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Old 30th August 2011, 04:32 PM   #1
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Default power tube power scaler

Ok, i found a schematic for a power scaler, and i wanted to check with someone this is correct. i also have a question. does this change the voltage of the entire amp? so power amp, PI and preamp? and is there a way i can only scale the powe amp? or is this not a good idea?
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Old 30th August 2011, 07:24 PM   #2
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no - it doesn't scale "power". It sets a bias voltage (in this case 12V) and so would only supply a VERY small amount of current (effectively none, but in practise a few micro-amps dependant on where the trim and bias pots are set)
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Old 30th August 2011, 08:09 PM   #3
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Looks like a good way to turn pots scratchy

Unless you're obsessed with having the latest and greatest fad circuit in your amp, I would go with the more traditional techniques of scaling power in a guitar amp. But judging by your previous thread maybe you are obsessed have fun!
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Old 30th August 2011, 11:42 PM   #4
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i asked a question, got an answer and did some research i wouldnt count that as obsessed! i simply wanted another opinion on this circuit

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no - it doesn't scale "power". It sets a bias voltage (in this case 12V) and so would only supply a VERY small amount of current (effectively none, but in practise a few micro-amps dependant on where the trim and bias pots are set)
from what i gathered it changes the B+ voltage going to the power tube, and the 2nd half of the circuit then adjusted negative bias accordingly, wouldnt this in turn scale power?

as for pot scratchiness, how so?
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Old 31st August 2011, 12:35 AM   #5
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doh! Yeah, early in the morning (for me) when I commented so yeah, it DOES change the B+ voltage then the bias... mea culpa. Sorta cheap and cheerful voltage regulation if you will...
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Old 31st August 2011, 12:40 AM   #6
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thats what i thought, but would it change the preamp voltage too? if it does, then is it possible to take the preamp voltage from the input of that circuit with a simple RC filter?
and you mention the bias at 12v becasue of the zener, is this the lowest it will go? for example would it not work on some EL34's biased at about -30v?
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Old 31st August 2011, 06:59 AM   #7
tomchr is offline tomchr  United States
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Let's take half a step back here. What are you trying to do? Why would you scale the power of the amp?

By changing the B+ and bias point, you change the operating point of the tube(s). This will result in a change in power, THD, and a bunch of other things - likely sound quality as well. Why not tweak it until you're happy with the sound and leave it there. No pots needed. I wouldn't recommend tweaking both B+ and bias simultaneously on the fly unless you have a good understanding of what you're doing, the hazards it involves, and the possible ways it can wreck your circuit.

~Tom

Last edited by tomchr; 31st August 2011 at 07:01 AM.
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Old 31st August 2011, 11:12 AM   #8
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bearing in mind its a guitar amp and THD isnt great anyway, and the aim is power amp distortion at a lower output. so i can have 1 amp for practise and just raise the power when needed for performance. i know there will be drawbacks and most f them i understand, this isnt something that will be altered like a tone pot/gain pot. and if its tried and tested i wouldnt mind giving it a trial in a laney LC15 to see how it worked first. the only problem i can see is less current draw from the power tubes therefore a rise in B+ but if i do this with a rotary switch i can switch in some dropping resistors in the PSU.
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