Metropoulos Forum

Anybody know where to find a JTM45/100 dual output transformer schematic?

It's same as 45W JTM45. Only difference is behind phase inverter, when connecting coupling capacitors. PI drives two 45W power stages with 2 output transformers, Output transformers got secondary winding wired in parallel. That's all.

It's simple paralleling power stages.
But I'm not absolutely sure with speaker load for this circuit. Maybe it's half compared to 45W amp.

LordAli wrote:It's same as 45W JTM45. Only difference is behind phase inverter, when connecting coupling capacitors. PI drives two 45W power stages with 2 output transformers, Output transformers got secondary winding wired in parallel. That's all.

It's simple paralleling power stages.
But I'm not absolutely sure with speaker load for this circuit. Maybe it's half compared to 45W amp.

I was pretty sure the two transformers are wired in parallel primary and secondary, acting as one higher power transformer.

Otherwise, I don't see how you could wire the outputs in parallel.

Throw me a clue, please. Oddly enough, I am considering doing this.

No no,
There are two power stages possibly driven with one phase inverter.
Each power stage got 45W power output,got one OT, two power tubes and own bias.

Output transformer secondary is connected in parallel, that what makes it 100W amp. Each power tube pair got own primary winding, same like 45W amp.

Differently: cut off JTM45 schematic (#1) before PI coupling capacitors. Get new schematic (#2) and put cutted power stage near by #2. Connect #1 schematic's coupling capacitors to PI and connect OT secondary winding in parallel with OT secondary winding in #2. And you've got JTM with dual OT.

I am sure both output stages are driven from a single PI, the same as the original KT66 loaded 100W with a single OT.
But I am puzzled about whether the secondaries of both OT's are parallel or that it is a 2x mono 50W configuration with each OT having 1 output jack.

From the Marshall 40th anniversary 100W amp owners manual:
"It is imperative that:
a) at least one of the speaker outputs of the amplifier is connected to a load whilst in operation and
b) the impedances on the output selectors matches the impedance of the extension speaker cabinet(s) being used.
The following combinations are our recommendations:

1x16 Ohm cabinet - Selectors on 16 Ohm. Use either speaker output.
2x16 Ohm cabinets - Both selectors on 16 Ohm. Use both outputs.
1x8 Ohm cabinet - Selectors on 8 Ohm. Use either output.
2x8 Ohm cabinets - Both selectors on 8 Ohm. Use both outputs.
1x4 Ohm cabinet - Selectors on 4 Ohm. Use either output.
2x4 Ohm cabinet - Both selectors on 4 Ohm. Use both outputs."

In case of parallel OT's:
Two OT's on 16 Ohm see a total load of 16 Ohm
Two OT's on 16 Ohm see a total load of 8 Ohm
Two OT's on 8 Ohm see a total load of 8 Ohm
Two OT's on 8 Ohm see a total load of 4 Ohm
Two OT's on 4 Ohm see a total load of 4 Ohm
Two OT's on 4 Ohm see a total load of 2 Ohm

In case of 2x mono output:
Both OT's on 16 Ohm, one loaded with 16 Ohm, other no load!
Both OT's on 16 Ohm, each sees a load of 16 Ohm
Both OT's on 8 Ohm, one loaded with 8 Ohm, other no load!
Both OT's on 8 Ohm, each sees a load of 8 Ohm
Both OT's on 4 Ohm, one loaded with 4 Ohm, other no load!
Both OT's on 4 Ohm, each sees a load of 16 Ohm

I think in both configurations, half of the Marshall recommended settings results in somekind of impedance mismatch, or am I wrong here?

One can clearly see here that the purple feedback wire is connected to both impedance selectors (don't know which tap on the OT), but I don't see any wires between either both impedance selectors or both the OT taps. Would this then be a 2x mono 50W configuration?
Same here

It clearly shows that the both output stages share the same bias circuit (only one pot in the picture).

Aaah. I get it back, my fault. These pictures are more clearer to me.

D-Max wrote:But I am puzzled about whether the secondaries of both OT's are parallel or that it is a 2x mono 50W configuration with each OT having 1 output jack.

I thought it's two monoblock, connected in parallel on secondary windings. But from pictures we can see that tubes drive transformers from pair (plates are connected together for each pair - pin 3). So all magic is that it's common JTM45/100 schematic with two parallel OT's (on primary side).


But still there's question how to wire secondary windings ?

Following opinions :
As I still think about it and draw some possibilities on paper it seems, that outputs can be in parallel and very probably even separated as well.

Explanation (suppose EL34 output tubes)

Secondary windings in parallel :

Manual wrote:1x16 Ohm cabinet - Selectors on 16 Ohm. Use either speaker output.

50W output, primary impedance 3k4 ohms

Manual wrote:2x16 Ohm cabinets - Both selectors on 16 Ohm. Use both outputs.

100W output, 2x16 ohms gives you 8 ohms, impedance reflected on primary is half = 1k75 ohms like 100W amps

Manual wrote: 1x8 Ohm cabinet - Selectors on 8 Ohm. Use either output.
2x8 Ohm cabinets - Both selectors on 8 Ohm. Use both outputs.
1x4 Ohm cabinet - Selectors on 4 Ohm. Use either output.
2x4 Ohm cabinet - Both selectors on 4 Ohm. Use both outputs.

Other taps are the same, you only switch tap selector and connect second cabinet which halves primary impedance to 1k75 ohms and you "switch" amp to 100W mode.


Secondary windings separated :
You reflect 2x 3k4 ohms to primary. These acts like before -> parallel 3k4 gives you 1k75 ohms (100W mode) and you've got 2x 50W = 100W


Maybe i'm wrong somewhere. But I think that it works like this description. Thx for pictures.

LOL.

Like I said, anybody know where to find the schematic?

BTW - I think the picture clearly shows the primaries paralleled. It's the dang secondaries that I am not sure of... separate, parallel, or series? Parallel on the primaries makes perfect sense, as Raa then gets divided by two, which is needed for a quad.

I might do this with parallel on both primary and secondary - even though I would lose the use of one impedance setting.

For example 4, 8, and 16 would become 2, 4, and 8, so 8 and 16 would become 4 and 8 for EL34, or 8 and 16 for KT66 mismatched, assuming the 3.4K primary of each transformer.

I think that's correct.

One last note - this way of scaling the JTM45 may be the only accurate way, as so much of the magic really is the OT. Getting a bigger power handling output transformer, involving different wire and lamination sizes, probably would never sound the same.

Bummer that you need twice the output iron, the priciest piece.

One further question - Marshall didn't use a tube rectifier with this amp... but if you were being true to the JTM45...

Couldn't you use a 5U4GB rectifier tube? You would have slightly higher voltage drop, but it would handle the extra two KT66, with similar performance to the 5AR4, wouldn't it?

Any thoughts on that?

Mickey_C wrote:Like I said, anybody know where to find the schematic?

I'd like to know myself.......I know the new reissue uses two impedance selectors, but I'm sure I have pics of at least one original dual-OT Marshall and it had only one impedance selector. So it doesn't add up to me yet that the new RI amp is done the same as the oldies.

Well, actually it has no impedance selector at all, but the output jacks are plainly paralleled to each other, so the idea of two separately-run power amps doesn't seem right to me:

Using this picture of the 40th anniversary JTM45 100 as a reference point:

(click this thumbnail for a larger close-up image)


I clearly see both primaries wound together, going to pin 3 of the center two power tubes respectively, each of those then linked to it's corresponding outside power tube.

The secondaries are both wired to individual impedance selectors, and the grounds are tied to the socket lugnut at pins1/8... so that means the secondary grounds are common, indicating they are not series, or parallel, but actual separate outputs, as LordAli suggested could be the case.

Flames1950 wrote:Well, actually it has no impedance selector at all, but the output jacks are plainly paralleled to each other, so the idea of two separately-run power amps doesn't seem right to me:

Holy smokin' JTM100... you have an original?

Please take a close up of the power tubes and output jacks!

So did they just parallel both sides?

I wish that was mine, it was just in the collection of pics I had archived from building my single-OT 45/100.

It's time someone posts the schematic of this. Don't keep us in suspense

I'm sure someone on this board has one or at least drawn up by reverse engineering.

D-Max wrote:It's time someone posts the schematic of this. Don't keep us in suspense

I'm sure someone on this board has one or at least drawn up by reverse engineering.

Maybe I'll do it. But now I don't have much time and don't know, how to upload it.