Two VVR questions

[surfsup]

Guys, I am putting in the VVR3 from Dana into my amp and have two questions on the installation. Well, three really...because I am still wondering if its better to scale the whole amp or just the power amp.

To install for just scaling the PA, I have the extra diodes (UF4007) that would be required already but I assume I will need a different dropping resisitor to bring the voltages down a bit more for the PI and preamp tubes as per this installation diagram? Looks like 25k 3W is a good replacement for the 22k 3W:

[img:905:185]http://chicagocadcam.com/ChrisHahn/schem/voltages.jpg[/img]

My schematic pulls the OT B1+ off the first filter cap, with the VVR between that and the B1+, the VVR will affect the filtering of the OT/plates? Another filtercap isn't needed?

[img:956:519]http://chicagocadcam.com/ChrisHahn/schem/VVR3_PA.jpg[/img]

[guitarmike2107]

Just scale the screen voltage... easy.

you don't want to scale the whole amp

[John_P_WI]

Surf,

The performance will be more transparent IF you scale the plate and screen compared to changing the screen only - look at tube curves what happens when you only change the screen voltage.

You can install the way you have drawn or add a cap at the node. The way drawn will have a faster response as you do not need to wait for the cap to discharge as you dial down.

BTW, ask for Dana's input, some guys with simple amps like this like to scale the whole amp.

I have power scaled much larger amps / high gain amps and usually do the screens and plates and clamp the preamp voltage to be constant as the plates are dialed up or down. Think of a pre set or fixed power scaling circuit dedicated to the pi / pre amp.

Mind your grounds.

John

[surfsup]

I pulled off the second cap (there are two in parallel up front) and put the diodes on the board. Using a 25k 3W in place of the 22k 3W I get this for implementing the VVR. I think its right:

[img675]http://chicagocadcam.com/ChrisHahn/schem/x10_option.jpg[/img]

[guitarmike2107]

Surf,

The performance will be more transparent IF you scale the plate and screen compared to changing the screen only - look at tube curves what happens when you only change the screen voltage.

Interesting John, Have you compared both ways directly on the same amp?
I know loads of guys who just do the screens are happy with it, I have an amp that I changed from the whole amp to just the screens and pre amp and I couldn’t really tell a big difference (from what I remember), other than the mosfet running cooler.

[John_P_WI]

In my experience in lowering the screen voltage only whether electrically as in mosfet control or by using larger screen r's, there is a greater change in feel and there is more compression. Yes, there are many guys doing this successfully. Isn't this the idea behind amp builder gary moore's circuit, power brake or what ever he calls it, using a big ass rheostat to control the screen voltage?

Changing only the screen voltage will change the transfer curve and ultimately the tone and feel. I do feel a properly set up power scale circuit in which the plates and screens are scaled is transparent for the most part.

Yes, your mosfet will run cooler only scaling the screens, as the plates are still cranking at voltage. If the plate voltage is dialed down too, the mosfet will be working harder and running hotter, but the plates will run cooler - extending the tube life. Yes??

Like anything in electronics, there are many different ways to accomplish the same sonic goal. I don't think any are better or worse, I happen to like the method I mentioned before, but then again I try to keep things constant in the preamp for consistent tones, hence the reason I use a voltage clamp for the preamp supply. Yes, I like high gain.

John

[guitarmike2107]

cool, thanks for taking the time to respond.

I will have to go draw some transfer curves to see if changing the plate voltage/load line slope keeps the transfer curves more similar compared to just compressing the grid curves/lowering the screen voltage.

I am 100% with you on the pre-amp voltages.