New Marshall Project (‘77 1987)

[Raoul Duke]

Got a local offer for my Boogie MkIII in a trade for a 1977 Super Lead 50w. Sounded pretty good (through my Power Station - freakin’ loud - the 100w must be like an LRAD) - so I agreed. Played it a bunch yesterday and today and just pulled it out of the box to check it out. Seeing a few odd things, but no mods and the ST1 board doesn’t look hacked up - so that’s good. Output TX might be replaced - but it has the remains of the tag (no date stamp) and it looks correct right down to the mounting holes and wire colors; PT and choke are dated “8 77”. Checked the fuses and the mains was correct (2A), but the HT had a 5A which I immediately replaced with 500mV. Seems to be a Brit model based on the QC tag (has EL34s).

Hooked up a load and fired it up on the bench and got an average plate voltage of 378 with the tubes running at 39 and 30 mA. Then I noticed it was set to 110v on the selector. Shut it down, removed the power tubes, and moved it to 120v. Here’s the comparison:

110v: heaters run at 6.7, plates steady at 407 unloaded
120v: heaters run at 6.2, plates steady at 373 unloaded

Tried 120 with tubes expecting to have to adjust the bias, but V5 started running away as I was taking measurements - so I shut everything down. Going to 120 seemed to drive the tubes further apart though; the plate voltages fluctuated between 345 and 370 and one tube (V5) was running at ~50mA and the other at ~32mA and both were fluctuating (V5 more). The tubes are older GT EL34-M and they look a little roasty - so I expect a new set is in order here.

My plan is to give this amp a good servicing and cleaning like I did with my 2204 and set it up correctly for 120v operation. I understand Marshall made some low B+ PTs and I’m fine with that. My guess is the previous owner had it set up to get higher plate voltage for some perceived performance increase (or didn’t know it was previously done).

Here’s a couple of pics:

IMG_0304.jpeg

IMG_0305.jpeg

Should be a good project. Really liked how it sounded and is certainly a different thing than the 2204. More to follow…

[Roe]

it is a 1987, not a superlead/1959. low voltages are normal

[Raoul Duke]

Gotcha, thanks Roe! I appreciate your input as I’m still learning the subtleties of all things Marshall.

Seems to be a lot of nuance to Marshall’s production during the ‘70s. Variations in component brand, actual build values compared to factory schematics, things like this transformer variation etc. I think it makes it more interesting.

Finished my once-over and found a few things of note:

- V4 tube socket replaced with a top mount socket that looks way older than the amp - and is missing pin 6
- no screen grid resistors on V4/5
- no grid stoppers on V4/5
- had the cap cans replaced with LCRs (that are 30 years old); the main reservoir was dropped to a dual 32uf
- a few top-soldered board components all of schematic values (some are Pihers) - maybe drifted values, maybe put back to stock?
- all resistor values seem to be in tolerance - really a testament to the quality of the old Pihers (and one Iskra)
- also figured out that the correct mains fuse for 120v seems to be 3A, so I’ll do that first
- bias caps and V1B bypass caps all look original

Guess I’ll make some notes and a list…

[Roe]

add grid stoppers (5k6-47k) and screen resistors (1k is standard but you could try a tad lower with the low voltages perhaps)

[Raoul Duke]

Thanks again Roe - will do.

While tracing out the wires and where they go before I start disassembling - I see that the OT has two commons (orange wire) coming from the secondary side. Is this typical and can it be used to identify the transformer conclusively? It looks like a 783-139 and has the same dimensions and wire colors to the appropriate hook-ups - everything matches except the dual orange common. All the 783-139 wiring diagrams I can find only show one orange/common on the secondary.

Any advice is certainly appreciated!

Thanks!

[martin manning]

See if that extra wire has continuity with the other secondary leads. Here is a 1987 50W scheme that shows gray (16R), green (8R), yellow (4R), orange (comm), and purple ( NFB).

[Raoul Duke]

Let me see what I can find out…

I see both emerging from the same bell end on the OT and attached to the speaker jack in the same (sleeve) spot. Also a double grey going to the 16R tap - same as the orange - one thicker, one thinner. Odd.

I’ll check both and report back. Thanks Martin!

[Raoul Duke]

All right - so this is interesting.

The dual orange and dual grey consist of one solid core and one stranded core each. When out of circuit, continuity is present between the green, yellow, and solid grey and orange - but not the stranded grey or orange - which have continuity to each other.

Resistance readings from solid orange to each are:
Yellow - .7R
Green - .9R
Solid Gray - 1.1R

Stranded gray to stranded orange: 6.3R

This is new to me; but I’m sure there’s a lesson for me here, lol.

[martin manning]

The dual orange and dual grey consist of one solid core and one stranded core each. When out of circuit, continuity is present between the green, yellow, and solid grey and orange - but not the stranded grey or orange - which have continuity to each other.

Resistance readings from solid orange to each are:
Yellow - .7R
Green - .9R
Solid Gray - 1.1R

Those are likely the 4, 8, and 16R taps.

Stranded gray to stranded orange: 6.3R

This is a separate winding, and the high resistance suggests it might be for a 100V PA speaker.

[Raoul Duke]

Yes - regarding the colors/taps - that’s how it was wired to the switch - which agrees with the 784-139 wiring diagrams.

So keep the like colors together or cap off the stranded orange and grey? My gut tells me the high resistance wouldn’t be a good idea in circuit; but that’s how it was wired.

More evidence that the OT isn’t original I guess.

[martin manning]

Are the stranded wires connected to anything? If not, certainly cap them off.

[Raoul Duke]

They were doubled up with their like-colored counterparts on the impedance switch.

[martin manning]

They were doubled up with their like-colored counterparts on the impedance switch.

I think I would try to confirm impedance ratios with it connected the way it is. Some OT's require external connections to get different impedance ratios.

[Raoul Duke]

Based on all the Hiwatt research I’ve been doing of late - I’m wondering if this is an OT out of a mid/late 80’s Custom 50? Based on what I’ve discovered - those amps used Drakes that were similar to the Marshall winds and still retained the Hiwatt 100v line out?

Anyhoo, what’s the best way to confirm as you suggest Martin? I’ve tried Google AI but it comes back with the primary test. I might not be asking it correctly so I’ll keep trying - just figured I’d ask you as well. I always appreciate the guidance and subsequent lessons that accompany them.

I know this is probably “run of the mill” for you guys that know this stuff, but I really enjoy “solving these mysteries”, lol. Love this stuff…

[martin manning]

...what’s the best way to confirm as you suggest Martin?

See: https://ampgarage.com/forum/viewtopic.p ... 18#p211118