VVR Stiffness?

[MarshallPlexi]

Anyone else notice that the VVR mod makes the amp sound/feel a little stiffer? I confirmed this by shunting around the circuit and the amp loosened up a great deal.

Anyone have an idea how to get around it?

[vibratoking]

Maybe you could tell us what amp and if you are scaling the whole amp or just the power section. I currently have VVR installed in a 5E3 and an Express and I haven't noticed any added stiffness. I also can't remember anyone raising this issue in the many threads I have read. Is it stiff with no voltage reduction or only when reduced...and how much reduction? I can feel what you may interpret as stiffness with a lot of reduction, but I attribute that to not moving enough air. I hear and feel absolutely zero difference with no reduction.

[MarshallPlexi]

Oh sorry! It's a top boost ac30. Between full power and bypassed I notice it there. I am using the STW15NK90Z MOSFET. I wonder if that could be different than the standard VVR mosfet that Dana recommends.

[rock_mumbles]

Where do you have the VVR placed in the power supply ...

How are you VVR'ing the amp, whole amp, power only, PI/PA ???

[MarshallPlexi]

Just the plate and screen.

[teemuk]

I'm not surprised amps with VVR's feel "stiffer". A regulated power supply practically introduces no voltage sag at all unless fairly complex circuit additions are made to simulate that particular effect.

[roberto]

teemuk, the purpose of VVR for guitar tube amps is just the opposite of what you stated. Just try to build one.

[printer2]

teemuk, the purpose of VVR for guitar tube amps is just the opposite of what you stated. Just try to build one.

Kind of in line of how I thought they work. They are not a regulator but rather the MOSFET is acting as a resistor which value is set by a pot. So the MOSFET should act to add more sag the more it is turned up, or the voltage turned down.


Now looking at the power supply capacitor and the amp under lower voltage conditions when the VVR is doing its stuff, we have lower voltage to the output tubes and since the load has not changed good old Ohm's Law says that we will be drawing less current out of the capacitors when they are at a lower voltage. At 400V if we were drawing drawing 100mA at 200V we might be drawing 50mA. (Probably does not drop in a perfect ratio but I am too lazy to think it all through)

So while the VVR should introduce more sag the reduced demand by the amplifier should decrease sag.

But then again, I could be wrong.

[LOUDthud]

As the VVR is turned down, the power amp draws less current from the main B+, which still goes to the preamp, so the main B+ sags less. Less sag to the preamp makes the amp sound stiffer because the gain remains stable. Have you tried connecting the VVR to the whole amp?