JTM 50 Really Loud!

[JoeCon]

Gentlemen,

I had some parts laying around and found that they would work out nice for a JTM 50 build. So I went at it with the junk I had...
V1 A+B shared 820R and 10uF
A Shift pot, 50KB, with a 33K resistor (ala Miles idea thanks) for the slope
V2B no bypass cap
PI Plate both 100K
Mixing resistors 270K
Power tubes bias resistors 150K not 220K
Got Classictone 4.2 50 Watt OT
Edcor PT gets me 405 on the Plates and 390 on the screens.

And it is really loud, the shift for the tone controls works very slick, and for the first time, for me anyways, the Presence control really does something!

[norburybrook]

yeh baby! it's a great amp, probably one of the best non master volume amps around along with the JTM45. It's more usable than a trainwreck express IMHO and it's just a great amp. It took the bassman and beefed it up, turning a great amp into a killer amp.


I'm still thinking about building one....I have a JTM45 but the plexi has a little extra something as well...……………...I have too many amps...…………………………..




M

[JoeCon]

Norbury

I built a model 1987 20watt 6v6 combo too. That little bugger is also loud, but a different sound from the JTM 50. The plexi 20 watt has a switch to bypass V2a with a cap so that adds to the crunch there. Plus the 1987 had split cathode for V1 and I parallel the channels with separate volume for bright and normal...

Hanging around here makes you build too many amps!

[M Fowler]

Looks great and glad it's loud and fun.

Hammond 159R choke I had to look that one up is it new? Good specs on that one.
I don't think I've ever seen that choke.

Got a customer that wants 25w version of my 50w Hot Rod Plexi but doesn't like 6V6 so I'll try low voltage transformers with 5881 maybe.
Fun stuff, thanks for posting up photos, I like amp porn. Nice blue boards too!

Mark

[Reeltarded]

hah! Love that. It looks great! I like the voltage. More is typical but less is better!

Here's a fix for your volume nightmare. Look up the common Jose mod and leave the clippers out of it. CF master is GREAT!! Just need a 1M pot, a couple wires and a .22u cap. It will still sound like it's too loud, only it won't be.

ROCK!

[Littlewyan]

Great build. They are definitely loud! If I put my JTM50 up against the TW Express at similar volumes, the JTM50 has a thump that I can't quite get with the Express. I also think the older master volume amps are extremely flexible. I once played in a covers band with my JTM50 and it nailed songs from Baker Street by Gerry Rafferty to Poison by Alice Cooper! Try 470K Mixers if you want a bit more bite. Beware though, you will lose treble on the Normal Channel by doing this!

M Fowler - You could stick with EL34s but use Cathode biasing. I'm going to build an amp with 420V on the plates, 6.6K OT and use cathode biasing with EL34s to achieve roughly 25W. I believe Marshall use cathode biasing for their 25W amps.

[M Fowler]

Sounds like a plan thanks.

Mark

[Littlewyan]

I should add, I'm only using 420V as I managed to get hold of a replacement 70s Marshall PT pretty cheap . My ideal voltage would have been 400V. But, meh, beggars can't be choosers.

[JoeCon]

Miles

Sir, you're saying add this MV after the CF before the tone stack with no clippers! Yeah looks pretty easy...
Thanks...

[Reeltarded]

YES

It's the most sensible MV rig ever. The clippers are fun, but you don't need 'em at all.

[pdf64]

It just seems ass backwards to feed a tone stack via a 1M pot, as its source impedance will vary from 0 to 500k according to the pot setting, with a consequential affect on the operation of the tone controls; why not put the MV after the tone stack

[Reeltarded]

Try it. It's as if it's not even there. It's the best MV for a Marshall. The normal master sounds like crap, the PPIMV is for Van Heflen addicts, and the Jose sounds like the amp is cranked up and spitting tunes.

[JoeCon]

Miles,
Can’t believe there’s a Van Heflin reference here, so funny. No one knows him I’m sure...

Mark
I think that choke was supposed to go in a Vox AC50 build but it got dumped so I used it in the JTM 50. I got the blue boards from some of your early builds. In fact my favorite amp is a 2 x EL84 rocket that you posted. My first build and boy still sounds awesome, most guys can’t believe it’s only two tubes....

[M Fowler]

I built two of those 18w Rockets for my sons, loud and fun amps.

[Reeltarded]

Not to tah-tah-totally go off the rails, but my Liverpool can eat a Marshall 50. Why so damned loud!??