first stage.

I think the 6V6 plate (BLUE) is out of phase with the OT output (YEL)

[martin manning]

2: I'm not sure. It looks like negative feedback from output to cathode.

I think that is correct.

3: I think they are just to reduce the B+ voltage.

Could also be limiting current so the 6X4 can get along with the 40uF reservoir capacitor.

4: It's (global) negative feedback from the output to the first stage.
I think the 6V6 plate (BLUE) is out of phase with the OT output (YEL)

I think this is correct too.

[loudthud]

The bass control works just like a Resonance control some amps use. When the control is shorting out the .0035uF cap, all the bass gets through. As the control is advanced the high frequencies go through the cap but the bass has to go through the pot so it gets attenuated. Because this is a feedback circuit, when you attenuate the bass there is less feedback so bass is increased at the output of the amp.

I have a similar Voice of Music amp. Mine does not have the resistors in series with the 6X4 plates and has a separate winding for the 6X4 heater. There is no feedback to the 6V6 cathode and the 300 Ohm resistor goes from cathode to ground. The part number on the schematic is 8697-9 and the numbers rubber stamped in black are 857 1862.

[Gingertube]

The guys above have got it all correct, back biasing, cathode feedback, and tone controls in the feedback path.
Except maybe for the resistors in the anodes of the rectifier tube.
These 300 Ohm resistors may be there to drop the high voltage a little BUT as Martin says, they may also be there to meet minimum plate resistance required by the rectifier tube.

I checked the 6X4 data sheet and it says typically 10uF max for the 1st capacitor with 525 Ohms plate resistance. That plate resistance includes high voltage secondary winding resistance plus primary winding resistance reflected into the secondary plus any added resistance (like those 300 Ohms) BUT even so, with 40uF as the first capacitor, the added 300 Ohms is maybe on the lowish side - DEFINITELY leave them in.

Cheers,
Ian

[g1]

Sometimes resistances like that were used to save the PT in the event of rectifier failure. The 6X5 was particularly troublesome, and it's not that different from the 6X4.
The 6X5 - a problematic tube - Canadian Vintage Radio Society

[Dave H]

I checked the 6X4 data sheet and it says typically 10uF max

Thanks, for the info. I did know about the max capacitance rating of rectifier tubes but I didn't know it could be that low. I've only used GZ34. I've never seen a 6X4.

[Tonewood]

Thanks for all the help everyone! It all makes sense now & I wouldn't have come up with it on my own.

[Tonewood]

Thanks for the info loudthud-
Does your amp sound ok as a small guitar amp? If it does and you can recommend any mods that you did, let me know. Otherwise I'll get it up & running and just move the volume control & see how it sounds.

[loudthud]

I got the amp in the early 60's with an RCA 45rpm turntable. It would play a stack of 45s and then repeat the last one until you turned it off or put on a new stack. I remember playing the first (USA) Beatles record on it. The B side was I Saw Her Standing There. The turntable ran slow and the second note of the song really boomed out of the home made bass reflex speaker I had. I played guitar through it for a while but later rewired it like a Champ with a tone stack. About 68 I got a Vox Buckingham.