VVR distorting at low volumes ?

[Madison]

Plexi type build, hotswitch mod.
VVR on power tubes only.
Doesn't keep the same tone when the VVR volume is reduced;it starts to distort.
Should I go ahead and regulate the entire amp?
Change the MV?
Right now I have a 2204 style MV.

The amp sounded great before installing the VVR so I'd hate to change any of the MV stuff unless I absolutely have to.

Thanks

[LooseChange]

Its supposed to distort.. Power tube distortion.

[Madison]

I mean, I could have the amp with a pretty clean tone at reasonably high volume levels and when I reduce the VVR pot the amp starts to clip/distort.
I was under the assumption the amp would keep the same cleanish tone even at lower volumes?

If not, thit means I lose my setting and have to re-tweak the regular MV every time I back off the VVR pot?

I did VVR on an S-E amp and it worked perfectly.
I know, it's a different beast but was hoping for the same results with a P-P amp.

[PRR]

Are you varying the bias?

[Madison]

Are you varying the bias?

Yes.

[Madison]

I left out the 1st 20K dropping resistor on the B+ rail, that is where the split occurs;vvr/non-vvr
Maybe I should have split it there but left the 20K?
I inserted the VVR between the 1st two 50uf filter caps.

I eliminated the "under board" wire for VVR bias regulation.
Bias adjusts fine.

All regulated voltages seem to be dripping to about 10% when the VVR is rolled back.

http://www.el34world.com/Hoffman/images/Plexi50Old.gif

[kagliostro]

Should I go ahead and regulate the entire amp?

if you leave the previous stages not lowered in B+ their gain remain the same and they can more easily push the final tubes to distort (many people insert VVR for that purpose)

if you want a clean tone you must reduce also one or more previous stages (may be not the whole amp)

if you lowered only final tubes B+ (and don't lower the preamp stage B+) also benefits can came with a PPIMV this will help you to achieve the desired tone

many people like to use VVR for the final tubes + PI + PPIMV + add a drop resistor to the PS chain suppling the preamp (this to maintain the B+ level of the preamp the same of previously present)

all that is easier in cathode biased amps, with fixed biased amps a different VVR circuit (traking bias) must be used

sometime ago FYL posted  an interesting schematic

EDIT: I find the link to the thread where FYL posted the schematics http://www.el34world.com/Forum/index.php?topic=8424.0

Kagliostro

[phsyconoodler]

Quote:"I was under the assumption the amp would keep the same cleanish tone even at lower volumes"

I don't think anyone installs a VVR for that purpose.You can turn the volume down on the amp to get that.The purpose of a VVR is to get lots of distortion at lower volumes with the least loss of good tone.
 

[Madison]

Quote:"I was under the assumption the amp would keep the same cleanish tone even at lower volumes"

I don't think anyone installs a VVR for that purpose.You can turn the volume down on the amp to get that.The purpose of a VVR is to get lots of distortion at lower volumes with the least loss of good tone.
  

Yeah, I know.
I just HATE the "swirl" noise of grids being pushed too hard.
There is a dual potentiometer,is there such a thing as a triplet?
That might fix it,IMHO.

If I ever have a chance to make one of these for my own,from my limited experience thus far, I'd do the SE version; that freakin' rocked!!
It kept the setting/tone/feeling from dimed down to 10% perfectly.

Note: I haven't tried a P-P cathode bias VVR yet.
I am thinking and hoping that is the schmack.

[sluckey]

There is a dual potentiometer,is there such a thing as a triplet?

Yes. Sunn used a triple for tremolo speed.

[kagliostro]

With this you can compose pot as you need

http://it.rs-online.com/web/p/products/0393885/
http://it.rs-online.com/web/p/products/0393807/




Kagliostro

[stratman_el84]

I've found that on standard Marshall-type amps that regulating both the power tubes and the PI (but not the preamp or cathode-follower) works well in most cases. Without regulating the PI along with the power section, the PI can cause nasty blocking distortion from driving the power tube grids far harder than the power tube plate voltage allows. As the plate voltage on the power tubes drops, the threshold for blocking distortion at the power tube grids drops as well.

I would avoid regulating the preamp & cathode-follower stages, as that destroys the dynamics, clean/distorted preamp threshold, and tone.

Strat