Subminature JTM45 Practice Amp

[zendragon63]

Hey gang, just finished a subminature build based loosely--and that might be optimistic--on a JTM45. I've sort of had a recent born-again early Marshall tone epiphany. Subminatures can be kind of a pain to work with and I took license on what I couldn't find on data sheets or plagerize from other comparable builds. Surprisingly good sound through a 12. It has its limitations, the best being the lower sound pressure. The tone differences in the two channels make for some nice mixes. Attached is the schem as built; feel free to use what you want and critque as you wish--I am sure that I have overlooked lots and I can always use any and all insights.

I need to get a clip posted; I just haven't set up an account with one of the posting site. Regards

dennis

[eleventeen]

LOVE the IC sockets!! Nice touch!

Sorry, had to laugh at those. Looks pretty nice!

[Ritchie200]

That is too cool!  Please post a clip or two!

How do you change tubes?  Looks like a permanent installation!

What are the physical dimensions of the chassis?

Nice work!
Jim

[DummyLoad]

cool little amp! :-)

love the IC sockets - thought about using the same thing (augat brand w/ machined pins) if i couldn't find some NOS submini sockets - i got lucky and found a seller who had three.

--ISO

[zendragon63]

Thanks guys. Chassis is a 13x5x2. Yeah, it would have been nice to find the correct sockets for it but these worked ok. The 5902s are soldered in place but the others are the plug in style. They can be changed out if necesary but I figure that any tube stringently designed for the military and built for a...oh I don't know, maybe an ICBM guidance system--will probably be good longer than I will ever play this amp. regards

dennis

[DummyLoad]

dennis, try the 5639WA. it's listed as a video amp, however, it has similar power diss. as the 5902 but it has a gm of 9000 and since it has a higher gm, the g2 power diss. rating is 1W. the anode dissipation rating is for video service, so i would imagine that in AF service it would carry a slightly higher rating.

my thoughts are that the 5902 with a gm of 4200 would be kind of 6V6 sounding and the 5639 would sound more like EL34/84 family.

--ISO

[steven]

I built a 5F11 using 5902s. Well, it would have been a 5F11 but I never got around to doing the trem, so I guess it's a Harvard. It's a great sounding amp. I've got a 6021 and 6112 in the preamp.

They made real sockets for the subminis. I think I got the last six left in Europe. Good thing about them is, after you've built something with them, then go back to normal Novals, you suddenly feel like "woah, good to have all this space to wire the sockets up."

They were actually made for the missiles themselves, not the guidance systems - look at the specs - 80,000 feet, 400G impact acceleration, 2000G continual acceleration, have to take 5 hits from a hammer without damage (specs from memory, may be wrong). I'm not too worried about the expected lifetime in a guitar amp.

edit: what an idiot, of course the guidance system is part of the missile. not enough coffee

I still have two sockets left, and a bunch of tubes; a submini-marshall would be kinda cool ...

steven

[zendragon63]

Yeah, the 5902 seems to be the common power pentode subminature of choice for these class of amps. ISO, good idea as I really hadn't ever thought about the gm relationship and tone, harmonic distortion or rate of breakup--especially in the subminatures. If I run across a pair of 5639s, I might even be able to wade through PRR's one of many (great) explanations on plate load calculations and find a suitable output OT match. I usually can't leave well enough alone Thanks all for the input. Regards

dennis

[DummyLoad]

I might even be able to wade through PRR's one of many (great) explanations on plate load calculations and find a suitable output OT match. I usually can't leave well enough alone grin Thanks all for the input.

use same OT... tweak Rk for similar current flow.   

--ISO