You tell me! I was just trying to lure you in here.
In a 4 power tube fixed bias amp, is it smarter (safer) to lift two cathodes, keep the power tubes referenced to ground with a largish resistor (10K or so), and double the load.
-OR-
Cut signal from two output tube grids (need 4 output tube coupling caps for this).
I am a little hazy on the latter. Does everything still look the same to the OT?
I know half power switch is not that big of a drop in volume. It's a request for a mod. If it were me, I would probably just leave it alone. It's an 800 from a kit that I can easily mod, so no harm done.
Thanks!
1/2 power switch, the smart way.
Moderators: pompeiisneaks, Colossal
Re: 1/2 power switch, the smart way.
Well, I ain't smart, but I have an opinion.
You can also switch the screen grid resistors to the plates. Check out some of the Marshall JCM900 schematics--that's what they did for the half power switch. What it does it switch the tubes from pentode mode to triode mode.
You can also switch the screen grid resistors to the plates. Check out some of the Marshall JCM900 schematics--that's what they did for the half power switch. What it does it switch the tubes from pentode mode to triode mode.
What?
Re: 1/2 power switch, the smart way.
When cutting signal to the power tubes, you are messing with the input grids. When doing so, make sure that the bias voltage to the grids stays intact.
Re: 1/2 power switch, the smart way.
Lift the two outside tubes' cathodes.
Simplest, works great, maintains voltages.
Simplest, works great, maintains voltages.
Re: 1/2 power switch, the smart way.
Ah, whoops, admittedly that had not crossed my mind yet. I guess that method is only practical with cathode bias.jelle wrote:When cutting signal to the power tubes, you are messing with the input grids. When doing so, make sure that the bias voltage to the grids stays intact.
Okay, will do, I was just wondering if there was some real reasons not to do this. All I can think of is rising PT voltages, and leaving the cathodes floating. I figured sticking in a resistor across the switch would be good. Do commercial amps that do this (if any) instruct on double the load? I know you should, but just curious if they actually say to do this...Teleguy61 wrote:Lift the two outside tubes' cathodes.
Simplest, works great, maintains voltages.
Thanks.
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Leo_Gnardo
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Re: 1/2 power switch, the smart way.
I'm with Jana on this one. When I've done switches that "turn off" 2 of 4 output tubes, can't really hear all that much of a difference in amp power. The meters tell me but my ears don't.Jana wrote:Well, I ain't smart, but I have an opinion.
You can also switch the screen grid resistors to the plates. Check out some of the Marshall JCM900 schematics--that's what they did for the half power switch. What it does it switch the tubes from pentode mode to triode mode.
In triode (compared to normal operation), you typically lose 2/3 of your power, need more signal to drive the remaining tubes, and you'll deafinitely hear a rolloff of high frequencies. Gotta turn up the preamp to get more drive to the outputs - hey it DOES sound like what you'd expect a half power switch to do. Not at all half power for the meters, no relief for the ears, what you get by simply leaving out a pair of tubes..
down technical blind alleys . . .
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Stevem
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Re: 1/2 power switch, the smart way.
My ears always tell me that half power will just get the output stage clipping 50% sonner along with only a small drop in the sound pressure level.
The best way is to drop off switch wise 2 speakers if you strumin thru a 4-12 cabinet, or a attenuator placed in line!
It's also not easy if your using 34s as output tubes to do the switch thing to triode mode as to do it right a 100 ohm resistor needs to added / switched in to both the screen grid to plate and the suppresser grid to plate.
Another way to go if your need is to play the amp at lower volumes all the time is to pop in 4 JJ brand 6V6s and reset the bias.
My preferred way to drop out two outputs is to switch in 20k worth of resistance into each cathode , that way the two tubes are in place and pulling there needed heater current so you do not get high filament voltages that can kill the tubes and also make the preamp section sound like crap due to much higher cathode emissions!
The best way is to drop off switch wise 2 speakers if you strumin thru a 4-12 cabinet, or a attenuator placed in line!
It's also not easy if your using 34s as output tubes to do the switch thing to triode mode as to do it right a 100 ohm resistor needs to added / switched in to both the screen grid to plate and the suppresser grid to plate.
Another way to go if your need is to play the amp at lower volumes all the time is to pop in 4 JJ brand 6V6s and reset the bias.
My preferred way to drop out two outputs is to switch in 20k worth of resistance into each cathode , that way the two tubes are in place and pulling there needed heater current so you do not get high filament voltages that can kill the tubes and also make the preamp section sound like crap due to much higher cathode emissions!
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!
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!
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martin manning
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Re: 1/2 power switch, the smart way.
No need to mess with the suppressor grids for triode mode, just move the non-tube end of the screen resistors to the plates. See Marshall 2550. This is easiest since there is no need to reset the speaker load. It will sound different, though.
Re: 1/2 power switch, the smart way.
Like Mesa found long ago (in MarkI none the less), just switch two outer cathodes off and do not change impedance tap, which is the "modern"suggestion, perhaps correct from a technical perfectionist way but not useful at the Pub stage.
Why?
Because losing 50% (or less) power is still too little to tame a loud amp, that's why.
While pulling 2 tubes and not adjusting impedance makes loss more than 50% (I guess power drops to 1/3 or less) and that is more useful.
Sounds technically incorrect?
Probably ... but works better than the other option.
Why?
Because losing 50% (or less) power is still too little to tame a loud amp, that's why.
While pulling 2 tubes and not adjusting impedance makes loss more than 50% (I guess power drops to 1/3 or less) and that is more useful.
Sounds technically incorrect?
Probably ... but works better than the other option.
Re: 1/2 power switch, the smart way.
Really, the only good solutions are
(1) turning down the volume, perhaps with a fixed attenuator switch ahead of the PI or some kind of switchable master volume
or
(2) having a small auxiliary amp that's lower power.
This issue is stuck firmly in the triple conflict: good, cheap, easy (or fast), pick any two.
It's tempting to think that just a few diddles with the circuit would leave you with the same amplifier tone and sound, but at a lower volume. But the reality is that you have to cut the amp power a lot to make it half the volume at the same % of the amp's total power, and even then the speakers are not going to be producing the same set of their own quirks, which are output level dependent.
The best technical solution is probably to switch the B+ to some lower value, mucking about with the bias and other things to make the same tubes only go to some lower power output, perhaps switching out a pair of tubes in a 4-tube amp. This is a variant of the power-scaling thing.
That fixes the amp power, but still leaves the speakers not being driven as fully, and losing that contribution to the tone you know and love.
Perhaps another solution is to build a baby output stage into the same box and make the power-level change bring in a new output stage optimized for the lower power.
(1) turning down the volume, perhaps with a fixed attenuator switch ahead of the PI or some kind of switchable master volume
or
(2) having a small auxiliary amp that's lower power.
This issue is stuck firmly in the triple conflict: good, cheap, easy (or fast), pick any two.
It's tempting to think that just a few diddles with the circuit would leave you with the same amplifier tone and sound, but at a lower volume. But the reality is that you have to cut the amp power a lot to make it half the volume at the same % of the amp's total power, and even then the speakers are not going to be producing the same set of their own quirks, which are output level dependent.
The best technical solution is probably to switch the B+ to some lower value, mucking about with the bias and other things to make the same tubes only go to some lower power output, perhaps switching out a pair of tubes in a 4-tube amp. This is a variant of the power-scaling thing.
That fixes the amp power, but still leaves the speakers not being driven as fully, and losing that contribution to the tone you know and love.
Perhaps another solution is to build a baby output stage into the same box and make the power-level change bring in a new output stage optimized for the lower power.
Re: 1/2 power switch, the smart way.
Thanks, really appreciate everyone's thoughts. First off the idea is to keep the mod simple. I've done all sorts of stuff before with custom PTs, Mosfets for "power scaling", but those are harder to retrofit and expensive.
The idea with this is not to really lower the SPL, but shift the clipping more towards the poweramp at a slightly lower level. I'm well aware of the 3db difference, but I have to be a bit of a devil's advocate here and say that at stage volumes that seemingly small difference is more exaggerated.
What I mean, is that in a context where everything is already really loud, small differences feel big. Kinda like how even a 1W amp seems super loud at home, similarly, on stage, a 100W amp feels like it has a lot more headroom than a 50W. That's my theory at least and the reason you still see half power switches and 50 and 100W models of the same amp. 50W players think 100W is crazy loud despite the 3dB. I've personally had a lot of frustation trying to keep up with another guitarist in a band who had a 100W when I was using a 50W. We were going for the same tone, he did not like to turn down, and so I could just not match him. 3dB ruined our sound!
Anyway, thanks again for all the advice and suggestion Much appreciated as usual
The idea with this is not to really lower the SPL, but shift the clipping more towards the poweramp at a slightly lower level. I'm well aware of the 3db difference, but I have to be a bit of a devil's advocate here and say that at stage volumes that seemingly small difference is more exaggerated.
What I mean, is that in a context where everything is already really loud, small differences feel big. Kinda like how even a 1W amp seems super loud at home, similarly, on stage, a 100W amp feels like it has a lot more headroom than a 50W. That's my theory at least and the reason you still see half power switches and 50 and 100W models of the same amp. 50W players think 100W is crazy loud despite the 3dB. I've personally had a lot of frustation trying to keep up with another guitarist in a band who had a 100W when I was using a 50W. We were going for the same tone, he did not like to turn down, and so I could just not match him. 3dB ruined our sound!
Anyway, thanks again for all the advice and suggestion Much appreciated as usual