JTM45 Build

[Reeltarded]

Needs more capacity. That is around 1/3rd of what it needs.

I like 20h in JTM with almost zero filtering. If you split the preamp filtering you can go stupid on the first stage. 8u even. I think mine is 16u all the way across now.

Rawr

[Roe]

I prefer 440v plate voltage and the following specs:
- 32-47uf on mains
- 16-32uf on screens
- 20-32uf on PI
- 10+10 or 22+22 in preamp
I generally prefer the 3h choke and a 470r flying resistor. but the 20h rs choke may work well if you omit the flying resistor

[Roe]

Hi. Would 7h 40ma be enough for a jtm45? Or isi it too small. I do have beefier 4h one. I have the measurements of tht one but not with me at the moment. Thanks

40mA should be ok with kt66s (esp chinese ones) or 6l6s. but you need 75ma if you run el34s

[joeboo88]

Thanks Roe. I ws planning on using 6l6s. This choke is tiny and will look funny beside the big OT.

[Littlewyan]

I thought I'd carry on this thread instead of clogging up the forum by starting another.

I'm getting closer and closer to building my JTM45 at last! Almost finished the planning stage. Few questions regarding filtering. On the 45 V1 and V2 share the same filter capacitor, now if I leave this as is will I still be ok if I want to increase the gain? Looking at David Bray's 4550 I can only see one cap can on top of the amp and he has a lot of gain in that thing.

[pdf64]

Increase the gain as in cascade the V1 sections?

[Littlewyan]

Well thats a bit extreme. I was thinking more decrease the amount of NFB and maybe bypass the cathode on V2.

[Reeltarded]

Yes they share that node.

Mixer choice make a big difference. Peaking there makes huge difference. Oddly, I prefer the 270k mix with a fender guitar, and the 470k mix with humbuckers.

[Littlewyan]

I wanted to stick with the 270K Mixers as I believe using 270K makes channel 2 a lot more usable by itself. I remember my 1987xl I had which obviously used 470K Mixers and there was no way I could have used channel 2 with my Les Paul!

Have you listened to David Bray's 4550? Or his JTM45 mod clips? They'll tear your face off! Didn't think a JTM45 could sound like that!

[pdf64]

If the V1 section are staying in parallel, then I doubt that the shared V1 & V2 B+ node will be a problem.
If there does seem to be a tendency to motorboating type oscillation (it would probably be more of slow ocean swell type of effect than motorboating) then the V1 cathode bypass cap may benefit from being lowered in value.

[Littlewyan]

I'll keep V1 parallel. The most I'll do is move the feedback loop to the 8ohm lug on the impedance selector. Trying to keep the amp stock JTM45. Although from what I've seen the feedback loop wasn't changed until they moved to a solid state rectifier.

[Littlewyan]

Metroamp JTM45s have a fuse on the HT centre tap which isn't a particulary good place for it. Have you guys who have built these amps stuck with this or have you put fuses on the HT secondary before the rectifier?

[sturgeo]

I've put mine on the HT centre tap as per the metroamp instructions, if it's good enough for George it's good enough for me and my first build.

[martin manning]

Fusing the outer legs is definitely a better way to go, since breaking the CT connection won't protect the PT in the event of a rectifier short. Looking at the Metro layout, it would be easy enough to mount a pair of fuse holders (same style as the CT fuse) on the side of the chassis, and pass the HT through them on the way to the rectifier tube.

[Littlewyan]

Cool I'll do that then. What rating fuses would I need? As someone mentioned that I'd need higher rating fuses for the PT secondaries as its AC and not DC.