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Reducing output by removing tubes

#2
The tube pairs are identified by letter, and if you want to do this, you need to pull both "1" and "2" tubes of a given letter, e.g. V1A abd V2A, or V1B and V2B.
 
#4
If you like to deal with impedance mismatch and different anode voltage at the power tubes, yes it's a way to tweak the voicing of the amp. Assumed the amp design and the power tubes are capable to handle those mods without damage.

Mesa Boogie sometimes suggested those mods to try out the best voicing of an amp for personal needs.
 
#6
Sid Fang said:
The tube pairs are identified by letter, and if you want to do this, you need to pull both "1" and "2" tubes of a given letter, e.g. V1A abd V2A, or V1B and V2B.
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Err what?? When a tube is identified on a schematic as V1A and V1B it signifies a tube with two elements such a a 12AX7.

With an amp that has four output tubes it is possible to remove the outer pair of tubes to reduce the amps output capabilities. You also have to double the load impedance that the amp is expecting. With six tubes not so much.

As ThisBass correctly posted, optimum matching to the load will become well neigh impossible.
 
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#9
ThisBass said:
If you like to deal with impedance mismatch and different anode voltage at the power tubes, yes it's a way to tweak the voicing of the amp. Assumed the amp design and the power tubes are capable to handle those mods without damage.

Mesa Boogie sometimes suggested those mods to try out the best voicing of an amp for personal needs.
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I'm not sure I follow. I read an article that explained how to do it on a 4 tube guitar amp, but I'm uncertain as to how that applies to a 12 tube bass amp.
Turock said:
Your amp didn't come with a volume knob?
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Yes, and I can't turn it up beyond 3 in a gig setting, with the gain at noon. I can't overdrive the amp without the volume becoming unmanageable!
Jim Carr said:
So, why not just run it clean, and use a VT Bass or similar pedal to emulate "tube tone," "grit," "dirt," "break-up", etc.? ;)
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I don't want to emulate the sound. I've done that in the past and IME it never comes close to actual tube drive.
RoadRanger said:
You're really better off selling it and getting a 100 watt amp if that's what you need.
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Tried that. Nobody's biting in my area, and I have it priced HUNDREDS below retail; I'm not forking out shipping for a head of that size to sell it online.
 
#15
In any case if you can figure out the pairs, remove 4 pairs leaving 2 pairs and run into an 8 ohm cab on the 2 ohm output/setting and you'll be pretty close to having a stoopid heavy 100w amp :D . You'd probably be OK running into a 4 ohm cab also. You should check the biasing after removing those tubes.
 
#16
RoadRanger said:
In any case if you can figure out the pairs, remove 4 pairs leaving 2 pairs and run into an 8 ohm cab on the 2 ohm output/setting and you'll be pretty close to having a stoopid heavy 100w amp :D . You'd probably be OK running into a 4 ohm cab also. You should check the biasing after removing those tubes.
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Ok. So the pairs are lined up in-line with the bias adjustment knob on the back of the head. So, I just have to pull ANY 4 pairs? Or should I pull the 4 middle pairs? Or stagger them?
As for using the 2Ohm settting on the head, can you explain why this would be done? I'd like to understand more about how/why this is done. :) Great info to have! I'm running into an 8Ohm head on the 4Ohm setting right now.
 
#19
awilkie84 said:
Ok. So the pairs are lined up in-line with the bias adjustment knob on the back of the head. So, I just have to pull ANY 4 pairs? Or should I pull the 4 middle pairs? Or stagger them?
As for using the 2Ohm settting on the head, can you explain why this would be done? I'd like to understand more about how/why this is done. :) Great info to have! I'm running into an 8Ohm head on the 4Ohm setting right now.
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I'd pull out the pairs such that you get maximum cooling space between those left. As for the ohmage (<- term carefully chosen to piss off the cork sniffers LOL) you'd normally want to figure you need to decrease the setting by the same percentage as you decrease the tubes. So on the 2 ohm tap would become a 4 ohm tap with half the tubes - in this case with 1/3 the tubes it becomes a 6 ohm tap. As you've already noted you can go about 2x safely so anything from 6 to 12 is fine for the cab. Also, as you're not going to be stressing the output transformer with the reduce power you can pretty safely go 2x the other way so figure 3-12 ohms on the 2 ohm tap with 2/3 of the tubes pulled :) .
 
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#20
Oh, and since a big part of the "overdriven tubes" sound is actually the output transformer saturating don't be surprised if this doesn't give you the sound you're looking for :( . OTOH if you were happy with what the 4 ohm output sounded like into an 8 ohm cab you might be fine :) .

Now I'm psyched to try this with my Bugera BVV3000 (SVT clone) that I normally run with the master on "1" LOL.
 

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