DVM,
The mods I'm using are a combination of those found on 18W.com and S2amps.com.
There are some very good suggestions on both websites. The 18W website even has spreadsheets to suggest what mods are recommended for different results. The S2 site explains what to do to "fix" the amps shortcomings as it comes from the factory, and what each stage does. Very interesting and FUN!
Some resistor and cap swaps do alot for this amp, but personally, the first thing I would do is invest in a Hammond output transformer and some good tubes. The 125DSE tranny and good tubes should run around $50.
After that, one of the best mods I've found is to replace R1 w/ a 1M and R2 w/ a 10K resistor. Seems the stock amp is wired to shunt 50% of the input signal to ground, resulting in a drop in signal/noise ratio that is like a low input jack signal.
Also, changing R6/R7 to100K helps open the amps gain up.
You can also lift one leg of C3 to add some negative feedback, which helps balance the clean to crunch, and if you don't like it, just solder it back.
There are ways to quiet the heater noise if you haven't already done that, and you can also wire the amp for more speaker outs (combo version).
I am still trying to voice the amp to my guitars, and I'm getting closer w/ each mod. Another thing I'd suggest is to check heater voltages, as well as screen grid. The amp is biased a little warm from the factory. Adjusting R10 to 1K 3 Watt, and R14 to 240-270 1 Watt will help reduce the B+ voltage to around 300-310 volts at the EL 84 plate pin 7, and reduce the EL84's diispation, the stock is over the limit of 12 Watts. I'm still waiting on parts to do this, but I think reducing these to a "browner" sound will be one of the last links in the chain, running on the edge of A/ AB.
I've only done 1 or 2 mods at a time, and it's slow, but I don't want to do a dozen mods and not like the sound, and have to go back and trace what didn't work!
So far, every mod has helped, IMHO. If you'd like a little screamer, (ala Marshall), I'd go for the "gold" mods on 18W.com. Zaphod Phil helped every step of the way, and he knows his stuff.
Be forewarned... if you want to read it all, it's over 300 pages of information... that's where I've been when I wasn't at work.
It was worth it.
This amp circuit is similar to a Champ and a Bad Cat Mini, with a few variations, and the PCB, which is perfect for a new modder like myself.
Check it out, and let me know what you think.
(BTW, your ZYS makes this thing scream now...
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