Rebuilt Transformers

[Reeltarded]

I have a 100w Super Lead that was run on a movie set without a load. It made me so angry I left it for a decade.

I have not tested the amp except for noticing no pilot light and no heater light. I can't remember testing the rectifier and never the OT. Just the PT sent me into fits. The quad of tubes amazingly survived and still kick (lots of) ass.

I have never had transformers rebuilt. What are voltage and capacity contraints if rebuilt on original cores? It's plac says 115v as any 71/72 US export model.

I would like for the PT to be 380-400v compared to the 465ish it used to be. Needs primary and secondary adjustments just not sure what is possible.

Thanks, peeps!

[Roe]

ask Brian at MArstran?

Running w/o a load may blow the output transformer, but no pilot light and no heater light indicates blown mains fuse or blown mains transformer. Check first to see what the issue is

[Stevem]

Having it on with no load and no signal going in , or even with signal going in with the volume(s) down will not do damage to a OT even if left on for weeks because the speaker jacks are shorted out when no cable is plugged into the amp and that's perfectly fine.

The only catch here is if the amp has a cable plugged into it that is not plugged into a cabinet on the other end , or it is plugged into a cabinet but the circuit is open due to blown speaker(s) .

Atleast this has been my experience, I mean if your dealing with something that runs a output plate voltage of over 600 then things might be different.

Judging by what you have describes I think you just have a case of a blown mains fuse due to a crapped out/shorted output which is very very common!

[Roe]

Having it on with no load and no signal going in , or even with signal going in with the volume(s) down will not do damage to a OT even if left on for weeks because the speaker jacks are shorted out when no cable is plugged into the amp and that's perfectly fine.

not the case with old marshalls unfortunately

[bepone]

I have a 100w Super Lead that was run on a movie set without a load. It made me so angry I left it for a decade.

I have not tested the amp except for noticing no pilot light and no heater light. I can't remember testing the rectifier and never the OT. Just the PT sent me into fits. The quad of tubes amazingly survived and still kick (lots of) ass.

I have never had transformers rebuilt. What are voltage and capacity contraints if rebuilt on original cores? It's plac says 115v as any 71/72 US export model.

I would like for the PT to be 380-400v compared to the 465ish it used to be. Needs primary and secondary adjustments just not sure what is possible.

Thanks, peeps!

Usually good transformers survives open loop if there is no signal on the amp.. damage can be only with all cranked and no load
this makes few kiloVolts in transformer and insulation can blow making shorts between the layers or secondary.
Possible to suspect - remove wires anode from socket and Ub+, and measure OT resistance
Anode 1-centre tap/Ub+,
centre tap /Ub+ to Anode2, and see if are ok

then Anode - secondaries, how much in Megaohms?

Which can indicate shorts in OT

[Reeltarded]

Thank guys.. THIS IS NOT A FUSE ISSUE!

I WISH

[Stevem]

Here's a easy way to tell if the OT is toast .

Hook the amp up to a speaker or a load resistor with a voltmeter set for ac across it.

Turn the lights down in the room and play thru the amp with it turned up to like 4.

Look at the rear of the amp and into the center of the output tubes thru the plate holes .

If you see the inside of the tubes flash up brighter when you play a note yet only see millvolts on the meter, then yes it's new OT time.

[Reeltarded]

The amp doesn't do anything but look like a mint British anchor.

It was rented to a dumbshit movie by a friend and I told him to cut the wire off and pull the fuses.

He didn't. They ran it for the pilot light... no load. I get the amp back 6 months later.

Anchor

[Stevem]

So a decade later your still calling this uncaring A hole a friend?

[katopan]

Back to your original question, any transformer winder will be able to crunch numbers on the coil design to fit the existing cores and target the new loaded output voltage you are after. Only thing that gets negotiable is if they have the tools to and are willing to replicate the original construction or if they want to use a modern plastic bobbin.

Original old iron is generally better than new, so definitely use the old core.

[Reeltarded]

So a decade later your still calling this uncaring A hole a friend?

He had a terrible thing happen and I gave up on angry.

[Reeltarded]

Back to your original question, any transformer winder will be able to crunch numbers on the coil design to fit the existing cores and target the new loaded output voltage you are after. Only thing that gets negotiable is if they have the tools to and are willing to replicate the original construction or if they want to use a modern plastic bobbin.

Original old iron is generally better than new, so definitely use the old core.

Excellent! And uhoh.. this is going to cost a tiny fortune. At least I will be able to finish thinking about it.

Thank you!

[bepone]

rewinding olds transformer is 3 times more job than winding from 0..
1. baking to be able to remove laminations
2. try to remove first, probably destroy few lams
3. removing, if is not going, baking again, and try
4. removing old copper (if rewinding, again a lot of time to count the turns and remaking the scheme)
5. clean bobbin
6. winding the transfomer
7. connecting the terminal wires, closing
8. soaking and baking

steps 1-5 are additional P.I.T.A. jobs, and time consuming, so rewinding cost much more..

[pdf64]

Maybe @wpaulvogel would be interested in taking this on?

[R.G.]

I have a 100w Super Lead that was run on a movie set without a load. It made me so angry I left it for a decade.

I have not tested the amp except for noticing no pilot light and no heater light. I can't remember testing the rectifier and never the OT. Just the PT sent me into fits. The quad of tubes amazingly survived and still kick (lots of) ass.

I have never had transformers rebuilt. What are voltage and capacity contraints if rebuilt on original cores? It's plac says 115v as any 71/72 US export model.

I would like for the PT to be 380-400v compared to the 465ish it used to be. Needs primary and secondary adjustments just not sure what is possible.

Thanks, peeps!

That would be frustrating all right. I'll skip the testing advice - you've been around here longer than I have and know the drill: test the PT voltages under no load, apply the internal-shorts test; test the OT with an applied secondary voltage and the internal shorts test, then start working through the rectifiers, caps, etc.

As to what is possible on the transformers, then. The PT can be successfully rewound to any lower total power output. You're wanting to go down in power, so this works.

The primary could be replicated, wire gauge and turns. It was probably designed for 115Vac as you say. Today's AC line is a bit higher, often 120Vac to 125Vac, even higher. That fact argues for more primary turns, in the ratio of 125/115 = 1.087, or another 8-9 turns per hundred on the primary. It will be a major pain to recalculate the wire gauge, turns, turns per layer, interlayer insulation, etc. if you haven't done this before. I could help with that if you decide to go in yourself. An experienced transformer rewinder will blast right through it. The extra turns are not crucial, because that model transformer does survive in today's higher voltage world. So it's something to think about.

With the primary turns, wire gauge, layers, etc. decided, the secondary windings are pretty trivial. I have to side step a bit here and come back. In designing a trannie, you design for enough core-cross-section in the iron and enough winding window to hold the necessary turns. The core-area times the window area is a key specification number for choosing laminations in the transformer biz. Rewinding, the iron area and window area are pre-chosen, so this is easier. Designers usually allocate the space in the winding windows according to the power handled by that winding. The primary starts at half the window area because it carries 100% of the power. The secondaries get the other half. Multiple secondaries start with window areas proportional to the power they put out. After that, the frenzy of allocating wire gauge, turns, turns per layer, interlayer insulation, and "margins", making layers shorter than the actual window width to allow for what is possible and economical to wind. All of this is why transformer design gets complicated.

Back from the side trip: going with the same transformer core and less power out of it means that the secondaries will have slightly less turns, or alternatively they can be more sloppily wound and still fit.
So it's do-able. I recommend getting a winding shop to do it for you, not doing it yourself. I've been designing transformers since 1973, wound quite a number of them, and I would hesitate to wind a 120V power transformer myself. It's a lot of work.