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Old 25th August 2011, 08:35 PM   #1
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Default variable power control

I've started noticing a few variable power controls on a few guitar amps, mainly egnators, a few marshalls have them now, and this seems like a really useful feature, so does anyone have any idea where to find a schematic for this? or any idea on how to use a pot to control output power? i know about triode/pentode modes, but this seems like something else.
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Old 25th August 2011, 08:44 PM   #2
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Perhaps they are adjusting the screen voltage on the output tubes.
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Old 25th August 2011, 09:06 PM   #3
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The two main methods are (1) PPIMV and (2) Regulated variable supply for the output stage.

PPIMV is Post Phase Inverter Master Volume - dual pot from the output of the PI to the grids of the output tubes. Advantage - cheap, simple. Dissadvantage - does not allow for compressive distortion in the output stage. All distortion comes from the preamp and PI stages.
http://blueguitar.org//new/schem/misc_mod/pi-m-vol.pdf

Power control (VVR). More complex, but normally a zener ref supply with pot to vary the output voltage to the output tubes. Advantage is scaling of power with overdrive to the output tubes clipping at all levels. Dissadvatnage - more complex circuitry.

From teh AmpGarage:

http://ampgarage.com/forum/viewtopic.php?p=65052#65052

Last edited by TheGimp; 25th August 2011 at 09:13 PM.
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Old 26th August 2011, 12:40 AM   #4
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thank you very much, after looking up the VVR a bit, where can i find a schematic or place to get one? as for PPIMV, might be something to add to something else like a JCM800...
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Old 26th August 2011, 02:04 AM   #5
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Quote:
Originally Posted by razorrick1293 View Post
thank you very much, after looking up the VVR a bit, where can i find a schematic or place to get one?
London Power (Kevin O'connor) sells kits
London Power scaling kits

Apparently there is VVR info at the 18watt.com site, but it was down when I checked.
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Old 26th August 2011, 12:52 PM   #6
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hmm, wasnt quite expecting $150 any schematics out there? i found 1 VVR+in+18watt+schem.gif (image)

but im not sure if thats the whole thing, and how would it work for fixed bias amps? and do these things work at anyh power? or would you need different components if you was to add this to a 100/50 watt amp?
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Old 26th August 2011, 01:17 PM   #7
TheGimp is offline TheGimp  United States
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The symbol in the schematic is incorrect. The pass device is a MOSFET, not an n channel J-FET.


Here is some pictures of an installation with pwb layout and parts layout so it is easy to make a schematic from it.

It is simply a FET pass regulator with large resistance pot for voltage control.

The Amp Garage :: View topic - Master Volume in a Fender 5e3 Deluxe??

Search for "Vari-Watt", "Power Scaling", "VVR", "Guitar Variable Voltage Regulator", etc for more info. The "Amp Garage" has a lot of info, along with the "18Watt" site and other guitar amp sites.
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Old 26th August 2011, 03:52 PM   #8
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Note that for a low power guitar amp all you need to make an attenuator is a cheapo wall volume control, the multi-tap transformer kind, such as used in home A/V installations. I am pretty sure that I read that the bottom of the line Weber attenuator, I think it was 5W or 15W or something like that is just an OEM'ed wall volume control.
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Old 26th August 2011, 08:08 PM   #9
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ok, so do these things affect the voltage of the whole amp or just the power section? or can it be made to only affect the power section?
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