New Marshall project - 1970 50w Lead Restoration

[sluckey]

Turn the amp on and measure the B+ at the rectifier positive terminal. If OK, then you really don't have to replace anything.

[Raoul Duke]

My decision is to punt, lol.

I ordered a KBPC5012FP and some Keystone 1563 turrets from Mouser and will re-evaluate the way forward once I have everything in front of me (to include the original block) and have thought about it more.

As for the other bits; I have a .68 mustard and NOS Ero MKT1813 .022/400v on hand to replace the obvious cap changes. I know the Ero isn’t “correct”, but I think it’s an appropriate sub for now. Heard back from Marstran a few days ago and just waiting for my invoice from Brian; and working on some resistors from our man in Spain.

Meanwhile, more cleaning and “putting back to normal”. Still thinking about trying 32/32uf on the pre…

[Raoul Duke]

So I’ve been chipping away here and there when I can and have cleaned up a fair bit of chassis, re-done the heater wires that weren’t original (with some OT off cuts that match pretty well), installed a Bulgin IEC replacement (not as “drop in” as advertised) because the brass plate that held the power cord strain relief was chewed to $4*t, added an impedance selector, filament ACT, 1R/1% bias resistors, removed the power tube sockets and cleaned and inspected them thoroughly (which was a job because the previous “tech” shrink tubes every pin with a connection), removed the front controls and lifted the board to clean underneath and re-do the filter cap connections, and got the beastly Thorardarson PT out.

I have everything I think I’ll need ordered except the PT. Hopefully that will happen some time this evening. Some other parts just showed up - so I guess I’ll open ‘em up and see what I’ll be working on tomorrow.

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[Roe]

your amp looks far cleaner than the '68 100w I am rebuilding from scratch (even getting the PT rewound)

[Raoul Duke]

Thanks Roe, I guess it could always be worse. If these things could only talk and tell us what they’ve seen, lol.
It’s still fun though right? Like solving a puzzle I guess?

I think it is.

[sluckey]

Don't forget, when you start to reconnect everything, stretch that white wire and connect it to one of the center lugs of the standby switch.

[Raoul Duke]

Thanks Steve, great reminder!

On that note, the amp came with SPST Fender type switches (polarity and power) plus a six terminal switch for standby. The 6 terminal switch didn’t work reliably, so I got two heavy duty Carling DPST for power and standby. I’ll have to review the power section hook-ups because I don’t think what I took apart was 100% correct - but I know there’s a way that will be safe and reliable - to include getting the bias flowing before the HT hits the tubes. I just haven’t figured it out yet, lol.

[Roe]

Thanks Roe, I guess it could always be worse. If these things could only talk and tell us what they’ve seen, lol.
It’s still fun though right? Like solving a puzzle I guess?

I think it is.

Yes, my amp has been thru a lot in 59 years or so. I like rebuilding amps with good iron. It is not just cheaper than buying a pristine amp but also more rewarding. and you can fix a few things (in this case the OT has wrong orientation, which gives hum, and the PT was not raised at all above the steel chassis, which then works as transformer laminate on its own).

A SPST power switch is ok with a IEC socket (I got an IEC socket that fits the old Bulgin sockets 99%). And a DPDT is good for standby. The polarity switch, however, is best used for something else

[Raoul Duke]

Thanks for the info Roe!

Did a little bit more today to include getting the buss bar off the pots and running the ground wires per the Larry scheme. The chassis had a few random holes in the right places - so I put them to use as the amp was pretty noisy when I test played it. Also did some cabinet work: clean up, checking/fixing the hole alignment (there was a gap on one side of the chassis in front), and adding some shielding.

Got my invoice from Brian at Marstran today and promptly turned it around. Hopefully it won’t be too long - but his stuff is definitely worth the wait. Got to get those voltages correct.

[Raoul Duke]

So plugging away today and I decided since everything was apart and mostly unsoldered, I’d check and measure some other stuff. Everything was going fine until I clipped test leads to the choke - at which time I got a reading of .8 ohms. Made sure everything was clear of other “stuff” and tried again - same result. Guess I need a new choke. Probably a casualty of the filter cap/power transformer incident? Who knows; but it’ll be right when I’m done with it.

It’s a bummer, but I’d rather discover it now than be troubleshooting hum after it’s back together. Looks like there’s a few from the usual makers that mirror the factory spec so I don’t anticipate it being a huge hurdle.

Onward…

[neskor]

you have one on UK ebay
https://www.ebay.co.uk/itm/236977617553

[Raoul Duke]

Thanks neskor, I appreciate the help!

A little steep on the price. I’ll have to check on the duties and fees etc. to see if it makes sense.

Based on what I’m finding I’m wondering when the last time this amp ran correctly - which makes the anticipation of lighting it up with everything put back right even more exciting.

[Raoul Duke]

Did a little on the board today. Rebuilt the bias circuit, got the .68uf and .022uf caps in, plate resistors, cleaned up some janky turret connections, V2 plate resistor in, and got some more funk up off the board. I think I’m going to go with the rectifier block since that’s how it was made. Got a KBPC5012FP - I think it’ll work fine and it looks like a modern version of the original. Ended up going with a Heyboer plexi choke with the same specs/footprint as the original. Should be here Friday. Last thing - going along with the “Fubar” streak - I tested the bias trimmer on a whim and wouldn’t you know: both extremes read ~34k with a low reading of ~9k at about 20% from the CCW end. Pulled it and re-tested; same result. Tried cleaning it; same result. Went searching for a 25k Marshall style Piher that I had on hand and apparently I must have used it on something else because I can’t find it. Now I’m trying to decide how many to order from Mouser to justify the shipping cost, lol. Oh well, no hurry - we’ll get there…

IMG_0558.jpeg

Oh, by the way: I got my resistors from Francisco faster than the time it usually takes to get parts from some CONUS vendors who are MUCH closer. He was super to deal with and fair pricing. Definitely going to grab more at some point.

[Roe]

the standby switches can be hard on the chokes. consider a 47k-150k power resistor across the standby switch

[Raoul Duke]

Thanks for the advice Roe! I was just reading up on the switching on these amps and came across a method to eliminate the “hot switching” that Marshall built into these. Let’s see if I can type it out correctly…

1. Connect the PT Center Tap directly to main chassis ground point.
2. Attach HT Leg 1 (Red) from the transformer secondary to standby “in” pole 1. Run bias feed directly off this AC leg.
3. Run HT Leg 2 (Red) to standby “in” pole 2.
4. Route Pole 1, Output of the Standby switch to one of the AC (~) terminals of the KBPC5012FP block.
5. Route Pole 2, Output of the Standby switch to the other AC (~) terminal of the KBPC5012FP block.
6. Wire the Positive (+) terminal of the rectifier directly to the input side of your HT Fuse Holder (0.5A or 1A slow-blow).
7. Wire the output side of the HT Fuse Holder directly to the positive lug of your Mains Reservoir Filter.
8. Connect the OT/CT and choke “in” to the HT fuse output

- The only hesitation I have is connecting 3 wires to that HT fuse out lug. Seems like a lot - maybe there’s a better way to bring them all together?
- Can I send the rectified DC to the cap, from the cap to the HT fuse in, then keep the OT/CT and choke on the HT fuse out lug?
- Does this help the choke getting smashed or should I still use the resistor?