New Marshall Project (‘77 1987)

[Raoul Duke]

Makes sense Martin. If all else is equal - let the sound determine which way to go.

Spent some time today looking at any 50w Marshall schematic I could find on the internet and none are showing an OT configured this way. Really puzzling. There’s got to be an answer to “what do I have here?” somewhere…

[martin manning]

There’s got to be an answer to “what do I have here?” somewhere…

Just to be clear, there are no markings whatsoever on this OT you have?

[Raoul Duke]

Here’s a pic of the marking and label shadow:

IMG_0308.jpeg

Actually, late last night I was searching the Marshall Amps forum and found some pretty solid clues. Turns out these are uncommon, but not unheard of. One theory has them as 70s/80s Unicord warranty transformers and another theory has them as “wound/rewound in house” at Marshall - with a couple that had them on brand new amps back in the late ‘70s. I thought the hand-written ID was from whatever shop pulled it out of something else - but I guess this is common for these “dual common, dual 16R” OTs. As we saw from my test data - aside from the separate winding they are considered equivalent to the 139. Kind of pleased that it’s period correct and not a later replacement.

The common explanation for the dual taps is a separate NFB circuit. The posts I read seem to indicate capping the separate winding as the accepted set-up with no degradation of performance or sound.

It’s funny because I’d been searching here and there by description; i.e. “dual common 16 ohm 50w output transformer” and variations of that - getting nowhere. On a whim, I searched “OT50M” and bam! I got a few hits immediately that pretty much painted the picture. Goes to show that “search” takes some perseverance I guess, lol.

I appreciate everyone’s brain power and advice on this!

Thanks guys!

[martin manning]

The common explanation for the dual taps is a separate NFB circuit. The posts I read seem to indicate capping the separate winding as the accepted set-up with no degradation of performance or sound.

Which is where we ended up ;^)

[Raoul Duke]

Never doubted you for a second Martin!
I was more curious why they were paralleled than anything. Just trying to make sure it wasn’t a “happy accident” that contributed to this amps’ tone.

Got all my grounding improvements and “pot dressing” done today. Yesterday I spent some time trying different dropping and pot tail resistors for the bias circuit. I got -30 to -47 with a 100k dropper and a 28k tail. I did this unloaded - so I’m hoping it still gets me in the ball park. The 220k/56k combo wasn’t even close on 120vAC - which makes me wonder if that played a part in the previous owner running the amp on the 110vAC tap (in addition to the low B+ nature of this PT)?

Looking forward to getting this amp running as it should and hearing it again.

[martin manning]

I was more curious why they were paralleled than anything. Just trying to make sure it wasn’t a “happy accident” that contributed to this amps’ tone.

Me too. I've never run into this configuration before.

[Raoul Duke]

Ok, got everything back together and fired her up. Everything behaving as it should, let it warm up and settle down.
Plate voltage settled right at 360vDC at 120vAC. The tubes are within a mV of each other and I biased it right at 41. Ran it, played it for about 40 minutes (carefully through a 1x12 Greenback cab) re-checked and everything is nice and stable, nice and quiet.

Voltages:
Reservoir filter - 362
Screen filter - 358
PI - 295
V2 - 255
V1 - 240

One odd thing that I think I understand: the bias pot swept from ~8 mV to ~50mV. Biased at 41 is a little past 3/4 of the way. To get it centered better I probably need to raise the 28k in series with the bias pot and maybe increase the dropper from 100k to 120k.

Sound about right?

[martin manning]

One odd thing that I think I understand: the bias pot swept from ~8 mV to ~50mV. Biased at 41 is a little past 3/4 of the way. To get it centered better I probably need to raise the 28k in series with the bias pot and maybe increase the dropper from 100k to 120k.

I don’t know exactly what the schematic looks like, but if you need more negative voltage I believe you should decrease the 100k or increase the 28k.

[Roe]

One odd thing that I think I understand: the bias pot swept from ~8 mV to ~50mV. Biased at 41 is a little past 3/4 of the way. To get it centered better I probably need to raise the 28k in series with the bias pot and maybe increase the dropper from 100k to 120k.

I don’t know exactly what the schematic looks like, but if you need more negative voltage I believe you should decrease the 100k or increase the 28k.

exactly. decreasing the 100k is a better option if the grid leak resistance is high (this is more an issue with 6550s than el34s in these amps)

[Raoul Duke]

Alright, got the bias sorted out and everything is running stable and quiet, so I gave it a whirl on my 2xG12H30 cab and it sounded pretty good. Nice, quiet, and stable even with both volumes dimed. I did opt to install the Lar-Mar MV, which works well and is easy enough to reverse if I grow to dislike it.

After playing it for about an hour, I shut it down and did some other stuff around the house for a couple of hours to rest my ears a little. Then I went back and connected the parallel 16R and common to see if I noticed a difference. Quick warm-up and reconfirm readings and bias, then back in to the same cab.

My immediate impression is that the sound seems slightly “fuller” in the lower mids. I forgot to mention that the cab is 16 ohms - which I imagine would be where the parallel winding would make the most difference. All things considered - I think I’d be inclined to leave it connected for now - as long as it’s not doing any harm - which we’ve already determined.

I’ll repeat my shake down process again tomorrow and if everything is still consistent - it’ll be time to go back in the cabinet.

[martin manning]

I did opt to install the Lar-Mar MV, which works well and is easy enough to reverse if I grow to dislike it.

Nice work getting that installed with a stable outcome. They can be prone to oscillation.

… I went back and connected the parallel 16R and common to see if I noticed a difference. …

My immediate impression is that the sound seems slightly “fuller” in the lower mids. I forgot to mention that the cab is 16 ohms - which I imagine would be where the parallel winding would make the most difference. All things considered - I think I’d be inclined to leave it connected for now …

Paralelling those two secondaries will lower the DC resistance of the secondary, which reduces damping.

[Helmholtz]

Paralelling those two secondaries will lower the DC resistance of the secondary

.

Yes, but only by 15% given the DCR numbers.
As total OT losses should be around 10% and around half of that is due to primary DCR, we're speaking of a power increase of less than 1%.

[Raoul Duke]

So given the numbers, this winding really shouldn’t really make an audible difference then?

My test yesterday wasn’t scientific by any means and my ears certainly change from day to day - and I’d rather err on the conservative side. So given factors like bass response and thermal management, - I’ll probably review the numbers and decide which way accordingly.

Unless there’s other factors I’m missing?

[Helmholtz]

So given the numbers, this winding really shouldn’t really make an audible difference then?

That's what I think.

[Raoul Duke]

Lol, confirmation bias is a thing I guess… (pointing the finger at myself)

Anyhoo, still trying to figure out what the “right” way to hook it up is then. Leaning toward disconnecting it now.

- One school of thought is since it’s a parallel winding - connect it to the like-colored wires to maximize utilization of the copper mass.

- Another school of thought is that paralleling this winding with the like colors was not the original intent of this design; so it shouldn’t be connected and really wouldn’t make and audible difference anyway.

I understand both sides and with all the examples I’ve found - it’s pretty evenly divided with both ways sounding “right”.

Maybe I’ll start a poll…

And thanks Martin - I try to really pay attention to neatness and lead dress just because I’m compulsive like that; luckily it pays off in other ways too. One of the many lessons I’ve learned while studying the expert builds here on this board!