1987 JMP with 6550's or KT88's??????

[randalp3000]

Just finished putting this together last night. It's a re rebuild of a Mojo JTM45 kit, Mojo Heyboer power trans is a 350-0-350 200ma and output is a 4k primary I believe. Voltages look good and seems stable thus far.

Right now I have 6550's in it with 463v on the plates with around 52ma for the bias. I really want to try the 6550's and KT88's but I'm concerned about the high current needed from the PT and going through the OPT. I'm looking for loud and clean with some cut.

Another question I have is about the 150k or 220k resistors coming off the bias circuit. The MKII schematic has 150k for the 6550's which I used. Will I also be able to use EL34's with the150k's?

So should I let her rip of should I be concerned about the extra current? I have a gig Sat night and might take it along to try some Marshally goodness.

I'll update the schematic soon with a couple cap tweaks and the SS recto.

thanks much
rp

[randalp3000]

here's a pic, it's a rebuild with some salvaged parts and a new board I made.

grid resistors on bright channel inputs are really sensitive and picking up all kinds of noise.

[Roe]

el34s can use 150k, but most prefer 220k

[martin manning]

Right now I have 6550's in it with 463v on the plates with around 52ma for the bias. I really want to try the 6550's and KT88's but I'm concerned about the high current needed from the PT and going through the OPT.

The standing DC current might be a bit higher than normal, but the output power will be set by the B+ and the Zpri. The real issue is screen current at peak signal. The 6550's want to be about 300V on the screen at 465V plate and 4k load. KT88's maybe 275V on the screen. If you can work that out I think you could do it.

[Structo]

The grid noise can probably be fixed by locating the grid stoppers at the socket.
I like to use a small terminal strip attached to the socket screw.

Then shielded cable from there to the jack(s)

[Reeltarded]

Stock Marshall PTs do ok as long as the bias is moderate. That is only 170mA to deal with too.

[randalp3000]

Thanks for the responses

got it biased around 52ma with 6550's. not as loud and clean as I thought it would be. Had it up to about half way and it was sounding pretty good with the channels jumped til it blew the HT fuse.

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