Help with VVR

[The_Gaz]

I just put VVR in an AC30 type amp for the first time, and is working for the most part save for a few issues.

Initially I tried scaling the whole amp, which seemed to work, however at bedroom volume the amp lost a lot of gain. Is this normal?

Next, I tried scaling just the power tubes, then the power tubes and PI. I liked this arrangement the better because it seemed to preserve the amount of gain, even at very low volumes. The only problem is that with either the the power tubes, or power tubes and PI scaled, there is a low hum present, which get louder the more I turn up  the VVR control.

I'm fairly certain this is because there is a lot less filterng for the preamp stages now that they are coming directly from the  1st B+ node. Are there any recommended fixes for this?

Thanks.

[zendragon63]

Hey Gaz, I typically get a higher hum in the preamp section--for the same reason--with the VVR scaling just the PA and PI. I add an extra RC before pulling the B+ off and it cleans it right up. See the attached, box "Extra RC".

Others are probably doing something similar or better. Good luck man. Regards

dennis 

[PRR]

zendragon is fast.

[The_Gaz]

You guys are the best. Works perfectly. Finally, I'm part of the VVR club!

Any ideas about scaling the whole amp, and losing gain? Normal when pushing preamp voltages that low?

[six el six]

Where do I find info about this VVR? Is this O'Connor/power scaling stuff?

[tubenit]

Where do I find info about this VVR? Is this O'Connor/power scaling stuff?

Did you try SEARCH feature and look at the extensive amount of information in the ARCHIVES?

http://www.el34world.com/Forum/index.php?topic=6899.0

VVR is by Dana Hall and is different than O'Connor's power scaling.

With respect, Tubenit

[PRR]

> an AC30 type amp ... at bedroom volume

If you are throttling a 30W amp down to 0.3W bedroom power, your 350V must be more like 40V.

> Normal when pushing preamp voltages that low?

Yes.

Techniques that work Ok VVR-ing 30W to 10W for different performance venues get out of bounds when turned to extremes.

Also since you aren't plucking much softer, you probably want to keep the stages before the volume control on nearly full voltage.

Are you really just VVR-ing the final tubes' screens and NOT their plates? That does work, but is unconventional.
 

[The_Gaz]

Thanks for the reply, PRR. I've settled on scaling the plates, screens, and PI. This seems to work very well even down to impractical levels.

In my researching VVR (et al.) online, I found many suggesting the addition of a pre-phase inverter volume control due to the fact that the PI becomes easier to drive at lower voltages. In all honesty, I don't really hear that effect, except perhaps at very very very low volumes, but even then it's heard to say for sure.

[kagliostro]

If you want very low power, in a 4 tube amp like AC30

in addition to the VVR you may consider also to have a mod

for disconnecting a pair of tubes cathode from ground or to connect at ground the input grids

for the mod to disconnect the cathode you can see the Matchless DC30 schematic

Kagliostro

[The_Gaz]

Thanks, Kagliostro. I actually already had that mod, but now I can't decided if it's redundant with the VVR in place. I'll have to play with it some more.

How would you ground two of the power tube grids to ground? Would you have to add two more coupling caps, so each power tube had its own? A schematic would be really helpful for me to visualize it.

[kagliostro]

A schematic would be really helpful for me to visualize it.

if I remember well Da Geezer posted such schematic in a thread sometime ago

EDIT: I've find this schematic posted by Da Geezer



that isn't exactly the schematic I was thinking, this one was to obtain an SE sound from a PP amp

for the purpose to halve the power in a 4 tube amp you must ground tha grids of a couple of tubes (one for each branch of the PP)

Kagliostro

[chocopower]

THAT schematic is wrong. You shouldn´t ground the grids in a fixed bias output stage. In a cathode bias one, with splitted cathode resistors, it works like a charm. (tested)

THATS the right one for fixed bias amps. (i know, i know... We are talking about a AC30)



topic: http://www.el34world.com/Forum/index.php?topic=12251.0

[kagliostro]

I'm not able to find the exact schematic to which I refer

today I had a big problem with my Computer and I lost big part of my archive

Kagliostro

[kagliostro]

I've just find the schematics I was looking for

If I'm not wrong author is Da Geezer (hope someone can confirm)

Kagliostro