Would this work? (adding gain stage on 5F6A)

[Bluefinger]

Hi,

My next project will be an amp roughgly based on a 5F6A narrow panel Bassman. I have an idea but I don't know if it will work. Here's what I have in mind:

there are 4 inputs in the standard configuration: bright hi, bright lo, normal hi, normal lo. I never plug in more than one instrument at a time and I can connect/disconnect the bright cap across the volume pot with a switch so there is no need for the two channels. I don't even need the lo inputs either. My idea is to use the second half of V1 as an additional gain stage. I thought about running both halfs in parallell like on a typical fender reverb driver (connecting pins 1+6, 3+8). Then Ican use a switch to either connect pin 7 to ground or to pin two so I can run it in standard configuration or simulate both channels jumpered. Did I miss something or does this make sense?

thanks!

Herbert

Edit: or would it make more sense to stack them ... like using the switch for either sending the signal coming from the volume pot through the second half of V1 and then to V2 or going directly to V2. Would adding one half of a 12AY7 make much of a difference anyway, stacked or in parallell???

[pdf64]

It will work but with the 2nd grid grounded, the gain from the 1st triode will be reduced (not quite halved) because the plate impedance of the 2nd triode is still in parallel with it.
Even with the switch in the other position, the benefits of triodes in parallel are questionable in a low gain amp (it reduces stage noise by about 3dB, and reduces the stage output impedance).
Pete

I think it would be better for the switch to put the triodes in cascade, the switch will definitely add 'something' then (assuming you want an overdriven tone).

[Bluefinger]

ok, I sure don't want to reduce the gain of the first triode when the additional gainstage is nor in use. I would rather have it close to stock. I will try to draw a schematic later but I will try to put it into words for now ... how about this:

DPDT switch should work. all based on this https://taweber.powweb.com/store/5f6a_schem.jpg

gain stage off:

o) signal from volume pot goes through 270K and then to V2/2
o) V1/6 stays connected to the power rail but decoupled signal goes nowhere
o) V1/7 goes nowhere
o) V1/8 grounded through resistor (value to be determined)

gain stage on:

o) signal fom volume pot goes through 270K to V1/7
o) decoupled signal from V1/6 goes to V2/s
o) V1/8 grounded through resistor (value to be determined)

does this make sense at all?

THANKS!!!!!!

[pdf64]

Looks good as far as I can make out, apart from V1 pin7 (and possibly V2 grid, gain stage on); a grid should always have a path to ground, to ensure that it stays referenced to the cathode. Otherwise it will float up, the triode will become a diode and potentially red plate.
Pete

[Bluefinger]

Looks good as far as I can make out, apart from V1 pin7 (and possibly V2 grid, gain stage on); a grid should always have a path to ground,

yeah, I forgot about that ... I need to put a 1MEG resistor to ground there.

ok, thanks a lot ... I'll give it a try

Herbert