Optimus Aquilón. Guitar amp conversion.

[chocopower]

Hi everybody!
I get this old (1960-65) public address tube amp for a really nice price (88€ to my door).

First i draw the schematic for uploading into radiomuseum.org  

Second i installed a 3 prong cord and tested the power transformer (unloaded) and all seems to be ok.

Next, cap job and others safety works.

I attach the original schematic and a first approach to what could work for guitar. The amp is gonna be used too, to power a solid state Leslie speaker.

My questions are.

.-What Bias schematic should I use to make that one variable?
.-NFB arrangement. Is ok as it is, or should i change it?
(since the power amp design is close to the 70s Orange, i was thinking in use them as inspiration for BIAS and NFB)


.- I know that 2 EF86 in series is gonna be too much gain, but i´m just gonna test it for know how it sounds.
I think i´m gonna try a 2 channel amp with a EF86 in each one voiced in a different way. Any other idea will be welcome.


The "V1" schematic files are the "mod" version.

Thanks in advance!

[chocopower]

.JPG  added.

[HotBluePlates]

-What Bias schematic should I use to make that one variable? ...

Replace the 39kΩ resistor with a 25kΩ pot, and maybe a 18-27kΩ resistor from that pot's ground lug to the actual ground. In other words, copy Fender's basic blackface approach, but use a pot/resistor combo to allow adjustment above/below the stock 39kΩ.

Since you're not changing anything in the output stage or power supply, it makes sense to leave the bias circuit as-is other than making it adjustable.

-NFB arrangement. Is ok as it is, or should i change it? ...

I'd assume it is good as-is, but maybe a bit more feedback than you'll want (you'll only know when you listen to it). If that's the case, just make the  82kΩ (and/or the 68kΩ) bigger to reduce feedback.

The 47kΩ/4.7nF is a fixed presence lift. You might want to listen to it before ripping it out. Make the 47kΩ bigger to reduce the presence.

If you make the 82kΩ bigger & change nothing else, you get less feedback overall and less presence; if you make only the 68kΩ bigger you do the opposite. If you decide to ditch the presence entirely, you can remove the 47kΩ/4.7nF and replace the other resistors with 100kΩ (for stock feedback).

(since the power amp design is close to the 70s Orange, i was thinking in use them as inspiration for BIAS and NFB)

The Orange bias circuit is close enough to be a direct swap into this amp.

The Orange feedback circuit is close, but doesn't have the presence adjustment (you may or may not want it), and also takes the feedback voltage from a different OT tap. If you try to copy the Orange circuit, make sure you move the tap on your OT to match what Orange used, otherwise you'll have very, very much more feedback.

[chocopower]

Thanks!  i´ll be back with the next moves.