Carvin X Series Boost mod

[cablercdd1]

I have a Carvin X-30 amp that I've just recapped and while I have it apart I'd like to find a way to modify the boost circuit to work better, maybe even make it adjustable. Right now the boost is turning off a transistor that grounds the tone controls. It boosts it little but also makes it very muddy. I'm enclosing some schematics for reference.

[Gtr_tech]

Thats the early version. I don't like that design....shared stages in both channels. Yucky. It'd take a *lot* of redesigning/rebuilding to make that sound good. Not worth the time/effort IMO. Off it and get a later version from the mid-late 80s.

My main "gig rig" is a X-100b that I've gone thru and made a ton of changes to. It sounded like ass stock, but its a screamer now.

The option is always there to ditch the pc board altogether and have it rebuilt PTP with eyelet board to your specs.

[cablercdd1]

I like the sound of the amp. Guess I've got simple tastes. I was looking at some other posts about stage gain and it gave me some ideas to try. I'm thinking of using the boost transistor to change the resistance in the cathode circuit to change the gain of that stage.
Thanks for the reply

[BackwardsBoB]

You don't got no GEQ, do you? Sure about the schematic? If it matches...

The cathodes shared by the clean and lead channels are fully bypassed, so changing the resistor will pretty much just change the bias, not the gain.

You can provide a general boost, then use the transistor to provide a "cut". If you like the current un-boosted tone, it's simple. Disconnect the transistor collector from the tone stack. Short the 47K resistor. This enables the tone stack.

There's a big divider over on the upper right of page 1 with a 1M resistor feeding two 100K resistors in series to ground. Change the 100Ks to 220K. This provides the boost.

Now use the transistor collector to switch a resistance from the junction of the 1M resistor and the 100K resistor (at the eq switch) to ground. 390K might be nice, or 430K if you can get it, or try a 500K pot and adjust to taste. A smaller resistor provides more cut.

If the GEQ clips, increase the top 100K and decrease the bottom 100K by the same amount. If the GEQ switch boosts/cuts (I think it will boost a little when the GEQ is on), adjust the 10K feedback resistor on the GEQ's buffer opamp. lower values of the 10K are lower GEQ output.

Never tried it. Should work. Won't blow anything up.

[cablercdd1]

Thanks for the reply. I'll be using the amp this week but I can try it out in the next 2 weeks to see how it works. I'll let you know.

[cablercdd1]

Thanks Backwardsbob, I tried your suggestion and it worked great. Thanks for the help
Paul