Brown Concert Volume/Tone Controls

[ThermionicScott]

Bruce, was your friend's amp a 6G12A with a leftover 6G12 tube chart?

The only way I can see the channels interacting on a 6G12 is through that shared cathode on the input stage. The "other" channel's input triode will be driven by the cathode, producing an out-of-phase signal (sorta like an LTP) that you can shape with the tone and volume controls. The "other" signal will be weaker, so I'd expect to have to crank it to have much effect, like MWJB said.

- Scott

[Chuck H]

The only way I can see the channels interacting on a 6G12 is through that shared cathode on the input stage. The "other" channel's input triode will be driven by the cathode, producing an out-of-phase signal (sorta like an LTP) that you can shape with the tone and volume controls

Now that makes some sense. Perhaps Bruce didn't hear anything because the amp they tested has been serviced. You would need a shared bypass cap with an ESR so high as to be almost failing for this to work. But then it certainly would.

Chuck

[MWJB]

I have a 6G# circuit amp with seperate cathodes at V1/V2 pin 8...still works just like on a regular 6G# amp.

Bruce, I have many times had harp players pick up the difference in tone unprompted (I'm not the sort of guy to "lead" people) . Even guitar players have noticed it "Only works with the treble full up too" - BF Deluxe Reverb owner.

[MWJB]

"I had a friend bring his dead stock 6G12 over for this test (well it has been recapped and has new tubes) and between three of us... with respect to the unused channel's tone controls effecting the output tone.... none of us could hear any of the effects everyone is talking about. Rats" Thinking about it, the early, centre vol, 6G12 often has a different mixer resistor arrangement prior to the PI (1Meg on the normal channel, later amps & transitional 6G12/6G12A had 220K/2x470K from the trem) ...like yourself & Chuck I can't really see how a difference this late in the circuit could cause the effect, but maybe this is relevant to your results?

[Bruce / Mission Amps]

I don't know... I'm a bit baffled by this.
I think I have decent ears for this sort of thing, even if subtle.

Yes, it could just be this amp... I think it is a 1961 Concert. Even though I have serviced this amp a number of times over the years, I didn't slide the chassis out this time to see if there were any splitting of the cathode cap mods.
However, the shared cathode in the first stage is exactly the reason the unused channel's volume control effects the sound of the other in the early tweed amps.
Got me on this one.

[MWJB]

Last night I listened again to my own amp, which has separated cathodes at V1/V2 pin 8 & 220K mixer resistors at the PI - there is an additional 'presence boost' when all the unused channel controls are fully CW, I was even picking it up playing CD through the amp...it's like having the presence control go up to 10.5 rather than 10 (presence pot is typically fully CW at all times) if you get my drift - subtle, but tangible. Perhaps more noticable when running a harp signal/amp under drive. Also, was rebiasing a friends 6G12A last night...less noticable, but he is running a 12AY in the PI rather than a 12AX7, which is softening the amp generally anyway (he still remarked that he preferred to run all preamp tubes "and use the other channel controls to tweak the sound" rather than the common harp practice of just running V1 & V6.

One of my regular clients, who has 4x 6G#A amps, always asks me to ensure I give him the "mid mod on the other channel" when the amps are serviced/worked on, because he thinks that there is something I do to the amps that facilitates this effect...I keep trying to explain otherwise.

[pdf64]

A possible cause of the effect may be stray capacitence between the wiring (input and tone/vol) of the 2 channels, causing signal applied to the normal channel to appear in the vib channel. Ok it might only be 10s of pF, but with the vol and tone set high there's a lot of gain, so even with a -3dB of >100kHz (impedance at 1st stage grid is 34k with nothing plugged in), the vib channel could still output some of the normal channel's signal.
As lead dress is a variable factor, this might explain why some amps might exhibit the effect more than others.
Also if the amp was modded with screened cable from the input sockets to the 1st grid, the effect might be greatly reduced.