[Question] Marshall Major 200 Watts

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Stevem
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Post by Stevem »

They run about 620 plate volts when biased in right.
Adding the proper value cap and resistor across the plate lead and the UL winding protects these OTs quite nice and they should also have about a 270 ohm screen resistor added in series.

As usual with these amps a poor contection on the plug in type impedance selector took out a lot of the OTs in these amps!
If there was ever a amp that needed a 250 ohm 20 watt safety resistor wired across the OT as a supliment load , this amp is one with the cost of that OT!

Proper wiring on the tube sockets is very important to keep arcing at by, and any pointy cut off wires or component leads on the output tube sockets with this amps voltage will lead to arcing!

It's my understanding that Marshall contracted out to another company to build these amps.
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martin manning
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Re: I

Post by martin manning »

Stevem wrote:Adding the proper value cap and resistor across the plate lead and the UL winding protects these OTs quite nice and they should also have about a 270 ohm screen resistor added in series.
The schematic shows a 250R 5W screen resistor on each KT88. Are you suggesting adding another 270 ohms?

What are the details of this cap and resistor mod on the OT primary, and what values are proper?
Bombacaototal
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Re: [Question] Marshall Major 200 Watts

Post by Bombacaototal »

Thanks for the link. I may be wrong but I think this schematic is from the PIG.

The one I am referring to is the pic below
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Stevem
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Post by Stevem »

Sorry , I was referring to adding a resistor to the UL lead to each tube, as it seems most UL type guitar amps lack this!

The cap and resistor I found mentioned in the book "an approach to Audio Frequency Amplifier Design"
In this book on page 9 they state that to absorb surges, a cap with a reactance equal to the load impeadance at between 10k to 15k and a resistor equal to the load impeadance and a wattage of 1/10 the full output of the amp is what they call for.
They show these used in 4 different UL output stage schematics in the book, but in 3 of them the resistor goes first off the UL lead and then the cap in series to the plate lead, but in one schematic the parts are reversed.
I do not know if the order matters or not!

In the schematic for a 100 watt KT88 amp each cap is 1000 pf and comes off the UL lead, and the resistor after the cap is anywhere from 470 to 1500 ohms and a 1/2 watter.

In regards to that photo of that 1969 Major, I have never seen one with a filter can like that one has near the preamp tubes!
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Not screaming like the passengers in his car!😊

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Roe
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Re: [Question] Marshall Major 200 Watts

Post by Roe »

I recently tried a major turned up to 6-7. It had a glorious sound and response, with some compression and breakup. The amp ran 230-240v wall power into the 245v primary and used 470R screen resistors
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martin manning
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Re: M

Post by martin manning »

Stevem wrote:Sorry , I was referring to adding a resistor to the UL lead to each tube, as it seems most UL type guitar amps lack this!
Ok, so those are just screen stoppers, which the Major's power stage already has.
Stevem wrote:The cap and resistor I found mentioned in the book "an approach to Audio Frequency Amplifier Design"
In this book on page 9 they state that to absorb surges, a cap with a reactance equal to the load impeadance at between 10k to 15k and a resistor equal to the load impeadance and a wattage of 1/10 the full output of the amp is what they call for.
It's unclear what is meant by that statement on pg. 9 since it doesn't say where "across the output transformer" is, but it does say "a pair" of resistors and capacitors so it is likely as shown in some of the high power schematics. In those cases the series resistor capacitor pairs are connected from anode to screen to damp oscillation.

I wouldn't be at all surprised if some of the Major's circuitry came from that book.
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Post by Stevem »

Yup, I was thinking the same thing as I thumbed through that book the other night!
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Not screaming like the passengers in his car!😊

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caps8419
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diodes?

Post by caps8419 »

Could this help?
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Stevem
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Post by Stevem »

It depends on what you call help, as the diode string will help spare the output tubes at the expence of the OT!
I don't know about you , but I always bring a spare output tube or two on gigs, not a spare OT!

Roe, when you listen to a 100 watter and a Major side by side you can hear / feel the difference that the Majors higher current 12AU7 driver makes in the amp, if you pop a 12AX7 in the amp your lucky to get 30 watts of power out of a Major!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Roe
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Re: [Question] Marshall Major 200 Watts

Post by Roe »

yes, the feel is different. You feel it in your stomach
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Post by Stevem »

Yes, the punch difference, but the output stage reacts differently in terms of play!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!😊

Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
strelok
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Re: Y

Post by strelok »

Stevem wrote: Roe, when you listen to a 100 watter and a Major side by side you can hear / feel the difference that the Majors higher current 12AU7 driver makes in the amp, if you pop a 12AX7 in the amp your lucky to get 30 watts of power out of a Major!
Is that due to the 47k plate resistors and the way the driver is biased? Or do the KT88's need more current to drive them than other tubes? I wouldn't think they do but that's why I ask.
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Re: Y

Post by martin manning »

strelok wrote:
Stevem wrote: Roe, when you listen to a 100 watter and a Major side by side you can hear / feel the difference that the Majors higher current 12AU7 driver makes in the amp, if you pop a 12AX7 in the amp your lucky to get 30 watts of power out of a Major!
Is that due to the 47k plate resistors and the way the driver is biased? Or do the KT88's need more current to drive them than other tubes? I wouldn't think they do but that's why I ask.
To make the comparison between a 12AX7 and a 12AU7 as the driver, the circuit would have to be modified to correctly bias the 12AX7. Just swapping a 12AX7 for the 12AU7 with the same Ra and Rk will result in a bit more gain, but the bias will be off-center, with only ~1/10 the available Vg swing to cut-off. That would reduce the measured power at clipping severely. But, even if the 12AX7 were to be center biased, the available Vg swing will be much less than the 12AU7 (at 400V Va 12AX7 reaches cut-off at -5V, and 12AU7 at -30V). KT88 data sheets don't show Ig1, but it shouldn't be an issue in AB1.
strelok
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Re: [Question] Marshall Major 200 Watts

Post by strelok »

Thanks, that's what I thought. It's interesting to see a guitar amp with a driver designed specifically around a 12au7, don't see that all that often. Supposedly Marshall had GEC design the amp for them, which would explain why it looks more like a Hifi circuit than anything. However I have no idea if that's true or not.
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Marshall Major

Post by DocRocz »

I had a great amp tech that suggested we remove the KT88's and replace them with 6550's. Well the tone became rounder and fuller.
The reason for making the switch wasn't for tone, but for cost.
The tone just happened to be icing on the cake.
As for clean, we put it on the scope and it was pedal steel clean up to 180 watts, with no clipping at all!
My tech then suggested a master volume and that the channels be cascaded, and to keep the amp as close to stock as possible we removed one of the speaker out puts and used that hole for the master volume.
Had I wanted to put it back to stock it was as simple as some soldering and putting the speaker jack back in.
I played it for 15 years with no issues, other than the standard maintenance.
I do remember the one thing the tech did do when he started working on the amp, he went through it from top to bottom to make sure there were no cold solder joints.
Once that was completed, he then started the mods.
This all happened in the mid 70's so given the amp was a 1969, it wasn't considered a vintage amp as it would be today.
Oh, and my prize only cost me $200.00.
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