'68/69 50W Tremolo project

[sonicmojo]

Hello,

Here's an interesting amp just out of years in storage. I've just got this on my bench and the owner wants to make it stock again. It is a Plexi 50W Tremolo (a.k.a 1961 4x10 Bluesbreaker) from late 68 or early 69. The tremolo socket and controls are disconnected and it has a rectifier tube instead of a can cap and is running KT66s. The voltage selector is disconnected and it is hardwired to 120V in kind of a messy way. Footswitch jack is also disconnected.

I'm making the assumption there were no '68 50W Tremolo Marshalls that came from the factory this way with the tube rectifier.....and someone along the line wanted it to be more like a JTM45, unless anyone has heard of that happening. The socket itself looks just like the power tube sockets. There is a little tagboard next to the transformer that is empty and I am guessing that is where the rectifier diodes used to be??? I haven't looked at it enough to figure out what other modifications the circuit have been done but there seems to be an assortment of different types of resistors throughout so I am guessing some more things.

If anyone can point me to some decent pics of a similar era stock 50W Tremolo or any sort of a stock type layout you know of so I can figure out the correct places to put things back, I'd appreciate it. Also, any Marshall experts that want to chime in on anything else weird you see here, please do.

Thanks!

[martin manning]

50W schematic: http://drtube.com/schematics/marshall/1987tu.gif
100W gut shot: https://reverb.com/item/2358271-vintage ... ransformer
There must be some photos of a 50W out there somewhere.

[sonicmojo]

Thanks,

I've been digging and compiling some pictures. I think the diodes come right off the transformer from what I've seen. I saw a '67 JMP labeled that had the rectifier. This one has no label but who knows if the front faceplate is original or not. It looks slightly cleaner than the back one which is off since the back would have been more protected in the cab, although maybe exposed to more heat....

I saw a '67 that had a tube rectifier. I'm sort of wondering why this transformer even has the 5V tap. Pictures of heads are easier to find and I think they are pretty much the same.

This one has the single front weave style cabinet that would be right for 68/69.

I'll have to flip it and date the transformers if I can....

Edit: no luck on transformers, just have the standard codes for type. Rectifier socket is white ceramic like the power tubes but just slightly different...sticking to my theory this is a JTM45 mod.

[Reeltarded]

I would like to see both plates and if there is a check sheet on the other side of the chassis.

That amp has been totally reassembled.. if not built from bits. In the 67 transition from tube to silicon rectifiers they can easily go over into 68. I sold a similar head about 6 years ago. Too late but still tube.

Let me go get a couple gut shots of what you are looking for.

[Reeltarded]

You can see here how rare the tube would be and information on all general features. Weird that I don't see any in the string of /T models with a tube rec because out of the many I have seen there were a few.

http://marstran.com/Historic%20Data%20Base.htm

Except for the trem, this is what you should expect.

http://marstran.com/67%2010054.htm

[sonicmojo]

Sorry, no check sheet on chassis. Thanks for the feedback. It looks to me as if almost every solder joint has been touched (no red paint) so I agree with your assessment on the rebuild idea. I haven't seen any other chassis pics with all the shrink wrap on the components like in this one.

Here are more pics.

[sonicmojo]

Couple more pics of sockets for power and rectifier tubes.

The good news is that the amp works pretty well, at least on my bench, no extra hum or anything! The volume pots were locked up pretty good but I got them moving again.

[Roe]

Definitely looks like a 1967 el34 amp. some had the 784-139 output transformer and the gz34 rectifier. see: http://www.marstran.com/Historic%20Data%20Base.htm

[Reeltarded]

Roe is right again. Looks like it's a 67. That is the one to have.

[Roe]

Basically a jtm50. A great amp. Some used a ss rectifier

[sonicmojo]

The goal is to make this a fully working amp, hopefully mostly stock parts but not go too crazy. Some of the mustards under the tubing have been spliced in, maybe in an earlier attempt to bring this back to stock. I undid the wrapped up wires and they are for the tremolo socket so most everything is still there but I need to figure out how to hook it back up correctly and I'll need to add or move and replace a capacitor or two in that section. Ditch the orange drops for sure. This is the fuzzy schematic I am going by but I can't find any gut shots that help me out too much. Looks like there should be a wire that runs all the way to the wiper of the volume pot. I can't make out the value of the cap between the 10K and the wiper. I'm thinking this does some sort of grounding oscillation to create the tremolo effect. Is there a cleaner schematic for this that anyone has?

[Reeltarded]

The goal is to make this a fully working amp, hopefully mostly stock parts but not go too crazy. Some of the mustards under the tubing have been spliced in, maybe in an earlier attempt to bring this back to stock. I undid the wrapped up wires and they are for the tremolo socket so most everything is still there but I need to figure out how to hook it back up correctly and I'll need to add or move and replace a capacitor or two in that section. Ditch the orange drops for sure. This is the fuzzy schematic I am going by but I can't find any gut shots that help me out too much. Looks like there should be a wire that runs all the way to the wiper of the volume pot. I can't make out the value of the cap between the 10K and the wiper. I'm thinking this does some sort of grounding oscillation to create the tremolo effect. Is there a cleaner schematic for this that anyone has?

Hey, buddy!

Here is the Metro replacement board layout. This should work for you!

[martin manning]

I can't make out the value of the cap between the 10K and the wiper. I'm thinking this does some sort of grounding oscillation to create the tremolo effect. Is there a cleaner schematic for this that anyone has?

I'd guess it's 0.1 like the one connected to the transistor base terminal. Yes, the trem works by grounding the signal. Here's some info on that part of the circuit where someone else came to the same conclusion: http://el34world.com/Forum/index.php?topic=7261.0

[Reeltarded]

Heya, Martin!

There was a 30 page discussion on making the trem run best that I was involved in probably 8 years ago. In the end we all agreed the best thing to do was never use it lol

[martin manning]

Ha! Yes, lots of energy spent on trem circuits over the years.