Very interesting circuit. I see why PRR suggested leaving it be. Here's what I see that you have:
The dotted line box at the input that says "part of R1" is an RIAA equalizer circuit. This was used to compensate for the shrillness that comes with a vinyl cut. V1 is a 6AU6 pentode. Perhaps the most common of all audio tubes. I think it's making a gain somewhere between 90 and 120 based on the RC chart located here.
http://www.mif.pg.gda.pl/homepages/frank/sheets/093/6/6AU6A.pdf That's all pretty straight forward old school hifi stuff. Next is the treble 1/2 of a Baxandall tone stack followed by 1/2 of a 12AU7 gain stage. Counting on thumbs, we'll assume the signal loss of the treble stack .1 and the gain of V2a is 10. They offset each other for an effective gain of 1. Next is the bass 1/2 of a Baxandall followed by the other 1/2 of the 12AU7. Same dealimabob as before. The loss of the bass stack is offset by V2b and again call the gain 1. I've never a Baxandall split up like this before.... I'm gonna say COOL! V3 is another 12AU7 and this circuit is commonly referred to as a paraphase inverter. This is the section that splits the signal for the push-pull output. Paraphase is not the most common PI in a guitar amp, but they can be found in Valcos, Dan Electros, and many others. The last 2 bottles in the signal chain are a pair of cathode biased 6V6's. There's 1 more tube which is the 5Y3 rectifier. B+ is 310VDC which is lower than a Deluxe but not necessarily a bad thang.
Ok, so you have a cool little amp. You need to decide how much you wanna mess with it. It's best to work these thangs out backwards, or should I say from the outputs to the inputs. The very 1st thing you ought to consider is the health of the power supply caps. It looks like its a 40/40/20 can cap. Those thangs are a tad expensive and electrolytic caps do fail with age. If the amp is humming pretty bad, then that can needs to be replaced. Another check is to pull all the tubes except for the 5Y3 rectifier. Fire it up and measure B+. It's gonna be considerably higher. Now with the meter still attached, yank the power and see how quickly the voltage drops. If it drops quickly, replace the caps. A bad power supply can spoil everything.
I wouldn't mess with the output stage or the PI. 220 ohm on the cathodes of a pair of 6V6's is textbook. The paraphase PI is too cool for school so I'd leave that too.
Baxandall tone stacks are not very common in geetar amps because they tend to be very flat in response. Geetar amps like to jack with the mids a lot and typically there's a lot of low end roll off at like 150Hz. 12AU7's aren't very common in guitar amps either because of their low gain. You could fairly easily scrap this. Replace the 12AU7 with a 12AX7 and copy the Princeton preamp and tone stack. Of course you won't want to stick that high gain 6AU6 back in front because you'll have waaaaay too much gain.
A few easy experiments:
Scrap that RIAA circuit at the input. Leave C1 and R2 in place and hang the input jack right off of C1.
Change R23 to a 500K pot. Ground 1 end, hook the other end up to C11, and connect the wiper to Pin7 of V3. Viola! Master volume. If it operates backwards, swap the 2 end connections. (Clip leads are your friend).
Change V2 to a 12AX7. You've changed gain of each 1/2 from less than 10 to roughly 17. Expect distortion.
Put something like a 5uF cap across R9 and or R17. Expect more distortion. Your gain just increase to 55ish per stage. It may squeal. Now put the 12AU7 back in. Your gain will be back to about 15 per stage. This may totally suck whereas overdriving tone stacks usually does.
Better yet, leave V2 alone and put a bypass cap (something like a 4.7uF) across R3. Experiment with different values. It's only 1.6 volts there so you really can't hurt anything. Lower values will cut more bass. Marshall used a .68uF, Fender used a 15uF. Hifi's might use a 100uF. That's probably the simplest coolest thang to try.
Gotta run. Have Fun!
-Richard
Topic: Phono to guitar amp conversion (Read 16356 times)