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Reducing output by removing tubes

#21
RoadRanger said:
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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Awesome explanation! So, if the 2Ohm tap would become 4Ohms with half the tubes pulled, that means the 4Ohm tap should now be an 8Ohm. Maybe I'll start with pulling 6 tubes, instead of 8, and see how it sounds from there...or do you always want an even number of pairs?
 
#22
Unless you're 100% sure that this amp can be safely down-tubed, don't even think about it.

There are a number of design choices that the designers of this amp could have made that would either make this not work or even damage the remaining tubes or other components.

As far as why this would work with a guitar amp but not yours, there are plenty of guitar amps that won't work with a pair of output tubes yanked.
 
#23
awilkie84 said:
Awesome explanation! So, if the 2Ohm tap would become 4Ohms with half the tubes pulled, that means the 4Ohm tap should now be an 8Ohm. Maybe I'll start with pulling 6 tubes, instead of 8, and see how it sounds from there...or do you always want an even number of pairs?
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You can try just 1/2 pulled just fine :) . You'll get a touch more than half the power so maybe 2db less volume - hence my suggestion to go with 1/3 the tubes. But if you have the time why not try both :) .
 
#24
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?
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It really doesn't make any difference as long as you remove the same number of tubes from both halves of the power section. You always want to have the same number of "push" tubes as "pull" tubes.
 
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#25
awilkie84 said:
Why would a guitar amp be so different from a bass amp? They're effective the same circuits, just with different wattage, aren't they?
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It's the frequencies that make the difference. The lower the frequency the more power and speaker cone area you require.

A guitar player can easily play a gig with a 50W amp and an open backed single 12. Try that with a bass and you'll be replacing drivers for ever.
 
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#26
BassmanPaul said:
It's the frequencies that make the difference. The lower the frequency the more power and speaker cone area is required.

A guitar player can easily play a gig with a 50W amp and an open backed single 12. Try that with a bass and you'll be replacing drivers for ever.
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What has that got to do with pulling power tubes? There really is no essential difference between a guitar amp (the amplifier itself) and a bass amp of the same power. Speakers, yes, of course.
 
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#27
BassmanPaul said:
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.
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See the YBA-300 manual. It's a 300W amp with a power stage of 12 (count 'em 12) 6L6 or EL34 tubes, organized in 6 pairs, A through F. They've got protection circuits to take pairs out of the circuit in the event of failure, so I think it very likely the OP's plan is actually workable on this amp.
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.
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It's got 12.
As ThisBass correctly posted, optimum matching to the load will become well neigh impossible.
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Again, see the YBA-300 manual. They were apparently concerned about how long one could keep 12 power tubes happy under rock'n'roll abuse and put some complexity into the circuit to enable taking a bad pair "offline" (they call it "locked out"). I don't have the schematic (it's not a common one on the web), but I'd guess that an unpopulated tube pair would be considered bad and locked-out. The amp should function, with diminished power. As others have noted, this operation isn't necessarily a Good Idea, depending on the amp. On this one, I think the OP can get away with it, though I'll defer to anyone who's actually owned or worked on a YBA-300. I wouldn't be surprised if the tone shifted a bit, but whether that's a good or a bad thing is a question of taste.
 
#28
I know it has 12 6L6GCs in the output stage. That's why I was advising against removing a couple of tubes. If you remove half of them, three each from the push side and the pull side and doubling the OPT tap value you can reduce the output capability by a half. The voltages within the amp will rise due to less current draw. That might or might not be a problem.
 
#31
Ok...so I pulled 3 pairs of tubes & left the amp on the 4Ohm setting. Volume was still excessive, but I was able to control it better than with the full allotment of tubes. There's a little bit of hum that wasn't present before.
I'm going to let the tubes cool down & then try it with only 2 sets of tubes.
 
#33
RE: impedance mismatch...from what I can glean from amp designers, it's not as critical an issue as we might think. Most tube amps seem to be OK with a 2:1 mismatch either way, and I've heard many respected amp designers and techs say as much. It changes the sound, makes it a little more compressed to my ears, but these potential meltdowns I've heard about seem to be few and far between.

However, I would definitely check with Traynor about any possible long-term issues from pulling that many tubes.
 
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#35
JimmyM said:
RE: impedance mismatch...from what I can glean from amp designers, it's not as critical an issue as we might think. Most tube amps seem to be OK with a 2:1 mismatch either way, and I've heard many respected amp designers and techs say as much. It changes the sound, makes it a little more compressed to my ears, but these potential meltdowns I've heard about seem to be few and far between.

However, I would definitely check with Traynor about any possible long-term issues from pulling that many tubes.
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I emailed them. As it's the weekend, I don't expect a reply until Monday. For the time being, I'm done my testing and won't fiddle with it until I hear back. :P
 
#37
JimmyM said:
RE: impedance mismatch...from what I can glean from amp designers, it's not as critical an issue as we might think. Most tube amps seem to be OK with a 2:1 mismatch either way, and I've heard many respected amp designers and techs say as much. It changes the sound, makes it a little more compressed to my ears, but these potential meltdowns I've heard about seem to be few and far between.

However, I would definitely check with Traynor about any possible long-term issues from pulling that many tubes.
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I did that with my 70's svt several years ago. I did it because I tried to figure out out the effect of the antiparallel diodes at the input ot the power amp. Briefly worded, the sound was slightly different but still ok with remaining of 4 power tubes but terribly horrible with remaining onlya pair of push pull. By the other hand a pair of them would be just the right way to tweak the output section to something around 60..70 Watt at 4 Ohm load.

If I had the lack of far to much power with my svt I would look for a V4B.
 
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#38
awilkie84 said:
Just finished trying it. Worked well. The volume still goes up REALLY fast, but I can now push the gain stage and get a bit of an overdrive out of it. Also, the slots with no tubes don't lock out, they just sit with the bias light saying it's low.
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I don't know what the "lock out" circuit actually does, but it makes me a little nervous to think of running on only 4 out of 12 without some kind of compensation. You've done the right thing by reaching out to Traynor.

The alternative is to go for some kind of Power Brake / Hot Plate dummy load. Not as "green" though.
 
#39
Pulling 8 out of 12 tubes means B+ is going to be quite higher, which means every tube in the amp will be seeing higher plate voltage, and no longer operating under their original design conditions.

Oh, and the filament (heater) voltage has now increased as well. You might wind up replacing all your tubes a lot sooner than anticipated.
 
#40
nashvillebill said:
Pulling 8 out of 12 tubes means B+ is going to be quite higher, which means every tube in the amp will be seeing higher plate voltage, and no longer operating under their original design conditions.

Oh, and the filament (heater) voltage has now increased as well. You might wind up replacing all your tubes a lot sooner than anticipated.
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What would you suggest, then?
 

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