Old 100watt PA conversion schematic

[FranciscoPerez]

Hi,
I'm restoring an old PA amplifier from the 60's, and would like to turn it into a guitar amp, keeping as much of the original circuit as possible.


I left the PI and Power Section almost untouched, added a baxandall tone stack, and a jmp preamp.
Do you think this could work well for guitar? I'm specially concerned about the NFB and PI section.

What do you guys think?
Thank you!

[tubenit]

I would be inclined to change the phase invertor into an LTPI.  We know what the LTPI sounds like overall.


You could make it cathode biased to keep it simple if you wanted.  And you could add a PPIMV since it's 100watts to keep it more manageable and simply use two EL34's instead of four.


The Matchless Clubman uses cathode biased EL34's.     CLUBMAN.DSN - CLUBMAN - Page1 (el34world.com) 


With respect, Tubenit

[PRR]

> added a baxandall tone stack

James.

(Bax is wrapped *around* an amplifier stage.)

[Keppy]

I think you're missing a coupling cap between the cathode follower and tone stack.

[FranciscoPerez]

Thank you guys!
I added the missing coupling cap, a ppimv and a 50/100w switch.
Preamp circuit is all soldered on a PCB. Changing the PI into a LTPI would require getting rid of the PCB and installing a turret board.

For that reason, I think I will first try changing some components on pcb to match my schematic, and keep the paraphase PI, then hear how it sounds. Could I perhaps remove the local feedback loop around the paraphase tube? And the cathode bypass cap? Do you think this would make the PI more guitar friendly?

As per the James tone stack, I'd use a dual concentric pot, since there are not many holes available on the chassis front, and would prefer not to drill additional ones. This is the main reason I decided using this type of tone stack instead of a TMB

Cheers,
Fran

[PRR]

I would leave the whole power-amp alone. The delicious depth of distortions we enjoy at the 10 Watt level will be overwhelming at this 100 Watt level, most of the "must do it this way!" rules don't really apply.

I also think you could figure how to do Fender Bass-Treb on a dual concentric. Even 500k+500k. Implement the Mid as an internal trim.

[66Strat]

Thank you guys!
I added the missing coupling cap, a ppimv and a 50/100w switch.
Preamp circuit is all soldered on a PCB. Changing the PI into a LTPI would require getting rid of the PCB and installing a turret board.

For that reason, I think I will first try changing some components on pcb to match my schematic, and keep the paraphase PI, then hear how it sounds. Could I perhaps remove the local feedback loop around the paraphase tube? And the cathode bypass cap? Do you think this would make the PI more guitar friendly?

As per the James tone stack, I'd use a dual concentric pot, since there are not many holes available on the chassis front, and would prefer not to drill additional ones. This is the main reason I decided using this type of tone stack instead of a TMB

Cheers,
Fran

The PI is a self balancing paraphase. The difference in signal between the 1M resistors in the PI plate circuits is applied to the grid of the inverting triode. Applying the difference to the input of the inverting tube keeps the PI in balance. This circuit is similar to the Vox AC50 MKIII.

https://el34world.com/charts/Schematics/files/Vox/Vox_ac50_1.pdf

You could tweak the value of the 27K feedback resistor to suite your taste or leave it out. The AC50 had no NFB.

[FranciscoPerez]

This is great info!
I'm definitely going to leave the power amp as it is for now, and see how it goes.

Heard some youtube clips of the vox ac50 mkiii, and really liked that sound. Seeing that the power amp section is similar to my PA amp, I might try to fit this preamp without the bright channel cathode follower and tone stack. 


Thank you!!!