[Question] Marshall Major 200 Watts

[Bombacaototal]

Thanks, I just used a standard Marshall chassis. The board is a layout I did, with the Major circuit with the modifications to change to 100W, Superlead bias circuit, remove 2 power tubes, etc. If you decide to build one just give me a shout and I can help you out if you need it.

Thanks a lot for the offer. Your project indeed looks very interesting. I will definitely give you a shout if I decide to go Mini-Major route

[Bombacaototal]

Do you have something against the 12AU7? Replacing KT88 with 6L6 would be easy. Just replace the OT with a 100W unit and reduce the size of the PT accordingly.

Hi Martin, assuming the correct PT and OT, i was ust wondering if there are any other adjustments necessary like screen and grid resistors, etc

[martin manning]

All of the resistors would be typical 6L6 values, as in a Fender Twin e.g.

[Bombacaototal]

According to this article SRV had a super PA and a Super lead

I think the blonde twin is a 6g8-a https://www.premierguitar.com/articles/ ... odel_6G8_A

[roberto]

Here you go:

http://www.drtube.com/schematics/marshall/1967u.gif

This schematic is wrong:

The signal is taken between the 2k7 and the 100k on the cathode of the concertina, not from the input, and the 1M resistor is connected to the same point, even if the schematic says the opposite.

Furthermore, after the concertina the two resistors are 270k, not 270 Ohm as written.

[roberto]

Even at 100W with 2 KT88's, the Major clone is very clean, due to the UL taps no doubt. I got Chris Merren to wind me a custom OT. He's the Marshall transformer king and I asked him to make me an exact replica of the Drake OT, but for 2 KT88's instead of 4. An added bonus was that he told me he knew why the originals failed (described by Dawk as the Smoke on the Water effect) and guaranteed me that mine would not.

Another option to safely run 6550s is to get the UL tap on the cathodes of the tubes (of course the OT has to be wounded accordingly), so we can lower the voltage on screens to a safe value.

[strelok]

Good to know. That makes a lot more sense. I thought it was quite strange that they would do it the way shown in the schematic but I didn't really put more thought into it. Guess I should have lol.

[Krinkle]

The signal is taken between the 2k7 and the 100k on the cathode of the concertina, not from the input, and the 1M resistor is connected to the same point, even if the schematic says the opposite.

Yes, the 1M should be connected, that is a mistake on the schematic. With all due respect I'm not sure about your negative feedback statement, but I'm not an expert so I ask politely. During my build I had a LOT of conversation with a LOT of people, some of them had direct conversations with Dawk, Blackmore's tech, to correct the mistakes on the schematic, and that was never mentioned.

Also, would you have NFB of 47k/100k?

What about the presence control? Now it wouldn't be in the feedback path. Please let me know, if this is an error I would love to confirm and change my amp.

Furthermore, after the concertina the two resistors are 270k, not 270 Ohm as written.

I agree with this.

[roberto]

I never talked about NFB, but always about the concertina.
The NFB is applied on the stage before the concertina.

This circuit is correct: http://drtube.com/schematics/marshall/1966.gif
This one is correct too: http://drtube.com/schematics/marshall/200w.gif

[Krinkle]

I never talked about NFB, but always about the concertina.
The NFB is applied on the stage before the concertina.

This circuit is correct: http://drtube.com/schematics/marshall/1966.gif
This one is correct too: http://drtube.com/schematics/marshall/200w.gif

Then I misunderstood you, thanks for responding.

[roberto]

You are welcome!
I just wrote that to underline that most of the schematics of the Major that you can find online, have some errors around the PI.
In case some other users would like to build one.

[rp]

I don’t know anything about Marshall Majors and Pigs but reading back through this and the arcing issues discussed the TT21 popped into mind: http://www.r-type.org/exhib/aby0019.htm

Who knows if Marshall was even aware of these tubes but I doubt any sane company would put deadly high voltage on the outside of a guitar amp especially during the mad, drunk, tripping rock'n'roll circus era. In Dean Martin roast Foster Brooks drunk voice, “I see why your amp sounds funny, this little hat fell off, here let me just…”

But if someone has the patience to sift through German eBay for rare tubes these do show up and are much cheaper than NOS GEC KT88s. I went surfing around curious when I first read out about them. Anyways, it’s a tube amp forum so I thought I’d throw it out there. Seems with present day hindsight and foresight a bulletproof version of a Major could be built. Along with a lighting bolt death skull badge on the back, maybe use security screws and a momentary switch that kills the mains when the back panel is removed (...kind of kidding).

And, brought to mind corona dope, checked CE and voilà:

https://www.tubesandmore.com/products/c ... ating-2-oz

If the pins were soldered and filed nice and round and doped would any type arcing be 100% solved - i.e. does corona dope actually work? I actually remember seeing corona dope in e-stores decades ago, maybe even Radio Shack when they sold tubes and wondering what it could possibly be. Two fascinating, cryptic e-words I remember as kid: “Corona Dope” and “Blonder-Tongue”.

[Krinkle]

Seems with present day hindsight and foresight a bulletproof version of a Major could be built.

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.

[Mr. dB]

Who knows if Marshall was even aware of these tubes but I doubt any sane company would put deadly high voltage on the outside of a guitar amp especially during the mad, drunk, tripping rock'n'roll circus era. In Dean Martin roast Foster Brooks drunk voice, “I see why your amp sounds funny, this little hat fell off, here let me just…”

Well, Ampeg used 6146s in early SVTs...

[Bombacaototal]

Just wondering if anyone ever attempted a Major Lead pre-amp with a LTPI like a JTM or blackface type phase inverter instead of the ultra linear

What type of result should I expect if I attempt this