musicman rp-65 bias

[Amp Kat]

I always liked that Music Man cascode circuit, from a tech geek perspective at least. (I'm not so sure how it would sound.) I think they did it for simplicity and economy.

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These amps are loud and clean and used by many Violin,harmonica,keyboard and guitarist who prefer clean high power. After all in hi power mode theres over 700 volts on the plates and the preamp with it's SS circuitry can provide a pretty linear signal with not much thd+N. There are some flaws as Mark pointed out the zeners maybe should have been heat sinked 78 & 79 series regulators as they go out quite often but those transistors very rarely fail and in all reality shouldn't. I like cascodes for mic pres over guitar amps and you don't need transistors for that as an all tube works excellent

[ogeecheeman]

musicman 112 65 rd

The reason these are so cold it the amp is running pretty much in class B. They idle near cutoff. They idle at about 6ma.

The adjustment is 25mv across the 3.9 ohm emitter resistor. If they are not equal, set the lower one to 25mv as long as the higher one does not go over 55mv.

Note: use only 1458 op amp to drive this. Do not use 4558 or others.

hi man.I know this is old, 2007, but I don't get on the page enough.since you helped me out with some great info on that crate I thought I'd ask about this music man 112 65 rd G2A(6L6) solid state driver. a customer came in,gave it to me.said he found it in a dumpster.thought I could use it for parts. well it was missing tube sockets,speaker,power cord,grille,output tubes.I had everything right there on the bench.I love these so being in the market for a nice gig amp smaller and lighter than my super twin I figured it was just what the doctor ordered.so I replaced everything,including the B+ supply caps.even had a pair of vintage sylvania 6L6 tubes.sounds great but...the bias is stable but it seems like I have to jack the gain and volume in the clean channel to get a usable level.when I do there is that rustling sound like an old black face fender with a bad plate resistor. still has original op amps. I know these things have a history of breaking down and op amps have become a lot quieter. can you recommend a replacement. the over drive channel is noisy as well but the signal goes through the same IC 1 as the clean channel. 1458.thanks