JCM 800 with a SIR 34 mod... what is it?

[hermannbjorgvin]

I was hoping someone could enlighten me a bit about the JCM 800 #34 modification by Frank Levi.

I know it uses 6550 tubes and a beefed up output transformer (right?)
It also does some other things in the preamp section if I am understanding correctly.

Another thing I am wondering if there is a difference in the 2203 and 2203a models or if 2203 is just a shorthand people use for the 2203a

What are some of the things to look out for when doing a JCM project? Does anyone have the kind of tips that only come from experience?

Thanks in advance for anyone looking at this, would buy you a beer if I could and talk about this

Also yes I have been researching this on Google and will probably continue to do so but sometimes it helps hearing it from a person who knows

[Roe]

...6550 tubes and a beefed up output transformer ([NO!])
It also does some other things in the preamp section if I am understanding correctly.

have never heard of a 2203a. Levi added a .1u bypass cap over the 10k cathode resistor. and a .47u over the 1k probably as well as a 150k from gain pot viper to ground if memory serves

[hermannbjorgvin]

...6550 tubes and a beefed up output transformer ([NO!])
It also does some other things in the preamp section if I am understanding correctly.

have never heard of a 2203a. Levi added a .1u bypass cap over the 10k cathode resistor. and a .47u over the 1k probably as well as a 150k from gain pot viper to ground if memory serves

Thanks Roe for pointing me in the right path

[Littlewyan]

I think he also replaced the 470pF cap over the 470K resistor (Before the gain pot) with a 0.0022uF cap. If you google #34 schematic you should find a few sources with the actual schematic. I think there's even one posted by Santiago who designed the AFD100 amp.