New Marshall Project (‘77 1987)

[pdf64]

When I investigated the 100V winding on a Partridge OT, it seemed to be optimised / intended for a 200 ohm load
https://ampgarage.com/forum/viewtopic.p ... 40#p418740

[martin manning]

Yes, a separate 20-ohm winding seems an odd thing. Maybe a dedicated feedback winding? Old Marshal schematics (JTM45) show an OT with 100V tap like this:

[Raoul Duke]

Here is a JTM45 version: https://ampgarage.com/forum/viewtopic.p ... 60#p296260
You have access to the orange and green grid leads from the top of the board so you could break those and run the outputs from the dual pot back to the board. This would leave the existing grid leads running under the board in place. Lifting the board would be quite a bit of work, so no surprise people want to avoid that. Personally, I'd be reluctant to modify this amp.

...the bias lead from the 220k “junction” on the board. Can I tap either 220k trace since they seem to be connected or do I need to jumper those and tap from that? Been researching and one method has top soldering from the leg of the bias cap - which seems kind of lazy to me.

The inner end of the 220k's (nearest the middle of the PCB) and the "output" of the bias supply are all one node, so there are several options.

Thanks for the excellent diagram Martin - understood re: the attached board leads. Board’s coming up any way to redo a couple of dodgy component “grafts” to old leads and the bias caps - so if I were to do it - it would be all at once. If I do the MV, I plan to use a speaker jack hole for the pot so that if reversed - there’s no trace it was ever there.

About that OT - seems ‘67 was when that 128 transformer was used, and only for a short time. From what I’ve read, it only had 8, 16, and 100v taps (like the schematic you posted). The 1962 BB reissue had 4, 8, 16 and “buried 100v tap”; plus all the other specs match - which leads me to my conclusion. Not real sure how that would work - but the 20R load seems odd to me also.

[martin manning]

Marstran sells a 128 OT. The diagram shows only 4, 8, and 16-ohm secondary taps, with no indication of any other winding.

[Raoul Duke]

Saw that. I think that proves there’s no real practical reason for that tap - so the juice isn’t worth the squeeze for the re-creators. I actually use that website a lot to figure out Marshall color codes. Looks like real high quality stuff; but I think I read somewhere they went out of business recently?

[Roe]

Marstran sells a 128 OT. The diagram shows only 4, 8, and 16-ohm secondary taps, with no indication of any other winding.

the 100v tap is buried but affects the sound slightly. the 128 has slightly more bass than the common 139 but is generally very similar

[martin manning]

Marstran sells a 128 OT. The diagram shows only 4, 8, and 16-ohm secondary taps, with no indication of any other winding.

the 100v tap is buried but affects the sound slightly. the 128 has slightly more bass than the common 139 but is generally very similar

Are you referring to the Marstran/Heyboer reproduction or the original here? If Marstran, has that been confirmed? I'd think that would be worth showing it on the diagram or at least making some mention of it if they are going to the trouble of winding the extra turns.

I can see where a deviation from the original winding pattern might have some effect, and the manufacturer may have just taken the easy way out, keeping the extra turns without bringing the end of the coil out. That didn't last long though, most likely due to a desire to eliminate extra costs. The interval where the 128 appeared may reflect the time it took to define a revised winding pattern with equivalent sound quality and get approval for the change from Marshall.

[martin manning]

Saw that. I think that proves there’s no real practical reason for that tap - so the juice isn’t worth the squeeze for the re-creators. I actually use that website a lot to figure out Marshall color codes. Looks like real high quality stuff; but I think I read somewhere they went out of business recently?

The site is still up, and Google says they are still in business.

[Roe]

Marstran sells a 128 OT. The diagram shows only 4, 8, and 16-ohm secondary taps, with no indication of any other winding.

the 100v tap is buried but affects the sound slightly. the 128 has slightly more bass than the common 139 but is generally very similar

Are you referring to the Marstran/Heyboer reproduction or the original here? If Marstran, has that been confirmed? I'd think that would be worth showing it on the diagram or at least making some mention of it if they are going to the trouble of winding the extra turns.

I can see where a deviation from the original winding pattern might have some effect, and the manufacturer may have just taken the easy way out, keeping the extra turns without bringing the end of the coil out. That didn't last long though, most likely due to a desire to eliminate extra costs. The interval where the 128 appeared may reflect the time it took to define a revised winding pattern with equivalent sound quality and get approval for the change from Marshall.

Unlike the original Drake (which it is based upon), the Marstran has a buried 100v tap and an available 4 tap. The Marstran is made by Heyboer to Marstran's specs. By their nature, the 128s reportedly have a tad more bass than the 139s. However, some contend that many Heyboer transformers have more low bass than the originals upon which they were based. The measurements I've seen suggests a very minor difference at best

[Raoul Duke]

Thanks for the background Roe - I figured you would know more about this OT. I appreciate you sharing your extensive knowledge and experience with Marshall amps. I’m always referencing old posts of yours to learn these details, history and fixes.

So from a practical standpoint - they likely keep the tap (buried) for the subtle variation it provides from the 139 then? Although it’s probably a smaller piece of the market - given that these high quality Marshall/Drake replicas are their specialty - I can see the reason for offering it (128) for authenticity sake.

Solving more “mysteries”… good stuff!

Thanks guys!

[Roe]

Thanks for the background Roe - I figured you would know more about this OT. I appreciate you sharing your extensive knowledge and experience with Marshall amps. I’m always referencing old posts of yours to learn these details, history and fixes.

So from a practical standpoint - they likely keep the tap (buried) for the subtle variation it provides from the 139 then? Although it’s probably a smaller piece of the market - given that these high quality Marshall/Drake replicas are their specialty - I can see the reason for offering it (128) for authenticity sake.

Solving more “mysteries”… good stuff!

Thanks guys!

yes, the 128 is intended for jtm50s and jtm45/50s with el34s from 1967. These are the last jtm45s and the first el34 amps among 50w amps (1986s, 1987s etc).

[martin manning]

So from a practical standpoint - they likely keep the tap (buried) for the subtle variation it provides from the 139 then? Although it’s probably a smaller piece of the market - given that these high quality Marshall/Drake replicas are their specialty - I can see the reason for offering it (128) for authenticity sake.

Still surprising that Marstran does not mention or show that detail on the wiring diagram if they are going to all the trouble of including it.

The 20-ohm secondary you have is still a mystery. too. Any chance you miss-measured something there?

[martin manning]

PS - I went for a Marstran OT for my 2204-ish amp in Trainwreck chassis. For whatever reason, that amp is a favorite around here.

[Raoul Duke]

The 20-ohm secondary you have is still a mystery. too. Any chance you miss-measured something there?

That’s always possible, lol. I will say that I measured everything twice - starting fresh on two separate days - repeating each measurement twice and writing everything down as I went. The cross-measurement of the gray and orange (solid to stranded each way) was especially puzzling (to me). I thought that would be the missing piece to the puzzle there…

The amp sounded pretty good with the like-colored wires joined and connected (both grays on 16R tap, both orange on speaker jack ground) though - even without screen grids or grid stoppers.

[martin manning]

The amp sounded pretty good with the like-colored wires joined and connected (both grays on 16R tap, both orange on speaker jack ground) though - even without screen grids or grid stoppers.

I realize now that I needed to swap the labeling on both gray leads in the diagram I posted above, so now that's fixed. Connecting the 19.5R across the 16R is less weird, but still not a match, so connecting them in parallel would cause some current to circulate in the secondary. Still a mystery as to how this might be equivalent to a hidden unused 100V tap. You could try it with and without that extra winding connected, and decide for yourself if it should stay as it was.