Simulating bright cap tonal quality?

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Simulating bright cap tonal quality?

Post by Alfi27 » Mon Jan 28, 2019 8:46 am

Hi fellas,

I have been quite obsessed with amp mods and circuits the last month or so, and to various degrees been interested in it for a year. Ever since I purchased a used Ceriatone 2204 and first played it stock, and then added a 1000pf bright cap I have been aware of the significant effect the bright cap actually has. My problem with it is, I can't take it out because the amp becomes really muddy if I do, but when it's there it pretty much makes the gain pot useless. It turns the gain pot into a "bright cap mix" knob instead of an actual gain knob, because I really dislike the tone with the bright cap below 4 and above 6 and I never run it below 5 or above 6. That being said, I do use a Suhr Reactive Load and I can control the gain to a certain degree by adjusting the master volume and changing between the high and low sensitivity inputs.

Over Christmas I had an epiphany when I got a 1972 JMP Lead 50w, and did the one wire mod: volume 1 could be set to 12 o' clock for ideal bright cap mix, and then volume 2 would control the overall gain beyond that, like a built in and great sounding boost pedal. It also demonstrated how much gain three gain stages can produce, which is far beyond any (stock) JCM800. I wouldn't have worried too much about it if I didn't have a 1981 JCM800 2204 in the UK, where I'm studying and the sweet sweet JMP is back home in Norway... One obvious, and not too comprehensive mod would be to just mod the 800 into a one wire JMP. But there's two things, first of all these two amps will one day be united and if I'm lucky sound great in stereo, and secondly I'm really digging the unique, aggressive JCM800 voicing as well.

Didn't manage to keep it quite on topic, but the way I see it there are two main options:

1. Keep the gain pot as is, experiment with cathode resistors and caps on the first gain stage until satisfied. Eventually putting an extra resistor and capacitor on a push pull switch, effectively "pull for gain boost".

2. "Model" the pot set to 5-6 with two resistors and a bright cap at the appropriate location, and wire the gain pot to control the first gain stage. Ending at a more or less a hybrid circuit between an 800 and a OWM Plexi. Does this work the way I think it does, by the way?

There is one additional thing that I am really really digging with the JMP, which I won't get with the 800 without drilling some holes (which I won't do). With the OWM, plugging a blank jack into the bright channel high or low gives a very nice gain boost slightly different from just turning up the gain. Or if you can't be arsed with adjusting anything and just want a bit more sustain for your solo, plug it in and it sounds great :rocker:

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Re: Simulating bright cap tonal quality?

Post by FourT6and2 » Thu Jan 31, 2019 10:22 am

Or put a few different cap values on a switch.

Or put in a second gain control like most Jose mods. Then put your bright cap on that Gain 2 pot and leave Gain 1 without the cap. That way you have a Bright Gain and Normal Gain.

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