JTM45 build vs OT impedance and PT voltage

[Badside]

So my initial goal was to build a 20W-ish JTM45 using 6V6 power tubes. I was thinking about using a 18W set of transformer to get a lower voltage (350-ish) and a higher output Z (8k) compared to your typical Marshall, in order to get lower power (I want to DRIVE this thing with the band, and 15-20W seems like the sweet spot). This using fixed bias (I don't want it cathode bias, although I might put it on a switch for giggles).

But then I was looking at Hammond's replacement iron (they don't make a 18W set, but I have their Plexi 50W set in my 2204 clone and it's the bomb) and noticed their JTM45 set is actually low voltage/high Z (600V secondary, for about 380 B+, and 7k7 primary Z). This should give 20-25W tops which sounds pretty good!
I was pretty much wondering why their iron doesn't seem to follow the usual JTM45 sets, but of course these things have been known to change across years, and 600V PTs seems to be a common option for those wanting a quieter "browner" JTM45.

But then, I come across a bunch of comments on Metroamp forum about the low-voltage sounding… boring, and I go listening to a bunch of Youtube videos and definitely the higher-voltage JTM45 clones seem to have an extra bite which is pretty nice to hear.
But then, this is with KT66, perhaps 6V6s would appreciate the lower voltage.

And in all cases, higher voltage = louder amp, there's just no way around that (my 2204 runs 480V in a 3k4 PT and it is so loud I can't open it even on big festival stages).

So I see 2 options:
- Build it with the lower-voltage tranny set, experiment with 6V6, EL34 and KT66 output tubes for best tone
- Build it the "right way", high-voltage and KT66, and invest in a good attenuator.

I must mention I don't like the sound of a PPIMV (yes, the LarMar kind), it mushes things up.

[Phil_S]

I'm an amateur, not an amp designer. I won't pretend to comprehend all that goes into a design, but I've built about a dozen amps and have some first-hand experience going off the beaten path.

My take on what you say is that we don't really know how things will sound and what the optimum voltages are. It's my view that the exact outcome of an amp that hasn't been built before can't be known with certainty. You've got to build it and see. Even then, your speaker choice (number, open or closed, size of the box, etc.) can have as much influence on tone as the amp itself. If you are up to it, I suggest at some point you've got to dive in and give it a try.

I have a few thoughts on what you say you want. I think I hear you saying you want to, as best can be done, duplicate the JTM45/2204 at lower power. I think you can get close.

My first reaction is, why don't you build an 18-watt TMB? This is a documented design, essentially the JTM45 pre-amp with a cathode biased EL84 power amp. Forget for a minute that you like/don't like EL84's. This amp when mated to the right speaker cab will give you classic Marshall tone. OK, you want a variant? Build it with 6V6's. I'm told making the 18W fixed bias somehow neuters it.

OK, so you don't want to do the 18W? Build the JTM45 with this OT20PP http://www.musicalpowersupplies.com/3.html scroll down, it offers 6600 and 8000 primary options, so you get to experiment.

For the PT, I'd look for a PT 275-0-275 -ish. With solid state that will give you B+ of about 385, with GZ34 about 355, and with 5Y3 about 300. The GZ34 (the JTM45 design) will put a pair of 6V6 a hair above where they should be, but it will probably be OK if you drop screen voltage a little. I would be disinclined to push a pair of 6V6 into the 380v range. More volts won't necessarily translate into more/better tone.

Well, that's my 2¢. I'm sure others will chime in, too. Good luck.

[Reeltarded]

no! Biuld a normal JTM then move down one click on the selector for 8 instead of 16 with the same speakers and bias it for whatever tubes you want back and forth from normal to 6v6.

It's that easy.

Are you sure 20w works with a band? AC30 minimum for me. I require bare headroom, but a snare eats a 20w amp in a single bite! Nommy nom..

[billyz]

JTM 45's I have seen are about 400 to 420 VDC. Most 6v6's will handle that. 400 is better for a 6v6GTY . 380 is sweet.

A pair of 6v6's running fixed bias would like about 6K on the primary side of the OT as well. pretty much the same as the EL34. 8K will be nice for your tubes too.

Why are you stuck with Hammond iron , there are several better options. like Mercury or Marstran or Heyboer. Personally I do not care the sound of the Chicago Mag trannys.

Actually, a set of Trainwreck Express Trannys from RJ would be perfect. you will need the 5v tap for the gz34.

[Badside]

Why are you stuck with Hammond iron , there are several better options.

I can get them locally for cheap, being in Canada.
And I really like the amps I've built with them (a 2204 and an AC30).

That said, their JTM45 set is actually quite expensive for some reason.

[Badside]

no! Biuld a normal JTM then move down one click on the selector for 8 instead of 16 with the same speakers and bias it for whatever tubes you want back and forth from normal to 6v6.

But with the typical 6K6 Z of a JTM45, that puts it at 13k2! Might not be the best zone for tone.

Considering the usual B+ of 430V, running 6V6s in a JTM45 with no other changes would actually put it in the same operating region as a Deluxe Reverb.
JJ 6V6S are rated for 500V anyway.

But my aim is to get less power, 6V6s are only a mean to an end.

[billyz]

no! Biuld a normal JTM then move down one click on the selector for 8 instead of 16 with the same speakers and bias it for whatever tubes you want back and forth from normal to 6v6.

But with the typical 6K6 Z of a JTM45, that puts it at 13k2! Might not be the best zone for tone.

Considering the usual B+ of 430V, running 6V6s in a JTM45 with no other changes would actually put it in the same operating region as a Deluxe Reverb.
JJ 6V6S are rated for 500V anyway.

But my aim is to get less power, 6V6s are only a mean to an end.

Like I said , the Trainwreck express Trannys would be perfect all around. I do not care for the JJ 6v6s( not a true 6v6 more like a older 6l6) the New Tungsol 6v6s have been excellant in tone and durability. RJ has a good price on them too.

[Badside]

Like I said , the Trainwreck express Trannys would be perfect all around. I do not care for the JJ 6v6s( not a true 6v6 more like a older 6l6) the New Tungsol 6v6s have been excellant in tone and durability. RJ has a good price on them too.

Where can one find more information about those "RJ transformers"?

Only thing I can find quickly are the ClassicTone (which I've been wanting to try for a while) which have a good price, and the PT has dual secondary taps (520 and 600V) which can be useful.

[billyz]

you can reach him at info@rjguitars.net

Web RJ guitars .

he is a member here .

[statorvane]

Kind of late to the party here, but if you want a low power JTM45 via 6V6 tubes, I'd just swap the tubes and rebias using the same x-frmr set. You can always go back to the KT66/5881 tubes if you want. I think it'll be that easy. You do need to ensure with the 6V6s your x-frmr is around 8K, compared to 4K for the KT66s.

I have swapped between EL34s and 6V6S tubes in my JMP50 clone. The power difference isn't overwhelming, but the headroom is bigger w/EL34s. Plates are ~ 465 volts. I haven't tried the TungSol RI 6V6 tubes yet, so I can't comment on those, but the JJ 6V6S have plenty of low end and good definition. I typically reduce the bass with these compared to the EL34s I have tried (Mullard RI and Chinese EL34B).

This worked out so well I am thinking of building a SuperLead variation for my son, using 6V6S for lower power, and Mullard RI EL34s for higher power.

I would think springing for an 18watt x-frmr set and running cathode bias - well you have a 6V6 18 watt - not a JTM45.

Good luck whatever you decide.

[rooster]

I like the Magnetic Components Inc iron and I think you should give this company a try. They offer a variety of PTs, you should be able to find your target voltage there. Btw, what is this Hammond PT # with the 600VAC secondary? My experience is they don't make one in a 200ma + version. ?? Did you confirm this on the Hammond site or is it possible a supplier has posted incorrect information?

[pdf64]

Choose a PT to give about 420Vdv B+ with a GZ34, then you have the option of maybe 400V with 5V4, 380 with 5R4 about 340V with 5Y3.
All voltages off the top of my head, not checked.
The regular drake OT was 8k p-p, so clone of that would be perfect for 6V6.
My preference is to keep the voltage on 6V6 lowish, say below 380V (fixed bias), otherwise they can sound too 'tizzy'.
Even then I don't think they sound like KT66, closer to EL34.
Tubetown has IGPW PTs with taps on the B+ winding, for regular and lower VB+.
Pete

[Roe]

try the classictone PT with dual taps for high and low voltage. and use a standard 103 8k OT. this way you can use 6v6s and lower voltage or kt66s and higher voltage (I prefer the latter)

[Badside]

I would think springing for an 18watt x-frmr set and running cathode bias - well you have a 6V6 18 watt - not a JTM45.

Except it will be fixed bias, and have the cathode-follower driven stack.

So it won't be a 6V6 18W, it will be... something else (not a JTM45 per se either of course)

[Badside]

I like the Magnetic Components Inc iron and I think you should give this company a try. They offer a variety of PTs, you should be able to find your target voltage there. Btw, what is this Hammond PT # with the 600VAC secondary? My experience is they don't make one in a 200ma + version. ?? Did you confirm this on the Hammond site or is it possible a supplier has posted incorrect information?

It's the 290LX (JTM45 "replacement")
600V at 230mA

http://www.hammondmfg.com/pdf/EDB290LX.pdf