OK, I've finally found some time to look at this design.
A few comments:
- Overall it looks good, and should work well. Congratulations on not using high values of cathode resistors in the preamp, especially after the first stage, which will help avoid fizzy tones.
- IIRC the Big Muff tone stack is quite lossy, and should work more efficiently when driven from the low impedance of a CF. The CF also adds some nice soft clipping, providing some thick Marshally crunch to the amp's tone. So that's good too.
- You've got a DC path from the cathode of V2B through to the grid of the 6V6
You need to add a 0.022uF between pin 8 and the tone control.
- I would recommend increasing the 220k resistor at the 6V6 grid to 1M, to increase power tube drive. A 1M grid reference in parallel with the 1M pot gives you effectively 500k, which is about perfect for a cathode-biased 6V6. Having a tone stack in front of the 6V6 means you have to overcome all the losses in the stack when driving the power tube. With a regular James stack BTW those would have been real high - hence the recovery stage. Otherwise it's usually preferable in higher gain preamps to have the CF and tone stack at the end of the preamp.
- Also consider using a larger cathode cap at the 6V6, since low values can make it sound rough when overdriven. 100uF is a popular value in cathode-biased amps. Or try a huge cap in the 1000uF to 2200uF range to really stiffen up the power amp for high-gain playing.
- I'm not sure what voltages you've got in your preamp, but aim for around 300V+ at the last two stages, to help drive the 6V6 (ideally running at around 350V). Also then consider reducing the 3rd stage's 1k cathode resistor to a more Marshally 820 ohms, to keep the stage warmish-biased at the higher rail voltage.
- The 22k resistor in the heater/elevation bleed, needs a cap across it, or you will get increased buzz in your amp. Something like 16uF shuld be good.
- Since this is a Class A SE amp, that vacuum state rectifier will have practically zero benefit in terms of sag and compression. Unless you simply want to use the tube for cool factor, you would be better using solid state rectifiers, preferably fast ones such as UF4007s. This would also allow you to improve the power supply filtering with bigger caps. Just a suggestion....
- Since your preamp is quite similar to the High Octane's, I would recommend also inserting an additional gain control on a ganged 500k pot between the 2nd and 3rd preamp stages. This helps the HO manage its gain much more effectively, so it can go all the way between clean and dirty.
- Do you really need/want the dual Hi/Lo input jacks? If not, I suggest reducing to one jack, and using a 10k input resistor instead of 68k. This will help the preamp not to pick up buzz from the power supply.
HTH.....
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Took me long enough to get around to this. Projectus Interruptus, you know? I've made the corrections and mods to the schematic...I THINK I got 'em all. If anyone's interested, I've posted a JPG in addition to the HTML file this time.
