I'm wondering...

[jackhawkins]

I own a fantastic JTM45 clone that sounds incredible and is highly responsive. However, I wanted to explore additional options for its performance. Initially, I installed a switch to toggle between the stock and split cathodes, using a 2k7/.68uf configuration on the bright channel. While this yielded a usable outcome, I desired a bit more character. To enhance the setup, I added a .68uf capacitor to that switch (utilizing an available pole), which bypassed the 820ohms on V2a. The result has been a reduction in bass and some unwanted noise. I'm wondering if the 50-watt version had more preamp filtering, causing this noise issue.

[Reeltarded]

820r is hot. Bypassing adds hiss from the top all the way down to just under 300Hz.

Is it frittery noise or just hiss? Is that stage wired with shielded wire to the tube socket?

For a normal plexi amp, best to move the 68k input pairs to become single 33k resistors right on the grids of both sides of the first tube and use RG-174 or similar. The world is a radio these days. bzzzzhhhhhhhhhhh

This described setup only uses the Hi inputs.

[Stevem]

Please focus in more from your general description of “ more character “

[Fischerman]

The 0.68uF cap on V2 can be increased all the way to 330uF and it still sounds good. To me 330uF sounds better than 0.68uF. Allegedly EVH's favorite 12-series had a 330uF on V2 stock as well as a 50k mid pot. Just doing those two things adds a noticeable amount of gain.

However, imo those mods above are better left for a Super Lead type of thing. A JTM45 isn't built to deliver the kind of gain a Super Lead is. If you try to turn a JTM45 into a Super Lead you usually end up with an amp that is neither. So... tthe obvious solution is to build a Super Lead clone.

EDIT: you can also put a low value resistor in series with the 330uf (or whatever) bypass cap. Something like 470R to 820R will reduce the effect but still give some boost. I have my Super Lead clone on a switch that is bypassed with a 10k resistor. So Boost ON (pull the Normal volume) is 330uF in series with 470R and Boost OFF is 330uF in series with 470R+10K so its essentially out of circuit. And no pop or anything.

Second edit: And you can do the same with the bright cap. My SL has a 4700pF bright cap and when you pull the Bright volume (labeled Pull Bright) you get the full 4700pF bright cap but when you push the Bright volume in it puts a 220k resistor in series with the cap. IMO that works a million times better than using a smaller bright cap. A smaller bright cap is the WORST solution there imo. It keeps the shrill highs but doesn't add those karrangy high mids that are essential to the tone.

[WhopperPlate]

EDIT: you can also put a low value resistor in series with the 330uf (or whatever) bypass cap. Something like 470R to 820R will reduce the effect but still give some boost. I have my Super Lead clone on a switch that is bypassed with a 10k resistor. So Boost ON (pull the Normal volume) is 330uF in series with 470R and Boost OFF is 330uF in series with 470R+10K so its essentially out of circuit. And no pop or anything.

Second edit: And you can do the same with the bright cap. My SL has a 4700pF bright cap and when you pull the Bright volume (labeled Pull Bright) you get the full 4700pF bright cap but when you push the Bright volume in it puts a 220k resistor in series with the cap. IMO that works a million times better than using a smaller bright cap. A smaller bright cap is the WORST solution there imo. It keeps the shrill highs but doesn't add those karrangy high mids that are essential to the tone.

Better yet …insert series potentiometer for further control