1959 Pilot AA-908 Amp into Guitar Amp


2008-03-16 10:12 pm
Hello all,
I came across this thread http://www.diyaudio.com/forums/showthread.php?postid=389321 and realized I could do the same thing with an old Pilot monoblock amp I have kicking around.

I have hosted the schematic and a couple pics of it here: http://www.zeromassstudio.com/pilot.html

I'm hoping to alter the preamp stages to a Fender 5F6-A type (or similar, if I can get away with using as much as what's currently in the Pilot that would be great) but not sure it will be possible using the current transformer combination (they're huge!) it says the output is 40W which should be great for a guitar amp. I tried a guitar through it and it sounds interesting a little low volume right now and muffled but I think that's due to the gain stage and negative feedback.

Any help I could get with this would be great, Iv'e been wanting to try my hand at a tube guitar amp (5F6-A clone) and figured this would be a great way to start.


Indeed that amp can be a very good guitaramp.
One clue when it comes to EQ'ing a circuit for electric guitar is to get it brightened up. 'So bright it hurts' one bluesman told me, and it is true since a flat EQ sounds very muffled and dull.
Gainwise you can get very good clean tones, with a nice hint of breakup with a single 12ax7 gainstage, but you must remove all feedback, also the 'AC balance'.
Also, when the input impedance is low like here, you get a muffled tone from the pickups, 1megohm will help brighten it up.
For a basic gainstage with 12ax7 try this for starters: 1kohm cathoderesistor bypassed with 1uF, 220kohm plateresistor, 10nF couplingcap to phasesplitter. You can switch in more coupling caps for a deeper (jazzier) tone.
A volumecontrol should be placed after the gainstage, or simply use the guitarvolume.
I would keep this amp simple, and use various effects pedals to very the tone/gain/overdrive.


2008-03-16 10:12 pm
Thanks Semper Fi, I've been able to get the guitar input sounding pretty good just by omitting the feedback to the first gain stage and adding 1m impedance. I also ommited the AC Balance half of the 1st 12AX7, with little change. However I'm having trouble with hum, and am confused with the phase inversion as it appears on this schematic. I tried ommiting the AC balance pot which was fine, then the DC balance pot which got rid of the hum but made the input sound like crap, I'm assuming this is part of the phase inverter and should remain, but comparing to a long tailed phase inverter (ala 5F6-A) I'm confused. I think next I'll just copy the 5F6-A inverter using the same 12AX7, but still not sure if it will fix the hummm...
what do you think?

A closer look and I see the AC Balance is part of the phase inverter's actual phase inverting, so my bad, untill you build the phase inverter you mention, which I agree is a better type, you need to reaply the AC Balance connection.
You probably get som opposing powersupply ripple in that way as well, which helps with reducing the hum.


2008-03-16 10:12 pm
So I replaced the inverter with the 5F6A type and it took care of the hum, but now the guitar breaks up in a really ugly way, similar to the way it sounded with the original circuit without the DC balance pot. It sounded really nice and Fender-like with the original inverter - just noisy. I might go back to the original inverter and try to track down the noise elsewhere. Any suggestions?

...I guess if all else fails I can bring the whole thing back to original and sell it as kevinkr suggested.