Qestions re: adding clean channel to JCM800 preamp

[RobBozic]

Hi,
Just have some questions about adding a cleanish channel to a JCM800 build. I'm going to mix the channels prior to the PI and use an A/B box to go between channels.


Q) With the mixing resistors prior to the PI, should I go with 220K or 470K mixing resistors prior to the PI?


Q) Is there a voltage loss in the mixing resistors prior to the PI? If so, how much loss is there?


Thanks in advance
Rob

[d95err]

I think it will work as drawn. Passive mixing circuits will always have a bit of loss. Probably something like 2-6dB for each channel. The loss for the clean channel will depend on the MV setting for the JCM800 channel.

I have one concern though. The classic JCM800 (2203) preamp is sort of the "missing link" between non-master and master volume amps. It has a bit of gain and a master volume, but really needs to push the PI and power tubes into at least a bit of distortion to get to its sweet spot. At this point, I don't think the clean channel has enough level to keep up and if it does, it will no longer be completely clean (if that's the intention).

Another option would be to use a more high-gain crunch channel, to ensure it can give a decent amount of distortion on its own, without pushing the poweramp.


EDIT: Sorry, misread your schematic; thought it was a classic Fender clean channel. Your "clean" channel should be able to keep up with the crunch channel in level, but still won't be clean when the crunch channel is in its sweet spot.

[pdf64]

Bear in mind that a classic non master vol clean sound doesn’t tend to be absolutely distortion free. Generally, pushing the power amp into some degree of subtle added ‘harmonic interest’ and compression is beneficial for most tones.

[d95err]

Bear in mind that a classic non master vol clean sound doesn’t tend to be absolutely distortion free. Generally, pushing the power amp into some degree of subtle added ‘harmonic interest’ and compression is beneficial for most tones.

Absolutely! I hardly ever use a completely clean sound. Always want at least a bit of tube compression. Usually just back off the guitar volume on a crunchy channel for cleans.

Lots of people seem to want that ultra-clean (sterile, IMHO) tone though.

The ”clean” channel above probably works well for a dirty light crunch tone. If cranked, it should produce quite a lot of distortion.

[PRR]

Passive 2:1 equal mix is 6dB loss.

However your 4-tube channel has about 10 times the gain of your 2-tube channel. I doubt you want to mix equally. If nothing else, the ever-present hiss from the high-gain path will dirt-up the clean path. You may ultimately want switching. Meanwhile I'd be trying 150k:680k mix, and tack-in rather than nailed-solid so it can be tweaked on test.

[d95err]

The ”clean” channel will have higher max output than the ”crunch” channel.

The crunch channel looses quite a bit of signal level in the tone stack, while the clean channel output (before the mixer) will be the maximum headroom if the second stage.

[Backwoods Joe]

$.02..... One of my fav clean sounds is the 800 dialed up to where its sounds fairly clean but has lots of harmonic content. Somebody on here (this board) has like a 2 channel 800 schematic that is foot-switchable with a MV for each channel. Why not use that schematic and change the tone stack on one of the channels if you like?

[nandrewjackson]

If your set on proceeding that way, your drawing is a good starting point.


Looks like it will play as-is.


+1 on PRRs mixing R values tip.


A couple well placed relays, and a volume pot can get you there without adding another input jack, 12ax7, and surrounding components. 

[RobBozic]

Thanks for the replies. PRR’s supposition of mixing the ratio is handy to know, dial it in with a 1ML pot used as a mixing pot.

Backwoods Joe , do you know what the schematic you’re referring to is called?

Thanks again



Rob

[Backwoods Joe]

No i'm sorry I don't, but's it's been posted several times through the years.