Master Volume on Fender Bassman AC568

[saint gillis]

I'm servicing a Bassman (AC568), here's the schematic
I agree on the fact that adding a master volume is not necessarily a great idea, but the hole is already there and the potentiometer too..

The person who did the mod dis this :



Which looks awful..

I could consider the Lar-Mar/Type-2 Master Volume, with A 100K log dual potentiometer, something like that (the 1.8M safety resistors are missing on the picture) :



Or the Trainwreck Type 3 Post Phase Inverter Master Volume :


And what about something like that :



Thank you for your thoughts !

 

[musipol]

Well, none will give you the distortion you get from the output tubes. If you're ok with that, the post phase inverter type comes closest(IMHO) to that sound. Never liked the sound of the pot shorting the 2 phases together ( what you termed the trainwreck version).

 

[GeorgeToledo]

Post phase inverter effects loop, then using a volume pedal at the end of the effects loop, can be a nice approach. The player can also then quickly bring everything down in level during practice if someone needs to speak.

 

[aiyiadam]

I experimented with PPIMV. It really was a lot of fun.. It was surprising for me to hear, how much distortion actually comes from the PI.

Two things come to my mind:

1. Getting a good and balanced dual pot is probably important (and also hard if you are using regular log type pots).
2. PPIMV messes up with the negative feedback.. Finding a way to compensate for the NF reduction would be nice.

 

[saint gillis]

I experimented with PPIMV. It really was a lot of fun.. It was surprising for me to hear, how much distortion actually comes from the PI.

Two things come to my mind:

1. Getting a good and balanced dual pot is probably important (and also hard if you are using regular log type pots).
2. PPIMV messes up with the negative feedback.. Finding a way to compensate for the NF reduction would be nice.

I don't understand how PPIMV is supposed to mess up with the NF..

 

[aiyiadam]

I don't understand how PPIMV is supposed to mess up with the NF..

Well.. My understanding is this:

If you reduce the volume using your PPIMV, you also reduce the amount of NF going back to your phase inverter. However, the signal level from the preamp going into the phase inverter stays the same.. With less NF, you reduce less gain in the PI and this effects the sound of the amp.

 

[saint gillis]

Well, none will give you the distortion you get from the output tubes. If you're ok with that, the post phase inverter type comes closest(IMHO) to that sound. Never liked the sound of the pot shorting the 2 phases together ( what you termed the trainwreck version).

Thank your for your (positive) feedback

 

[saint gillis]

Lets try to compute the gain,
I don't know if my logic is correct, but this is what I find for the circuit without the master volume :
Api : Gain of the Phase Inverter
Apo : Gain of the Power Tubes
Vin : signal from the preamp
Vf : signal from the negative feedback
Vpi : signal at the output of the Phase Inverter
Atot : Total gain

 

[saint gillis]

Same calculations with a Master Volume :



From the last formula, if I'm not totally wrong, we can see that the total gain we had without the master volume is now attenuated by the contribution of the master volume attenuator, and we can also see that the NF contribution is affected too by the attenuation of the master volume.
So I guess your are right

 

[saint gillis]

Side by side :

 

[Whoops]

not wanting to disturb the nice conversation here,
but just to provide some extra advise.

The best forum for Guitar Tube Amplifiers and Mods is the EL34 forum,
it's completely dedicated to Guitar tube amps and there's very knowledgeable and helpful people over there in all these subjects:
(PRR is also a regular there)

https://el34world.com/Forum/index.php

 

[saint gillis]

not wanting to disturb the nive conversation here,
but just to provide some extra advise.

The best forum for Guitar Tube Amplifiers and Mods is the EL34 forum,
it's completely dedicated to Guitar tube amps and there's very knowledgeable and helpful people over there in all these subjects:
(PRR is also a regular there)

https://el34world.com/Forum/index.php

Hey, sure, I should maybe post there about guitar amps
Thanks