1987/1959 channel mixer (channels interaction)

[framos]

Hi everyone,

this weekend I was playing with my 1987 clone normal channel.
I changed the cathode from 820/220uf to 1k5/220uf, and added a 500pf bright cap to the normal channel volume pot.
This gave me a JTM45-ish tone which worked very well with a strat. Loved the result!

However, I was a bit concerned if the bright cap on the channel pot would somehow affect the other channel, which made me revisit the schematic (mixer section) and try to understand how the channels interact.
Basically I want to confirm that I got it right.

Assume a scenario with a guitar plugged to the bright channel alone, no Y cable, no jumpering.

Right after the bright channel volume pot, I see that the mixer is actually a voltage divider (470k , 470k//560pf). The 470k//560pf acts as a treble peaker and the signal follows on the next stage (in red). The voltage divider is formed as the 470k branch (in blue) goes back to the normal channel pot wiper (and then to ground).

10513 1987 pre.png

1) Let Rx be the resistance between the normal channel pot wiper and the ground tab. Then the voltage divider formed at the mixer is in reality (Rx + 470k, 470k//560pf), as Rx is added in series with the 470k - and that explains why the normal pot setting affects the bright channel even when the normal channel is not in use. Did I get this right?

2) If so, then I understand the bright cap I added to the normal volume pot does not affect the bright channel at all. Is this correct?

Thanks ins advance for your time!
Fabiano

[RockinRocket]

The cap on the volume pot shouldn't effect anything but the mixer 560p cap on the bright channel is acting as a treble dump for the normal inputs.

[framos]

But again, wouldn't the amount of signal attenuation and/or treble bleed through the mixer (with or without the cap) depend on the "other channel"´s vol pot setting ? Apart from the main question (cap on vol pot, thanks for the answer), that´s basically what I am trying to understand.

[RockinRocket]

The setting of the unused pot can add a small bit of gain but I believe it wont have a effect on the mixer 560/470k network.
You should be able to try this for yourself and see if you like it. If your still unsure try swiching the cap in and out and see if there is a effect. Report back on your findings.

[Roe]

for extra gain raise the unused volume to 7

[framos]

yes, in practice I noticed the gain on ~7 (on the "other" channel vol pot) yields the most gain.

But again, I am trying to understand why.

I see the mixer (voltage divider) goes through the other channel's vol pot before going to ground; ie, as I understand the vol pot adds some (variable) resistance in series to one side the voltage divider and thus changes the voltage ratio, and thus the signal level. That was my (attempt) explanation for this effect.

However the practical observation that the perceived gain increases as you open the other channel's vol pot up to about 7, then decreases again, means that I am missing something...