[Question] Marshall Major 200 Watts

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sonicmojo
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Re: [Question] Marshall Major 200 Watts

Post by sonicmojo »

Krinkle wrote: Mon Apr 10, 2017 3:54 pm I agree. I was pretty happy when Chris Merren guaranteed me that my Major OT would NOT fail. I didn't ask why but I believe him, he said he knew what the issue was and his reputation is impeccable. Being able to crank pedals through it without worrying is great.
Does anyone have any thoughts on what measures could be taken on these to prevent any OT failure? My buddy has three of these and I'd like to help him get them going again after many years (carefully).
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Krinkle
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Re: [Question] Marshall Major 200 Watts

Post by Krinkle »

sonicmojo wrote: Thu Apr 27, 2017 4:40 pm
Krinkle wrote: Mon Apr 10, 2017 3:54 pm I agree. I was pretty happy when Chris Merren guaranteed me that my Major OT would NOT fail. I didn't ask why but I believe him, he said he knew what the issue was and his reputation is impeccable. Being able to crank pedals through it without worrying is great.
Does anyone have any thoughts on what measures could be taken on these to prevent any OT failure? My buddy has three of these and I'd like to help him get them going again after many years (carefully).
I didn't push Chris as to what he did, but I believe it was done during his manufacturing process so my guess is you probably can't do it after the OT is built. I've heard of people adding insulating materials between the primary and secondary, but when I mentioned this to Chris before he built it, he pretty much said that he was doing something else. Sorry if that's not helpful but at least you know what I know.
Krinkle
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Re: [Question] Marshall Major 200 Watts

Post by Krinkle »

Maybe email Chris and just flat out ask him, what do you have to lose?
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sonicmojo
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Re: [Question] Marshall Major 200 Watts

Post by sonicmojo »

Good idea. I found his website. Thanks.
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Re: [Question] Marshall Major 200 Watts

Post by Krinkle »

Good luck, let us know if you figure something out.
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Re: [Question] Marshall Major 200 Watts

Post by sonicmojo »

My own question got me to researching ultra linear design a bit although I'm no engineer. Interesting stuff with the screen taps on the primaries and what I've read is that KT88s are a good tube for that design. The original OT was likely wound specifically for them, may not have had the best insulation between primary and secondaries in the manufacturing process and it may explain why there is a big red plate on some Majors warning not to use 6550s (although I know people do). I wouldn't want to be the one who experiments with circuit tweaks on an original amp and then loads up the front end with boosts and overdrives to see what happens. It's probably best to accept the amp for the loud clean machine that it is! Unlikely I'll be turning it to 10 anyhow.

It got me to thinking that it would be interesting to try an ultra linear power section with a SSS!
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Re: [Question] Marshall Major 200 Watts

Post by tweedeluxe »

I've built and rebuilt a few majors, with original/rewound transformers, as well as new transformers from heyboer. On the first Major, I couldn't decide what to do after hearing all the horror stories. Should I run regulated screens, etc. I wanted to hear what a cranked stock major sounded like with the 250ohm sg2 stoppers, so after doing a lot of research, I decided to run the power amp stock but with low capacitance 1kv MOVs straddling each OT leg, with ~1k resistors in series (a bit less than half the primary Z). This was a 69 with the original OT. I CRANKED it for an extended period of time with no issues. Have done a few since in the last 7 years or so and none of them have ever come back.

The HT fuse is situated right between the 2 sets of power tubes, so its pretty convenient to wire in the MOVs. I attached a phtoto of how I did it in a complete rebuild on a '67 that someone had hacked to pieces.
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M Fowler
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Re: [Question] Marshall Major 200 Watts

Post by M Fowler »

Maybe I need to reread this thread, but what are the PT and OT specs needed for a Pig amp?

Heyboer made custom trannies for this build?

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Roe
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Re: [Question] Marshall Major 200 Watts

Post by Roe »

the tranformers differ from the major's trannies. The pig uses partridges, more similar to a hiwatt 200, whereas the major uses drakes. The former has lower screen voltages, the latter higher, with an ultra-linear Ot
Last edited by Roe on Tue Sep 12, 2017 7:20 pm, edited 1 time in total.
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bluesbreaker62
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Re: [Question] Marshall Major 200 Watts

Post by bluesbreaker62 »

To further complicate things, not all pig were created equally and most surviving specimens were modded at some point.
I’ve seen them with both C and EI core transformers (and combinations thereof).

Best information I could find on the transformers is this bit, shared by Barry of Ampmaker I believe:

“The power circuit is quite traditional (NOT an ultralinear type). HT volts is 750VDC ( ouch ). Screen grids are 550vDC from four grid resistors. HT is generated from center tapped transformer through three diodes in series from each tap. Smoothing caps are four 33uf 450V wired as two parallel pairs in series. Screen grid is from a seperate supply with two smoothing caps in series. The transformers are made using grain oriented continuously wound strip C cores. These cores I believe was made by a company called Seddon and Bramall in Sheffield UK. Who wound the transformers Ime not sure. The output transformer primary was 550 0 550 turns of 25swg (british wire guage). These were split into smallergroups and interspaced to achieve a better balance. Output winding was 150 turns for 15 ohm and 75 turns for 8 ohms. Guage Ime not sure I guess 20SWG. On full output into 15 ohms I once measured 235V AC . Hope this is of interest. I will post full circuit when done as it appears ime the only one whos got it. It had one problem the output NFB was fed into the ECC82 phse splitter and there was some crossover distortion. I notice later versions take the feedback to an earlier stage to have more effect.”

For further reference, a good place to start would probably be here…
http://www.marshallforum.com/threads/sc ... pig.40968/
And here…
http://www.marshallforum.com/threads/sl ... one.78770/

Too bad most of the pictures went down after photobucket chanced the 3rd-party sharing policy.
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Re: [Question] Marshall Major 200 Watts

Post by Stevem »

I recall reading some where about the need for diodes in the screen taps of a UL OT !
I will see if I can dig it up tonight.
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martin manning
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Re: [Question] Marshall Major 200 Watts

Post by martin manning »

Given the turns in the OT primary and secondary for the Pig (above), you can get primary impedance: (1100/75)^2 * 8 = 1721. That is way too low for 4x KT88 at 750V Va. I would make it at least twice that, better yet 4k, and run the screens at 275V.
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