Eine Textdoku von Larry:
Assuming that your amp is a Mastervolume model with two Inputs and it's a bluesy tone you want to obtain with it, I'd recommend you a complete adjustment of your circuitry, beginning at the first gain stage as follows:
1-st stage: 1.8K cathode R bypassed with a 1µ film cap
After 1-st plate swap in a .0047 coupling cap to the 470K, but bypassed with a 1000p Silver Mica instead of the common 470p Ceramic.
Important!!! Swap in a 250K log Gain pot (instead of the 1M type) and don't bypass it with a cap - then put in a additional 220K...470K grid stop R to the 2-nd stage.
Diminish the 2-nd stage's cathode R to 5.6K and DON'T bypass it with a cap.
The voltage divider to the 3-th stage is 680K after the cap, bypassed with a 220p Silver Mica - then with a 220K to ground. But now you have to add a 220K grid stop R to the tone amp's grid.
Keep the resistor values around the tone amp, but increase the slope resistor in the tone stack to 56K and swap in a 220p or a 250p Silver Mica treble cap.
If the tone amp's cathode resistor (820 ohms) is bypassed with a cap - remove it!
Twist the wires running to the tone controls including the ground wire and run these near the chassis or behind the control's housings!
I guess, you have to remove the MV's and Mid's ground wire from the back of the pots and instead of this you better run a ground wire from that point, where the tone amp's cathodes are grounded on the board. Otherwise you can't twist it together with the control's wires.
Go from the Master Volume's wiper with a 100K R into the input cap of the PI tube.
Increase the PI's common cathode resistor from 470 ohms to 1.5K and also the tail resistor from 10K to 47K.
Replace the 47p cap between the PI's plates with a 220p cap.
Use 2 FB resistors in series. A 39K, 33K or 27K followed from a 56K, 68K or 82K and bypass the larger one with a 1000p cap. This device runs off the 4 ohm tap.
The supply voltages on the preamp tubes shouldt be:
- 290V...300V on V1
- 320V...330V on V2
- 345V...360V on V3
If your OT isn't a rasor blade type and your preamp tubes didn't come from the factory "I'll be cutting your ears off", then you'd have your expected "bluesy sound" - but w/o any warranty!
Much success!
Larry
