Major Marshall & mods

[10thTx]

I had some trouble discerning the original Marshall Major 1967 schematic. It was very illegible in places and the phase invertor was drawn incorrectly from what I can tell?

Can anyone verify if this redrawing of the original schematic is correct or needs edits/corrections?

I included a SCH editable version if someone needs one or wants to use this for edits. There is another SCH version drawn by Pete (on another forum) with some slight modifications to lower the "ice-picky" tone on one channel.

With respect, 10thtx

[10thTx]

Delete

[Reeltarded]

Has to have some kind of master. An unattenuated one-wire would take the amp apart.

Not sure on schematics. Is the original one from Dr Tube? I have 68 and 70 schemes around somewhere.

Please tell me you aren't building one. heh

From memory, everything looks good to me. All of my Majors have 560p or 556p treble caps. (Err used to before I started robbing them for 50s)

[10thTx]

Thanks for the response! The original schematic was from the Hoffman EL34 World site:

http://el34world.com/charts/Schematics/ ... d_200w.pdf

No, I am not building one. Trying to preserve what little hearing I have left. I just liked the tone in the YouTube so well, that I was curious what kind of amp was creating it.

With respect, 10thtx

[Stevem]

A big part of the tone difference in the Majors is the fact that they do not drive the tone stack from a Cathode follower!
You can take a 50 watt metal face and rewire it onto the Major tone stack without too much doing if you want something not quite as loud!

One mod you should do while its open is to buss all the output tubes Cathodes down and run it into a inline fuse as the HT fuse is too slow to respond to a shorted output tube and then the OT goes south!
l
Let me tell ya I have done 3 of these amps in the last 5 years and Mercury gets near 600 Bucks to rewind a OT and even a off the shelf new one is 400 dollars, so the fuse is piss cheap insurance!

God how I hate going thru the ordeal of bolting those chassis back in the cabinet!!

[Dr-Joned]

The bias is a little funky looking. Should it have a ctr tapped ground ? Is it a separate winding?

[10thTx]

The bias is a little funky looking. Should it have a ctr tapped ground ? Is it a separate winding?

Yes, you are correct. It should have a center tap. That is a good catch. I will redo the schematic and hopefuly repost it later today or tomorrow.

One mod you should do while its open is to buss all the output tubes Cathodes down and run it into a inline fuse as the HT fuse is too slow to respond to a shorted output tube and then the OT goes south!

Thanks for that info!

Thanks, 10thtx

[10thTx]

OK, here is my proposal ........................ DRAFT IDEA. I have NOT built this.

OWM & PPIMV & cathode biased with 375 v on the plates. Based on my own Tweed BluezMeister B+ rail, I think I have made a reasonable guess on the dropping resistors for this idea. Should get pretty close to the right Major preamp plate volts?

I am understanding this to be 30 watts based on page 3 of this data sheet:
http://www.mif.pg.gda.pl/homepages/fran ... k/KT88.pdf

Edcor has a 275-0-275 PT with 325ma that would work just fine and is reasonably priced:
https://www.edcorusa.com/xpwr004

With the 250uf (individual) cathode caps, I think it would emulate a reasonable fixed biased tone?

IF this works out to produce 30 watts of KT88 tone, then it may (at some level) honor the Major KT88 tone at a much more useable and realistic volume level.

This is only a DRAFT IDEA. With the changes mentioned it is NOT a true Marshall Major but simply a 30watt idea that may be a great amp that pays some respect to the KT88 tone.

With respect, 10thtx

[martin manning]

I guess this is your OT, then?
https://www.edcorusa.com/cxpp30-ms-5k

[10thTx]

Thanks, I think that OT could be a good choice! I appreciate the info.

With respect, 10thtx

[oj]

A big part of the tone difference in the Majors is the fact that they do not drive the tone stack from a Cathode follower!

cabinet!!

Yes that is true, but A bigger part is that the feedback comes in on the cathode on the stage before the eq.
This makes the eq kind of semi active.

[jazbo8]

I think we had some discussion on the mini-Major over at Hoffman awhile back, anyway by making several key changes to the power amplifier configuration - significantly lowering operating voltages and changing the fixed bias to cathode bias. IMO, this can no longer be considered a Major, so please call it something else and remove the "Marshall Major" title from the schematics to avoid causing any confusion to the others in the future. Just me 2c...

[rp]

One mod you should do while its open is to buss all the output tubes Cathodes down and run it into a inline fuse as the HT fuse is too slow to respond to a shorted output tube

Steve, is this the same / different / better? Than just putting the fuse at the beginning of the power string?

[oj]

I think we had some discussion on the mini-Major over at Hoffman awhile back, anyway by making several key changes to the power amplifier configuration - significantly lowering operating voltages and changing the fixed bias to cathode bias. IMO, this can no longer be considered a Major, so please call it something else and remove the "Marshall Major" title from the schematics to avoid causing any confusion to the others in the future. Just me 2c...

Well I don't know how it is in your country, but here in Sweden Major is very much associated whit Blackmore and his Major was modified by Marshall factory so in that sense what is a Major ?

But I see your point , and that is why my major build will be called: Minor

[jazbo8]

Blackmore and his Major was modified by Marshall factory so in that sense what is a Major ?

Yes, they were modified to have a higher preamp gain, but they left the outptut stage just as GE designed it (except for may be upgrading the OPTs later). After all, Blackmore wanted all the 200W and more... Yes, no one these days want or need that much power, so I fully agree with the logic to lower the output power, but by doing so, I speculate that the "tone" will be irrevocable changed, thus calling it something else like the Minor makes much more sense - having "Marshall Major" on the schematic or suggesting that it could somehow sound like the original crank'd up is a bit of a stretch.