1973 50 Watt Lead - Japan??

[Deric]

Marshall Gurus-

Just got this Marshall to service, clean-up, possible re-cap. 1973 50Watt Lead with Japan written on the label. Serial # S/A4794E . Sounds quite good as is - lots of hiss when cranked but that's pretty normal from my experience.

Looks like a few mods were done and reversed. There are several solder blobs scattered around the chassis that look like they were once used as grounds. Also an extra jack in the back that has been disconnected but left in place.

Anyways...I really don't know my Marshall history. Can anyone point me to the correct schematic for this amp??

Pics attached. Any info appreciated.

[Deric]

More...

[Smokebreak]

I imagine the '71 1987 one will be the closet to your amp
http://www.drtube.com/schematics/marshall/1987tu.gif
Of interest are the 82K Output gid leaks and the .68 presence cap. I imagine the PT was changed at some point?

[Stevem]

It's time to recap it and do the owner a solid and replace the PI plate resistors with 1 watters as they need to be to handle over 350 volts being applied to them!

[ToneMerc]

It's time to recap it and do the owner a solid and replace the PI plate resistors with 1 watters as they need to be to handle over 350 volts being applied to them!

No need to change those vintage correct 1/2 watt Iskra and Piher resistors, as they already spec for 350v. Nevertheless, 350v is not dropped across them anyway so why do they need to changed?


TM

[Cameron]

This amp looks like it may have had a Jose mod at some point in its life..

[Deric]

Thanks for the info guys. Pretty much the 1987 schematic with only a couple of variations. V1a cathode is 220u/820r. V1b is .68u/820r - the 820r has been changed. V2a plate resistor looks stock but has drifted WAY high to ~1M cathode is 1K with no cap. The presence is stock and I guess not uncommon. The 82K grid leaks and the PT seem to be stock.

Plan to recap, replace the V1b cathode resistor and V2a plate resistor. Should I change the grid leaks to the more common 220K? The owner plans to only run EL34s. Also, I saw a lot of recommendations to add 5k6 grid resistors on the output tubes. Good idea? Will also clean up the power/standby and AC cord wiring.

Any other suggestions?

[Smokebreak]

I would think you'd need to get that V2a plate resistor off the socket to get an accurate reading .
Nothing wrong with the 82K leaks. See what sounds best in that amp. Sometimes it's real hard to even tell. Grid stoppers can't hurt. Maybe try the more common .1 presence.
What voltages do you have?

[Deric]

Haven't measured voltage yet. Ordered new caps for now. Will measure before and after the cap swap.

Curious why the plate resistor would need to be off the socket. With tube pulled and meter clipped directly to each end of the resistor how could it measure high?

[pdf64]

Yes that should measure accurately.

[Smokebreak]

I figured something else maybe was going on for it to read that high. Caps not fully discharged or something. That's quite a drift!

[Bob S]

Thanks for posting Deric - nice pics.
That's a labor of love.
I bet she screams when you're done with her.

[Deric]

The high plate resistor is weird. I've never seen a resistor drift that high though I suppose it's possible. Even though I had test leads directly on the resistor I'm still curious to see if there will be any change when I get it out.

Should be a pretty easy/quick job once I have the parts. Going to clean up the power cord and power switch today. Also get rid of all the solder blobs scattered around the chassis. A few of them fell right when I poked at them. Then I'll do a voltage check as is.

Any reason to keep the carbon comp 68k resistors on the inputs as opposed to swapping them out for metal film?

[ToneMerc]

The high plate resistor is weird. I've never seen a resistor drift that high though I suppose it's possible.

Deric, it would't happen to be in the range of 300k would it?

TM

[Deric]

Nope, definitely 1M+. Just checked again and measures 1.15M.

Why?

Thanks