Metropoulos Forum

i built a jtm45 style plexi out of an old chassis and parts i had laying around. its set up more like a 50watt plexi, but with the tube recto. here is the spec im using so far:

50uf mains/screens- no choke on hand, so i used a 390r power resistor- amp stays tight.....
16uf PI
16uf preamp
normal- 330u/820r
brite- 2.7k/.68u/2n2 brite cap
270k mixers
.47u/820r
22n couplers- 100n PI

the power transformer is from an old hilgen bass amp, and the OP tranny is from a 50watt peavey valvking. im running el34's and i have 500vdc on the plates and i biased at 32mA, seems to be a bit cold, so im thinking ill warm it up some, although it sounds nice and crunchy as is, but im wondering since this OPT was originally spec'ed for 6l6gc's if i would be better off tone wise to stick some 6l6gc's in in it instead. thats the closest i have to kt66's right now.

any suggestions on how to make this amp scream a bit more would be great......

Mixer change. 470k mixers sound more open, and forward... and just better.



Hmm.. 500@-32 sounds about right. Try the 6L6 if you like... I like 34s probably. After changing the mixer arrangement, I bet you would be close to where you want to be.

i only have a 16ohm tap, would changing the 27k to 47k give more power amp gain? less clean, more dirt?

A little more, but the ratios at 16 might make me start by throwing 100k or more at it to see.

I like 100k on the 8 for a balanced thing. Maybe 150k is close on the 16 and an easy to find value. Try them all!

some changes i made......tried 47k and 100k nfb, but prefer 27k, as the others made the amp too fizzy....
presence is now .68u....love it
prefer to have pre- PI master volume, as i have more control of the tone with it
added a 16u on PI filter.....might add a 10u on the preamp filters if needed

havnt changed the mixers yet, what will that do? will it make the amp smoother, not as ice picky?

gonna try my old ruby 6l6gc's, as those should bring the amp back up to a full 60watts.

so far it sounds really good with a clean boost, nice and crunchy.......this is starting to morph into a 1987 plexi....

changed the mixers to 470k and removed the cap on v2's 820r, much better sounding, more open like you said it would be, i love it........changed tubes to the 6l6gc's and now im runnin with a plate voltage of 380vdc, biased to 55mA........sounds much much better.....

Reeltarded wrote:Mixer change. 470k mixers sound more open, and forward... and just better.



Hmm.. 500@-32 sounds about right. Try the 6L6 if you like... I like 34s probably. After changing the mixer arrangement, I bet you would be close to where you want to be.