I learned something new today, or Everything I thought I knew is wrong.

misterwhizzy

misterwhizzy

Well-known member
So my complaint with my JCA50 head has been the drive channel. The amount of gain is just ridiculous, and I don't think there's any way I could find a way to make it musical, even dialed down to 1 or 2. So I decided I'd try to put a lower-gain tube in the cold-clipping stage, thinking that it would lower the gain of the drive channel while leaving the crunch channel the same as it is now, which is definitely high gain for a clean channel. The crunch channel is reminiscent of a JCM800 as it stands.

So I checked an online schematic real quick and decided V3 was the cold-clipper, counted the third tube from the input, and swapped in an EHX 12AU7. I fired it up, and it had neutered the gain on both channels, and it's AWESOME. Basically, this is what I wanted when I bought the amp. The clean channel is clean, and the dirty channel is usable. I'm almost positive the real V3--please stand up--is not in the crunch channel's preamp topology at all, so either I can't read a schematic, or the third tube is not V3.

Either way, I'll take it, because this gives me a second channel, and I've got drive pedals in case I want to be stupid with it. I guess I can take a look at the board to see what's what the next time I swap and bias power tubes, but I'm usually not pleasantly surprised by my mistakes.
 
Re: I learned something new today, or Everything I thought I knew is wrong.

It is always great when something works out without you having to spend the $$ on another amp. A tube swap is a great way to fix things.
 
Re: I learned something new today, or Everything I thought I knew is wrong.

In the JCA and soldano derivatives its is the second half of the second valve the cold clipper belongs to. The third valve would be the last stage and CF. That's why the gain dropped for the clean as well.

Be that as it may, the cold clipper by itself isn't the big gain producer. It clips radically for sure, but the voltage gain is only about 1.8 to 2, versus the other stages that have an AV of roughly 70.

The cold clipper is more responsible for the refined crispness and harmonic content, its not whats responsible for the mush. The last stage, the one you actually changed, is the one that is the most heavily clipped. so you kind of serendipitously did what you would want to.

On Soldano amps, the first thing I do is ditch the last cathode bypass, and lower the first two coupling caps so that their -3db point is around 80hz or so. Depending, I also reduce the last stage plate resistor. these small things give a punchier less mushed out distortion character with still great gain on tap.
 
Re: I learned something new today, or Everything I thought I knew is wrong.

Bonus points to anyone who recognizes the reference in the thread title.

Also, thanks, Joey.
 
Re: I learned something new today, or Everything I thought I knew is wrong.

Also I change the gain pot of the od channel to about half the value, 500ka for the jet city, (you can easily just throw a 1M resistor in parallel with the existing pot, but I think I already told you all that months ago. It gives the treble peaking filter that’s part of that network a better slope.

No clue about the reference.
 
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