Mod for Marshall Amps in 60's Rolling Stone magazine?

[Steve A.]

Someone just posted this on another forum and despite the obvious errors (like "dual power triode"?) I was wondering if anyone was familiar with the mod...

"I went to the Fillmore to see Cream when they first arrived here, but they were so loud I couldn't get past the foyer. A few months before Rolling Stone had published an article showing the unused half of a dual power triode in the Marshall, and they reproduced a copy of the bootleg schematic that was being handed around showing how to connect it up to double the power. My ears have been ringing ever since!"

I knew that at that time Randall Smith was modding Fender amps for more gain, but didn't realize that amp techs were modding Marshall amps as well. :eek:

Steve Ahola

[Enzo]

"Double the power?"

[Gtr_tech]

Uh...yea....I was going to comment on that and forgot about it. Schea....heright......
Tube manuals don't lie.....

[Fretts]

Sure, I remember that. That model amp design left half a ECC83 (12AX7) unused. Clapton wanted more sustain out of the big amps like he was used to from his Bluebreaker amp days. The amp tech looked in there and noticed the unused half-a-12AX7, namely a triode, and cascaded it into the other triode. The way he set it up, one of the input jacks fed to that double-triode gain stage and he painted the washer red. If Clapton wanted the overdriven sound, he would just plug into the "red" jack. I don't remember it saying so, but it sounds like that implies that he would have the stock amount of gain if he plugged into any of the other jacks.

[Steve A.]

Sure, I remember that. That model amp design left half a ECC83 (12AX7) unused. Clapton wanted more sustain out of the big amps like he was used to from his Bluebreaker amp days. The amp tech looked in there and noticed the unused half-a-12AX7, namely a triode, and cascaded it into the other triode. The way he set it up, one of the input jacks fed to that double-triode gain stage and he painted the washer red. If Clapton wanted the overdriven sound, he would just plug into the "red" jack. I don't remember it saying so, but it sounds like that implies that he would have the stock amount of gain if he plugged into any of the other jacks.

I was browsing through my Marshall schematics for the amps built in the 60's and didn't see any with unused triodes- do you happen to know what model number that would have been?

Thanks!

Steve Ahola

[Fretts]

No Steve, I don't remember.
But just putting two and two together, I would expect that they were 1959 plexi jobs since that was the default choice in those days for a loud band.
I wonder if googling around wouldn't dredge up that quote from the article?

[Gtr_tech]

I think what happened was he robbed the input stage from the other channel and reconfigured it to one channel with an extra gain stage. But if you did that, it would reverse the phase and play hell with the feedback loop so you'd need to account for that and make the appropriate changes. They may have left out the NFB altogether to get a ballsier sound.

[martin manning]

Isn't this exactly what a 2203/2204 does? The input jack switch terminals are wired to cascade the two input stages using the high sensitivity, and the low sensitivity skips the first one. The 1987 only has an unused triode stage if there is no jumper or instrument plugged into it.

The phase reversal doesn't matter. The only thing that counts there is the phase relationship between the source of the feedback signal (the output transformer secondary) and the point at which the feedback is applied (in this case at the PI).

MPM

[Fretts]

No, that wasn't it. I distinctly remember the tech being interviewed saying something about looking at the chassis and going, aha, look at that, a whole triode going unused. So it would have been some model that didn't use that triode. Maybe it was a non-tremolo version of a model that was also available with it. Maybe that would have been a 50 watt head...
It's a long stretch, but I found a weird situation like that in my old Gretsch amp. There was a whole resistor and capacitor matrix to accept the signal from a second channel was not on that model of amp. But the parts were there anyway. So it's not that weird to suppose that there would be a tube that was only being half used.

[Steve A.]

No, that wasn't it. I distinctly remember the tech being interviewed saying something about looking at the chassis and going, aha, look at that, a whole triode going unused. So it would have been some model that didn't use that triode. Maybe it was a non-tremolo version of a model that was also available with it. Maybe that would have been a 50 watt head...
It's a long stretch, but I found a weird situation like that in my old Gretsch amp. There was a whole resistor and capacitor matrix to accept the signal from a second channel was not on that model of amp. But the parts were there anyway. So it's not that weird to suppose that there would be a tube that was only being half used.

Not that weird at all. I suspect that some of those early Marshalls might have used a separate tube for the input stage of each channel, which would be obvious in a layout drawing but not necessarily from looking at the schematic.

I always get mixed up with the various model names and model numbers in Marshall amps- like what was the model number for the Bluesbreaker amp? I think that would have been the JTM45 Model 1962 with KT66 output tubes. I believe that Clapton did use a Dallas Range Master pedal with Cream and Mayall.

Thanks!

Steve Ahola

[Gtr_tech]

Isn't this exactly what a 2203/2204 does? The input jack switch terminals are wired to cascade the two input stages using the high sensitivity, and the low sensitivity skips the first one. The 1987 only has an unused triode stage if there is no jumper or instrument plugged into it.

The phase reversal doesn't matter. The only thing that counts there is the phase relationship between the source of the feedback signal (the output transformer secondary) and the point at which the feedback is applied (in this case at the PI).

MPM


Pretty much the same as the 800s/JMP MV, yea.

Brain's stuck in SE mode lately.....disregard the phase issue. Got out a rainy day project the other day (old 50s tweed Epi SE 6L6 job) and have been going over ideas for the rebuild.

Carry on.

[Joey Voltage]

I have deffinately heard of the "one wire mod" marshall was performing, which was a very crude, and unstable version of what became a 2203 many years later. maybe this may be something similar

[txstrat]

I believe it could be the "1961 Lead 45" (see schematicheaven.com) cause if you look at the schematic you'll see seven triode stages of 4 ECC83 tubes.
I think the last (unused) one is not in the schematic, simply because it's not used.

[Steve A.]

I believe it could be the "1961 Lead 45" (see schematicheaven.com) cause if you look at the schematic you'll see seven triode stages of 4 ECC83 tubes.
I think the last (unused) one is not in the schematic, simply because it's not used.

That's the tremelo circuit that uses the single stage?

So now I wonder about the specifics of the modification- we *know* that they aded that extra stage to the input circuitry, but I wonder what values were used for the plate and cathode resistors, as well as the coupling cap and the cathode resistor bypass cap (if used).

Thanks for the detective work!

Steve Ahola

[txstrat]

If you look at the "1959 Lead 50W" schematic (schematicheaven.com) you find the section of the 2203/2204 master volume amps at upper right. That's what I would do with the extra gain stage. Low = skip the first stage. High = additional first stage. I believe thats why Claptons other input must have had a red washer.
For the additional first stage I would start with the values in the schematic (100k plate resistor, 2.7k cathode resistor with a 680nF cap) and see how it sounds. To alter the sound I would then decrease the cathode resistors value and/or increase the bypass cap value. After that all I would decide what sound I like best.
BTW you could also use a DPDT to switch the extra gain stage in at the same input. That way you would only have to use the switch for the extra gain stage instead of plugging your guitar cable out and into the next input.

[txstrat]

With a closer look at the schematic I found out that if you don't add the "gain pot" the amp would behave like a 2203/2204 with the gain (pre amp volume) fully open.
That for I would go for the DPDT switch and use the hole in the chassis (second input) for the gain pot.
This way you can dial in the amount of gain you like.
In other words you turned that JTM45 into sort of a master volume amp.
I would also tinker around a little with the caps to the pre amp volume pot. I did this a couple times because for my taste the channel was too brittle. I increased the values of the caps a little until I liked it.

...and yes, it's the tremolo section that uses the single stage.

Matt