Small valve mixing console

[C.B. - Boudio]

Could it be a viable option to use a 2K2 wire wound pot (connected as a variable resistor) as the cathode resistor of the PF86 as a way of gain control?

 

[C.B. - Boudio]

Otherwise stated: what would be the biggest drawbacks of doing this?

 

[tk@halmi]

You will constantly change the DC operating point with volume changes. This is not desirable because it produces changes to headroom, symmetry and distortion with each setting. It might be a cool effect, but it will not make for a predictable recording channel.

I would err on the lower overall gain side for the channel. Today’s audio interfaces are very high resolution, and we also have lots of inline gain boosting solutions for quiet dynamics and ribbon mics.

 

[C.B. - Boudio]

You will constantly change the DC operating point with volume changes. This is not desirable because it produces changes to headroom, symmetry and distortion with each setting. It might be a cool effect, but it will not make for a predictable recording channel.

I see! Maybe I'll make the g2 decoupling variable, then?

I would err on the lower overall gain side for the channel. Today’s audio interfaces are very high resolution, and we also have lots of inline gain boosting solutions for quiet dynamics and ribbon mics.

The overall concept of this project is a 'prosumer mixer in the early sixties', and I want to keep everything in line with that; I'd even like to mod a Revox x36 to 4-channel (in the distant future). Which is why I really want quite a lot of gain, even at the cost of a pinch more hiss and/or half a percent more distortion (maybe I should have said that earlier...).
It's a bit lo-fi (I do hate that term, though) punk-ish whilst trying to get results as decent as possible, if that makes any sense (well, it probably doesn't and raises some hair and eyebrows with some of you ;-))

 

[tk@halmi]

Another option is to substitute the 15K transformer secondary loading resistor with a 25K audio pot in parallel with a 39K resistor. The capacitor would go to the wiper of the pot and into G1. You will need a 1M resistor or something of your choice to bias the EF86.

Input transformer saturation on transients sounds much nicer to me than oddly squashed tube signals.

 

[C.B. - Boudio]

Another option is to substitute the 15K transformer secondary loading resistor with a 25K audio pot in parallel with a 39K resistor. The capacitor would go to the wiper of the pot and into G1. You will need a 1M resistor or something of your choice to bias the EF86.

Sounds like a good idea!
But I'd like to have the impedance switchable, so 15k/10M (or something a little lower, I'll see what works best in the end), for high-impedance consumer valve equipment, and mainly to eliminate the need for a DI-box and to be able (not nescesserily par default) to get something like a fuzz bass just straight from the mixer. So that's why I am also considering other options.