How would modifying an amp to take fewer power tubes affect tone?

BoomTexan

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I'm thinking about trying to make a modified Sunn Coliseum PA (just a schematic for fun for now) so that I don't have to pay 1.5-2.5k to get my hands on one, and so that it would fit my playing style/tone preferences.

I would cut out two of the 4 channels, have one with higher gain and one that's basically unmodified that has a normal gain level. I would make jumping these channels possible. I would possibly add a switchable tube boost circuit and a negative feedback control pot to the modified channel. Maybe a mid boost if I can figure out how to make one.

The real modification would occur in the power section. I want to remove two of the KT88 tubes, so it would be just a 2 tube push-pull section. Maybe I'd consider doing a class A KT88 build, but I think that the tone would be drastically altered, so I'll probably save that for a modified Deluxe or Princeton build and try to get 15 watts out of it. The push-pull dual KT88 setup is very similar to smaller Sunns of the time, which were good for 40-75 watts with two KT88/6550s. Still deafeningly loud, but I can at least attenuate it, as opposed to the mighty roar of 4 KT88s in the Coliseum. I think the only way to deal with that kind of power is to get an OX Box, and I'm not paying 2k for one of those. I'm planning on just dumping a Sunn Sentura or Sceptre power section in there and calling it a day.

My question is this: how will the proposed power amp modification affect tone? Given that one channel of the amp will be drastically changed from the original Sunn circuit, its not that important if its a little different, but I really want to get that sweet smooth Leslie West power tube distortion sound out of this amp. Would the sound be terribly different from a 4 tube setup?
 

uriah1

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Not sure it relates, but I remember back in the day I would remove the 2nd and 4th power tube
in my Marshall to knock it down a tad. Seemed to work ok.
 

BoomTexan

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Not sure it relates, but I remember back in the day I would remove the 2nd and 4th power tube
in my Marshall to knock it down a tad. Seemed to work ok.
Absolutely, that's what I was assuming would happen. I've heard so many stories of people doing that, whether with old Peavey Maces, Marshalls, or Fenders. A little quieter but no real issue doing it. I'm just checking to see if there's some technical reason that it wouldn't work (unlikely in my opinion) or some noticeable difference in the end product.
 

Phrygian77

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You get sort of a composite of the grid curves with two tubes in parallel since they are not exactly the same. So, it technically should have an effect on the harmonic distortion. I guess how much of that is perceivable is debatable.
 

2L man

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"Math" there is that output power drop to about half and volume about 3db. Then when you drive only half the power tubes to same volume sound distorts more.

Two tubes use less current than four so operative voltages rise which marginally gain power back. If voltage rise considerably it shorten power tube life and might cause sudden breakups if four already did have high voltage.

To keep tube load the same loudspeaker should be changed to lower impedance output jack.
 

Dacious

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That's a bit like 'I want an economy car so I'll buy a half ton truck and remove half the pistons and rods'.

Just get the amp that's right sized for what you need. Lots of people have pulled two power tubes and one speaker lead off a Twin Reverb and discovered it makes marginal difference to tone, headroom or unfortunately size and weight.

I think what you're referring to as Class A, is cathode bias. Which will reduce power and headroom due to the cathode bias resistor chewing up a lot of power.

Starrting with a PA and trying to make a guitar amp out of it. ... possible. But why would you?

Just get a SF Bassman head. Probably sound ten times as good, easier, simpler, lighter, easier. Very easy to mod. Bulletproof.
 




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