JTM45/100 Dual OT schematic

LordAli · Post by **LordAli** » Sun Sep 02, 2007 8:22 am

Suggestions:

Flames' vintage JTM:
1 impedance selector -> OT wired in parallel on primary even secondary

RI JTM:
2 impedance selectors -> possibly separated secondary windings. You need connect secondary winding to common ground 'couse you need to close negative feedback. Other terminal of secondary winding is connected to feedback, but are not connected together -> separate secondary winding.

That could confirm my suggestion, that you can run amplifier on secondary winding in parallel or separated.

But I think, that Marshall designed this RI with separated secondary windings couse it's more safe when running with 1 speaker cabinet (only 50W amp). Secondaries running in parallel cause amp running still on 100W power output what can break only one cabinet at full power (opinion). So if you want Dual OT Marshall then it's better to run secondaries separated.

Mickey_C · Post by **Mickey_C** » Sun Sep 02, 2007 1:17 pm

It's hard for me to believe that as a reason when marshall sells single OT amps that are 100w every day. Anyway, I need the ability to deliver full power through whatever is connected, regardless of impedance. 100W will not blow my speaker cabinets.

Mickey_C · Post by **Mickey_C** » Sun Sep 02, 2007 1:19 pm

LordAli wrote:
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.

mail it to me. Use the mail button. I will post it online, and send a link.

Post by **Brian Wallace** » Sun Sep 02, 2007 2:32 pm

On the original Marshall Dual OT amp, the selector was the common, 8,16 and 100V used on late 60's JTM45 amps. With the two transformer secondary's now wired to this selector, you will have 4,8 and 100V. I think I have a picture of one with the new label on it somewhere.

Mickey_C · Post by **Mickey_C** » Sun Sep 02, 2007 3:33 pm

Brian Wallace wrote:On the original Marshall Dual OT amp, the selector was the common, 8,16 and 100V used on late 60's JTM45 amps. With the two transformer secondary's now wired to this selector, you will have 4,8 and 100V. I think I have a picture of one with the new label on it somewhere.

BTW - What the dickens was the 100V connector for? I have NEVER understood that. I did try the tap, and it definitely worked with my cabinet (only for a moment, as it was curiousity, then I disconnected it).

Post by **Brian Wallace** » Sun Sep 02, 2007 4:09 pm

The 100V tap is for a PA setup. You have to have a 100V to 4, 8 or 16 OT at the other end for it to work properly. With this seup, you can run a boat load of speakers just like a store would.

Flames1950 · Post by **Flames1950** » Sun Sep 02, 2007 4:35 pm

Brian Wallace wrote:On the original Marshall Dual OT amp, the selector was the common, 8,16 and 100V used on late 60's JTM45 amps. With the two transformer secondary's now wired to this selector, you will have 4,8 and 100V. I think I have a picture of one with the new label on it somewhere.

I smell a new project in the makings.........

Mickey_C · Post by **Mickey_C** » Sun Sep 02, 2007 10:52 pm

Brian Wallace wrote:The 100V tap is for a PA setup. You have to have a 100V to 4, 8 or 16 OT at the other end for it to work properly. With this seup, you can run a boat load of speakers just like a store would.

You wouldn't have a schematic for that would you?

Would this be a practical way to scale an amp with this transformer? How does that work?

D-Max · Post by **D-Max** » Mon Sep 03, 2007 1:00 am

Searching plexi palace I found this http://www.plexipalace.net/plexiboard/v ... php?t=6704 topic. Some of the information has already been shared by the others.

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

Mickey_C · Post by **Mickey_C** » Mon Sep 03, 2007 1:12 am

All I've found is the Super 100JH schematic:

D-Max · Post by **D-Max** » Mon Sep 03, 2007 7:14 pm

Done some digging myself on Amp Archives and PP and came up with the drawings in the attachment below.

Please shoot on it to get the errors out.
My first question is: was the NFB resistor 27K and was is connected to both 16 Ohm taps?

Mickey_C · Post by **Mickey_C** » Mon Sep 03, 2007 8:48 pm

Yeah, I think that's about it.

Either way.

I think I would go the original way... though I can see some advantage in the dual output selectors. I've read some arguments that though you can parallel the primarys, you can't do it with the secondaries, because they are all about turns ratio, which won't change regardless of parallel or not. Functionally, though, you would still have to run the load higher than the tap, as the load would be shared by both... so I'm not sure if it matters or not. I guess we could find some old carver schematics, with the dual dynaco transformers, to figure out the right way.

Of course the original marshalls are a testament that it won't blow the iron.

Still sitting on the fence regarding this. I wish I had the source of the impedance and voltage selector switches, so I could print my own. They're a pretty pricey part.

I'd add the 5K6 grid stops, btw.

Not sure about the feedback circuit either. Can't tie it to both without adding some R between them.

rhinson · Post by **rhinson** » Mon Sep 03, 2007 10:30 pm

you don't need to take nfb from both output sections----one is all you need to achieve the intended purpose. there's only one p.i. here. rh

Mickey_C · Post by **Mickey_C** » Mon Sep 03, 2007 11:09 pm

rhinson wrote:you don't need to take nfb from both output sections----one is all you need to achieve the intended purpose. there's only one p.i. here. rh

I agree with you - but for some reason the Marshall RI pulls it from both.

Could it be that if you pull it from one that has no load on it, not reflecting back the impedance, it has no output on the taps?

Just a total WAG here.

If you look at the photo, you can see the NFB pulled from both. Looking at the left side (opposite socket from where the NFB wire snakes to the board), you can see what looks like a carbon film resistor in the shrink wrap on the end of that purple wire. Like I said, I don't understand it. (click this thumbnail for a larger close-up image):

D-Max · Post by **D-Max** » Tue Sep 04, 2007 4:25 am

Mickey_C wrote:I agree with you - but for some reason the Marshall RI pulls it from both.

Could it be that if you pull it from one that has no load on it, not reflecting back the impedance, it has no output on the taps?

Just a total WAG here.

If you look at the photo, you can see the NFB pulled from both. Looking at the left side (opposite socket from where the NFB wire snakes to the board), you can see what looks like a carbon film resistor in the shrink wrap on the end of that purple wire. Like I said, I don't understand it.

Yeah, I see on each impedance selector a resistor that is shrink wrapped.
But if only one OT is loaded, this circuit isn't going to be enough to relfect the load on the other OT.
Maybe LordAli's schematic can reveal that and hopefully some of the guru's overhere chime in to give us some clues.