power scaling wierdness

[daz]

Actually it's the other version called VVR, but same principal. I implemented it in a 18 watt converted to 6v6. I first tried it scaling the whole amp. It worked as it's supposed to, however the preamp loses a lot when turned very low. So i then scaled the PA only and for some reason i cannot comprehend, the VVR pot had no effect at all from full up to all the way down. I then set it back to whole amp scaling and again it worked.

I'm hoping someone can tell me what i'm doing wrong. By the way, i added a second cap can and resistor for the preamp because when scaling the whole amp you have to remove the can and resistor from the preamp. so to make sure the pre had the same filtering as before i added that second can/R you see in the schematic. So w/o further ado, heres the schematic showig how i scaled the PA only. note that this is NOT the same amp, but a schematic of my EL34 amp. I just didn't have a schematic of my 6v6, but they are thier PSU's are configured the same except for values.

[Tubis]

What doesnt happen with the PA scaling? The PA scaling makes the PA distort at lower volume levels, its not a volume control.

[daz]

well, VVR is supposed to drop the amp's volume. And it does when used to scale the whole amp. The volume goes down to where the amp's loudest volume is what normally would be about 8:00 on the master. Much much lower. But when i scale the PA there is zero volume reduction, and thats not right because why would scaling the whole amp do it yet not when the PA alone is scaled? after all, it's the PA that makes the actual volume of the amp happen. So it should drop as much as when scaling the whole amp.

[Ptron]

He says nothing happens from full up to full down so something is definitely wrong. Daz, did you measure B+ out to see if it changes at all? My first guess would be that you still have some connection between preamp B+ and PA B+ that you missed. You don't show a ground connection to the VVR board but I assume you have one or it wouldn't work either way.

EDIT: I started this post before Daz's reply. Daz, I think his point is that your volume won't change much or at all until B+ gets low enough that you start running out of headroom. So for instance if your preamp volume is really low, the VVR won't do much until it is really low. But if it doesn't do anything when all the way off then this isn't what's going on.

[daz]

I did measure it, and the B+ went way down. Can't recall how low but it was on the order of 60v or somewhere in there i believe. And yes, it literally makes zero difference in volume with the VVR know all the way down. I gave up on it because i can't make any sense of it.

[Ptron]

Oh, well then it sounds like the VVR is working. There is probably a resistor you can adjust or baypass to get the VVR to go lower or all the way to zero volts.

It's possible you don't have 60V (peak) of signal at the output: preamp volume is low, master volume is low, preamp doesn't drive the PA that much in the first place, whatever. The VVR just lowers your headroom. If your signal isn't running into this headroom, no matter how low it its, it won't have much effect.

There's also the matter of the nfb loop fighting the change in output and distortion. I don't know what kind of effect that has. I think it's been discussed around here.