New Marshall project - 1970 50w Lead Restoration

[Raoul Duke]

So this showed up today. A lot of originality intact for what I gave for it. The power trans is a Thordarson T22R34, the rest of the iron is correct. It’s had some poor work done to it, but I’ll correct it. Quick test confirmed that it makes sound and controls work. Didn’t want to push it as I see some weird stuff going on in there; for instance: I don’t think the PT is wired correctly and I think this has the bias voltage on the standby factory fault. Time to start making my list…

IMG_0554.jpeg

Hoping for constructive feedback from the Marshall experts here if they are willing. Thanks in advance guys!

[Raoul Duke]

Found some info on that power transformer. Seems it’s rated 385-0-385; which I think is too high for what this design needs (385x1.4=539). Probably been in there awhile I’d guess - which might explain the faint odor of a detonated filter cap and the petrified electrolyte stuck to the chassis between the two cans at the rear.

With this PT probably designed for 115-117v, plus modern wall voltage in my ‘hood usually averaging 123vAC, I’m starting to think a different PT might be in order.

Hmmm…

[Roe]

Found some info on that power transformer. Seems it’s rated 385-0-385; which I think is too high for what this design needs (385x1.4=539). Probably been in there awhile I’d guess - which might explain the faint odor of a detonated filter cap and the petrified electrolyte stuck to the chassis between the two cans at the rear.

With this PT probably designed for 115-117v, plus modern wall voltage in my ‘hood usually averaging 123vAC, I’m starting to think a different PT might be in order.

Hmmm…

yes lower voltages are necessary with the 3k4 Z of the output transformer. otherwise, el34s will redplate, and the sound wont' be right either

[neskor]

The Blue & Brown heater wires start to show up in 1970 around #8000A and continue right up to January 1st, 1971. The reason for the color switch appears to be a vendor shortage of Black & Red wire.
The early 50 watters had a flaw in the way the power was sent to the bias section. The way they were wired originally, the bias circuit only gets voltage when out of standby mode. This means the B+ and bias circuit get voltage at the same time. The bias circuit needs a sec or two to build up and make the tubes idle correctly, and as you see this also causes a current surge that helps kill these earlier 50 watt rectifiers and PT's
I can see some non original parts:
100K Plate resistor of V1 have 150pf across, not original
e-cap on 2K7 cathode resistor changed
coupling cap on V1 changed
100K plate resistor on V2 changed to metal film
tropical fish cap across 1k cathode resistor
two 10K dropping resistor shorted with jumper wire
added master volume pot on the back
bias caps changed
two snubber caps changed
probably pilot lamp, power and standby switch changed
screen grid resistor added
PT changed, it should be Drake 1202-164
Drake 1202-164 is a standup version of the Drake 1202-118.
The Drake 1202-164's in '69 tend to run the same rectified voltage (420-435Vdc) as the Drake 1202-118, but once you get to 1971 they vary quite a bit.
Some run as high as 450Vdc and as low as 380Vdc.

[Roe]

later in the 70s voltages were lowered to ca 380v

[Raoul Duke]

Thank you both, I appreciate the info and background! The serial on this is 96xxA and the tag has its final inspection on December 23, 1970. Definitely some odd choices made with component replacement but that’s easy enough to fix. Did you catch that the reservoir cap only has one side wired? Odd…

From my quick search yesterday I find that the lowest secondary off the shelf is the Hammond 290GX at 345-0-345. Everything else I could find quickly is 360. I’ll likely do some more searching for something period correct - but second hand seems like the planets have to align to get something in decent shape without paying an arm and a leg. I may try Marstran as well; they were great to deal with last year when I built my JTM45. Their 164 is lower voltage but may be toward the 380 B+ side IIRC.

I plan to go through it thoroughly today and have my parts list fairly complete - then time to search. Speaking of: anyone know if the switches should be black plastic flat bats or standard shiny chrome? The pictures I’ve dug up so far seem to favor the US export models showing chrome, but they could also be replacements.

Thanks again folks!

[Roe]

you need a drake 1202-164 replica, which Martran offers

[neskor]

for EU version switch should be with a plastic bat, for US model probably with metal bat

[Raoul Duke]

Thanks again guys!

Message sent off to Brian, evaluation continues.

Any opinions on the polarity switch hole? I was thinking remove and plug since it serves no purpose unless there’s a better use for it? I’m not blindly stuck to original for original sake - open to tweaks that make sense. Such as:

* lower value cathode bypass caps on V1
* replacing monolithic rectifier with diode string (looks like it may have been that way at one time)
* maybe 32/32 on the preamp filters
* remove the snubbers on the HT
* probably re-do the grounding scheme - I’ve had great results with the Larry grounding and this isn’t a museum piece

Looking for the best representation of this early metal panel amp I guess. This part is almost as much fun as hearing it the first time after it’s fixed up, lol.

[Roe]

Thanks again guys!

Message sent off to Brian, evaluation continues.

Any opinions on the polarity switch hole? I was thinking remove and plug since it serves no purpose unless there’s a better use for it? I’m not blindly stuck to original for original sake - open to tweaks that make sense. Such as:

* lower value cathode bypass caps on V1
* replacing monolithic rectifier with diode string (looks like it may have been that way at one time)
* maybe 32/32 on the preamp filters
* remove the snubbers on the HT
* probably re-do the grounding scheme - I’ve had great results with the Larry grounding and this isn’t a museum piece

Looking for the best representation of this early metal panel amp I guess. This part is almost as much fun as hearing it the first time after it’s fixed up, lol.

.68uf is fine but 220+uf is overkill

[Raoul Duke]

Understood, thanks Roe!

[Roe]

Understood, thanks Roe!

the only advantage to 220+uf is that it may reduce hum on v1 (even though this depends on the tube). On the normal channel, I prefer 1k8-100k-3n3 and even a small bright cap on the volume control, e.g. 100pf

[Fischerman]

Cool amp! I would also change out the cc plate resistors on v1.

IMO, higher voltages than 380-390v sound more punchy and better clarity. Just my opinion tho.

[Roe]

Cool amp! I would also change out the cc plate resistors on v1.

IMO, higher voltages than 380-390v sound more punchy and better clarity. Just my opinion tho.

+1. around 420v tends to be ideal for the classic 1987 sound. But you can easily adjust preamp voltages in any case. Also, screen resistors may be adjusted somewhat.

[Raoul Duke]

All very helpful info guys, I do appreciate it!

Roe, when you say 1k8-100k-3n3 - I’m interpreting that as 1k8/3n3 as the cathode bypass and 100k as the plate load? You gave me some values for V1 when I built my JTM that really made the difference in its sound, so I just want to get it right. I think I have a few 1W/100k Iskras here somewhere that I’ll likely replace those carbon comps with.

Also, is it possible that the rectifier is original? I’m finding pictures that show both the monolithic and the diode string. Which is preferable for reliability and longevity?

Thank you!