Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by IIIBOOMERIII, Apr 10, 2008.
PPIMV and Power Scaling, what is the difference?
PPIMV stands for post phase inverter master volume, which means a pair of pots are inserted before the power tubes to lower the volume.
Power scaling lowers the voltage to the power tubes, which decreases their power and volume.
They are different methods for lowering the volume and retaining tube distortion.
Some prefer power scaling because a master volume control results in distortion from the preamp only.
From the London Power website.
You are basically lowering the B+ on the output ( and perhaps others) so the amp has less output. It is controlled by a pot so is variable from full power to milliwatts. There is still a drive compensation (MV) to lower the drive to the tubes so as to not be over driven too hard when the B+ gets lower.
The drive compensation (usually a PPIMV) on a "power scaling" setup is so you don't let the power tubes distort, which is almost always nasty sounding. What we really like to hear is the phase inverter clipping which is accomplished by a carefully executed PPIMV. That's what most people refer to as "power tube distortion"
Speaking of PPIMV... I have been playing around with the variable-PI-cathode-load-as-PIMV thing on my "FrankenTraynor Deluxe Reverb Master" (i.e. Traynor Reverb Master modded in the direction of a Deluxe Reverb). In a recent thread someone called it "poor man's power scaling" which is obviously not accurate, but of course the term implies it's not supposed to be...
Combining that with a variable NFB knob gives a ton of tone variability and plenty of good tones. The PI cathode 'MV' isn't really such, as it doesn't range down to zero and instead changes the operating conditions of the PI halves... I'd use one of those vaguely defined terms like 'drive' or 'edge' or 'intensity' if I was to label the control (and I might, since I'm leaning towards keeping it). Not saying it's the best, but I like that better than the cross-line phase cancellation MV I was running previously, or the basic OEM pre-PI shunt-to-ground that was in there.
Here's a real poor mans "power scaler". Just lower your first filter cap value down, way down. It will lower your DC plate voltage and you'd be surprised how much AC ripple the plates can handle and not make any hum. I've seen amps with over 100 volts of AC RMS on the plates and no objectionable hum. That'll take an amp that normally has 450 volts DC on the plates down to about 300 volts. A cheap and easy way to turn a Superlead into an 18 watter. Not the best way to go but it works.