JTM45/100 Dual OT schematic

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LordAli
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Post by LordAli » Tue Sep 04, 2007 8:46 am

Schematic almost complete :D
D-Max wrote: LordAli, I shure would like to have your drawn schematic. I got some ideas to make a JTM45+100 (notice the plus), where the output stage can be switched between a JTM45/100 4-tube dual OT SS rectified output stage and a 2-tube, single OT JTM45 output stage with a GZ34. Possibly also high/low B+ switching.
D-Max


JTM45-100 RI got that switching (50W/100W) by connecting another speaker cabinet to second OT. But when you want vacuum rectifier, you can have it even in 100W amp.But you need at least 2 GZ34, better 3 rectifiers to run amp properly on EL34 (I don't know KT66 characterictics).

Another 2 or 3 GZ34s need another winding on MT (5V@4 - 6A = 30W on MT). Better is to have each GZ34 with own secondary winding (350-0-350) and in result you've got 2 times greater (even in size) mains transformer. If you run GZ34s from common winding in parallel, there's problem with tubes oscillations......

if you finish that amp, you can post results, I'm really curious :D

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Post by Mickey_C » Tue Sep 04, 2007 1:09 pm

Luckily for me, my amp (loosely based on a 1st generation JTM45) is really truly only 34watts - it only has a 150ma B+... so I could pump it up two more tubes, and still be under the 250ma max for 1 GZ34. I use FRED rectifiers for the SS mode (there's a three way standby to select "Tube" or "SS"), so I think I'm covered on that note. I realize there would be greater sag at higher volumes with the tube, of course.

Regarding the loading of the primaries, well I think the tubes are still on either way, unless you actually shut them off. The primaries being paralleled, all four tubes will see the total reflected load, from either one or both of the impedance selectors. Just a (slightly) educated guess.

I'm scaling my "Soultone Forty-Five" up to a larger version, complete with dual footswitch-selectable Power Scaling controls (both the Power and Drive/Master control). So far this looks like the most viable way to do it, without losing the gorgeous tone the amp is known for. I'm not confident of scaling the OT!

I hadn't planned on the dual impedance taps, but I'm starting to think you need them, being the secondary is determined by turns ratio, not impedance, and probably can't be paralleled with any good effect. Marshall got a lot smarter over the years from the original amp.

Thanks for the help so far,

Mickey

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Post by LordAli » Tue Sep 04, 2007 6:27 pm

Dear Mickey.
You're not right in some things.

Main mismatches:
1) 150mA is transformer RMS current. This is not DC current. This current flows when amp is turned on, but you don't excite him. DC current is for EL34s for example only 130 mA. When you excite amp to a full power, then your 55W amp will drain approximately 300 mA which is beyond the recommended operating level of GZ34 = is overloaded. So one GZ34 don't fully cover not even 55W. Amp will work with one GZ34 and 4 power tubes, but it will never run at full power and soon you could run for new rectifier tube. That's the reason, why 100W amp don't use vacuum tube rectifiers, but cheaper silicon diodes :wink: I saw some 100W amp constructions with tube rectifiers, but at least 2 paralleled.

2)Marshall used this OT's in dual he started building 100W amps becouse it's the same as one 100W transformer. See Dr.Tube. There're some information about it.

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Post by LordAli » Tue Sep 04, 2007 6:37 pm

So here's promised JTM45-100 Dual OT schematic.
I made it from collected information and pictures from this thread.

I hope it helps someone :wink:
Attachments
JTM45-100_DualOT.pdf
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Mickey_C
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Post by Mickey_C » Tue Sep 04, 2007 6:52 pm

LordAli wrote:Dear Mickey.
You're not right in some things.

Main mismatches:
1) 150mA is transformer RMS current.
My transformers supply a 400V B+ at 150ma, and I went through hell and high water to develop them as such. They're very low power. I am not talking about the VA rating of the transformer 340-0-340 RMS, that's a different spec.
LordAli wrote: 2)Marshall used this OT's in dual he started building 100W amps becouse it's the same as one 100W transformer. See Dr.Tube. There're some
information about it.
I've read it and everything else, on this topic too, so I think we have a language barrier.

It's my understanding that they scaled the amp using the two transformers until they could get a higher power version wound. The word scaled in this context means "INCREASED THE POWER CAPABILIITY OF THE OT."

Which is exactly what I want to do, without having to develop a new transformer. Once again, I use custom-wound mercury iron that's exclusively for my amps.

Perhaps you're confused about my tossing in the dual power scaling controls comment. That has nothing to do with the transformers, I was just mentioning another feature I will be adding, since I will be using a larger head case, and can make room for another two controls on the front panel.

Sorry, I just didn't follow your statements.

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Post by Mickey_C » Tue Sep 04, 2007 7:02 pm

LordAli,

Thanks for the hard work! Is this mostly guesswork, or has it been confirmed against an original unit? Just wondering.

Your schematic shows the secondaries paralleled, which I understand they did.

However, I think they may have been mistaken on that one point. I've been reading that while you can parallel the primarys without issue, the secondary is a function of turns ratio. Paralleling them together does not divide their impedance taps.

I think the split outputs are smarter.

What's your thoughts on this one?

Mickey

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Post by LordAli » Wed Sep 05, 2007 3:19 am

Marshall haven't changed his first amp constructions (JTM, JPM) until now. Only in details in some models like Dual OT wiring etc. For example Super JH100 is nothing more than common JTM45/100 with 33k/470p in tone filter. So I guess JTM45/100 Dual OT is almost same as JTM100 (or JTM45/100) which was my basis for schematic drawing.

I can't see problem with paralleling secondaries. I thought it got paralleled only secondaries, but OTs run in parallel both sides. Maybe Marshall realized, that separated secondaries is better solution. I can agree with you Mickey that it's better than original 60' constructions.

No, parallel secondaries don't divide impedance taps. With paralleling secondaries you raise output power by twice, so you need to go down with load impedance by twice too to save right primary impedance.

It's the same like going down from 100W amp to 50W amp. You go down with power by twice, so you need go up with load impedance by twice.


I must apologize for my previous post. I realized, that you possibly speak about amp with KT66, what could be 150 mA when running. I've got fixed EL34 values. I saw some transformers with 150 mA RMS for EL34, thats what confused me.

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Post by Mickey_C » Wed Sep 05, 2007 1:19 pm

Is there some other tube than the KT66? I refuse to believe it.

:lol:

But seriously, putting the rectifier to the edge as you say is probably not wise. The internal resistance will be peaked, and it won't have much headroom. I am sadly crossing it off the list. Less heat too - as even in a Power Scaled amp, the rectifier just sits there making heat, even when the power tubes are running cool at 40V. Removing it is probably the best move. Adding two would work... but that's twice the heat and hassle.

Somebody promised to send me a good Russian article on paralleling transformers... If I get it, I will pass it along. He stated you need to put the secondaries in series... which seems really strange.

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Post by Mickey_C » Wed Sep 05, 2007 7:21 pm

LordAli, you will be relieved to know I am definitely dropping the tube rectifier, and dual impedance selectors. There's just no room!

Something has got to go, if I expect to try this myself.

Image

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Post by LordAli » Thu Sep 06, 2007 6:24 pm

I wouldn't use tube rectifier for 100W amp. Next two tubes require more space and I would want cab. for 100W amp not a box long like a train :lol:

But when there were only tube rectifiers in 30' through 50' then there was not a choice.

Send some photos after you finish it :wink: Looks cool

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Re: JTM45/100 Dual OT schematic

Post by neikeel » Thu Sep 12, 2013 5:12 am

Resurrection of old thread (hope Flames id watching as he has a file on these

I have one of (if not the) oldest production 100w Marshalls on the bench at the moment.

It's components date from Feb (pots) March (diodes) and May 1965 (filter caps).

It has dual OTs, one is original (predecessor to 784-103) with an 8/16 and 100v selector on the side, the other is a later -139 (funnily enough another UK owner of similar amp has one blown OT too?)

It is clear the OT primaries were rewired at the time, the OTs are in parallel (again modern wire - well pre 1980 as that is when last owner acquired it).

Anyone else got pics or schematics?
Neil

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Re: JTM45/100 Dual OT schematic

Post by Roe » Tue Sep 17, 2013 4:25 am

Tommy wrote this on PP:

"The centre tap of BOTH OT's is connected to the B+ post at the choke in the usual place. The primary reds of both OTs are Y connected, and the combined primaries go to the innermost tubes as usual, with a connection on to the outermost tubes between the valve bases. There is a single swamp resistor between power tubes V1/2 and V3/4 just as you will usually see on a 100W.

The yellow secondary common (these are JTM45 OT colours) go to the sleeve connections of the output jacks (you can see these on the pics). The remainder of the secondaries are bundled together and go to the output selector, where they are connected together colour wise to give -

- the blues - 8 ohms (these are the 16 ohm taps - same ohms law as applied when using two cabs on a single amp - set it to 8)
- the greens - 4 ohms
- the whites - 100V

This is the reason for the Dymo tape under the output selector - there's no 16 ohm tap on these amps.

To explain the above in simple terms, the OT's are simply connected in the usual way but completely in parallel to one another, and the impedances halfed!

There are other (less authentic) examples where only the CT of the primary is parallel, and the secondaries feed an individual pair of power tubes each. In these cases there are 4 swamp resistors in use, and the green signal leads coming off the board from the PI go to tubes 1/3 and 2/4 rather than 1/2 and 3/4."
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neikeel
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Re: JTM45/100 Dual OT schematic

Post by neikeel » Tue Sep 17, 2013 7:35 am

Roe wrote:Tommy wrote this on PP:

"The centre tap of BOTH OT's is connected to the B+ post at the choke in the usual place. The primary reds of both OTs are Y connected, and the combined primaries go to the innermost tubes as usual, with a connection on to the outermost tubes between the valve bases. There is a single swamp resistor between power tubes V1/2 and V3/4 just as you will usually see on a 100W.

The yellow secondary common (these are JTM45 OT colours) go to the sleeve connections of the output jacks (you can see these on the pics). The remainder of the secondaries are bundled together and go to the output selector, where they are connected together colour wise to give -

- the blues - 8 ohms (these are the 16 ohm taps - same ohms law as applied when using two cabs on a single amp - set it to 8)
- the greens - 4 ohms
- the whites - 100V

This is the reason for the Dymo tape under the output selector - there's no 16 ohm tap on these amps.

To explain the above in simple terms, the OT's are simply connected in the usual way but completely in parallel to one another, and the impedances halfed!

There are other (less authentic) examples where only the CT of the primary is parallel, and the secondaries feed an individual pair of power tubes each. In these cases there are 4 swamp resistors in use, and the green signal leads coming off the board from the PI go to tubes 1/3 and 2/4 rather than 1/2 and 3/4."
That is helpful, the one I have with me at the moment has selectors on the OTs themselves, but halving the impedences makes sense. There are two swamps and one OT primary feeds V5 and V6 with the other feeding V7 and V8 (it is a PA).

An interesting point is where the centre tap of the OT is fed in relationship to the choke. Most Marshalls have the arrangement where B+, choke in, centre tap are tied and then the choke out, screens and first dropping resistor are tied. If you look at the earliest schems of both the JTM45 (actually a Model 1961 http://www.drtube.com/schematics/marshall/jtm45tr.gif
and same here:http://www.drtube.com/schematics/marshall/1959t-66.gif
I am pretty sure that we have discussed the significance of the relationship of the choke to the screen feeds but not the OT centre tap. I guess these earliest amps were running with a 20H 690ohm choke (70mA rated) compared with the later amps using a Drake 354-114 with 3H at 100ohms (not sure of its rating?).
Neil

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Re: JTM45/100 Dual OT schematic

Post by Roe » Tue Sep 17, 2013 11:06 am

that particular 1961 jtm45 schem is thought to have mistakenly placed the choke before the OT (instead of after), since there is no evidence of a plexi with the choke prior to the OT's center tap (CLC filter). I have actually tried this setup (the CLC filter) in my jtm45, using a 300ma choke, and it does not give the classical plexi sound at all.

vox used a CLC filter on the very early ac15s, brian may uses it on ac30s and fender used it on the 5E tweed series.

I've also tried a 500mA choke on a 100w amp w/CLC filter. it cures ghosting but sounds a little too clean
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Re: JTM45/100 Dual OT schematic

Post by maxbauer » Sat Mar 04, 2017 5:58 am

Roe wrote:Tommy wrote this on PP:

To explain the above in simple terms, the OT's are simply connected in the usual way but completely in parallel to one another, and the impedances halfed!

."

Hi guys, im new here!
I had to dig up this old thread, there is so little information on the Dual OT amps.

From what i understandt about transformers is, if you wire them parallel in primaries, and secondaries,
the impedances stay the exact same. because the transfer ratio stays the same, only the power
is divided on two irons causing less heat on either. The tubes are seeing the same impedance
as they do with one trasnformer. - given that they are the same irons.

Anybode want to chime in again on this subject?
I could not get any schematics of the old 45/100s, they are not available anymore :(
if someone could repost them that would be great.

max
:rocker:

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