Fixed bias VVR help

[6v6]

Hi all, new to this forum and looking for some technical help with a fixed bias VVR installation.

I have a fixed bias VVR2 board from hallamplification, going into a Fender Blues Junior.

I'm only scaling the power stage, and all the voltages seem to scale as expected, between 10% and 100% of the v+ and bias v-

However, when I turn the VVR control to very low settings (say less than about 50v), the tubes (EL84's) start glowing very brightly.

I've tried the following :
- Disconnect the VVR bias and leave the bias fixed at -14v - VVR works OK and tubes don't glow brightly at low settings
- Measure tube current when the tubes are starting to glow bright - they don't seem to be pulling too much current (calculated about 8w dissipation)

Looking inside the tubes (and I'm not an expert at tube internals), there is a coil of wire in the centre which is glowing brightly (is this the grid?)

Could the problem be that the VVR is scaling the tubes to to low voltage (I think they go to about 30v), causing the grids to go positive and pull lots of current from the bias supply?

Any ideas or advice very much appreciated, as I'm a bit confused as to what's going on!

[UR12]

Hi all, new to this forum and looking for some technical help with a fixed bias VVR installation.

I have a fixed bias VVR2 board from hallamplification, going into a Fender Blues Junior.

I'm only scaling the power stage, and all the voltages seem to scale as expected, between 10% and 100% of the v+ and bias v-

However, when I turn the VVR control to very low settings (say less than about 50v), the tubes (EL84's) start glowing very brightly.

I've tried the following :
- Disconnect the VVR bias and leave the bias fixed at -14v - VVR works OK and tubes don't glow brightly at low settings
- Measure tube current when the tubes are starting to glow bright - they don't seem to be pulling too much current (calculated about 8w dissipation)

Looking inside the tubes (and I'm not an expert at tube internals), there is a coil of wire in the centre which is glowing brightly (is this the grid?)

Could the problem be that the VVR is scaling the tubes to to low voltage (I think they go to about 30v), causing the grids to go positive and pull lots of current from the bias supply?

Any ideas or advice very much appreciated, as I'm a bit confused as to what's going on!

Check all the voltages BEFORE you put the tubes in. You should be able to check the VVR B+ and bias by setting the VVR control at full and measuring B+ and Bias. Then turn the control to 12 o'clock. You should see about 1/2 B+ and 1/2 of the previous bias voltage. With the VVR control turned all the way down, you should see 10% B+ and 10% bias. If that all works the VVR is functioning properly. If you see something else other than the above post your findings and maybe I can help. MAke sure the pot on the bias board is turned to MAX (Arrow pointing to towards the back of the board)

[6v6]

Hi Dana, thanks for the quick reply!

I did check the voltages before putting the tubes in, and they looked OK, here's what I measured :
V+ Vbias
356 -14 <Control MAX>
239 -9
96 -3.2
-56.6 -1.9
-33.1 -1.2 <Control MIN>

The dual pot is log taper so 12'oclock is actually getting down to about the 96/3.2 setting IIRC

Then when the tubes go in, things seem to go wrong for some reason, everything works fine until the VVR gets near the bottom of the range, then the tubes suddenly start glowing bright.

Will look into it more tomorrow, but I'm a bit confused, since everything seems OK voltage wise until the tubes go in?!

[6v6]

Hi all,

For the benefit of anyone else who gets a similar problem, I'm pretty sure I found the problem.

Basically I've cut the B+ in the wrong place so the screen-grid power-supply capacitor has the correct (scaled) voltage on it, but the actual connection to the screens themselves is still coming off the non-scaled supply.

So in effect I'm only scaling the plates not the screens, leading to the bad behavior

Not finished rewiring it yet but pretty sure that's the cause of the problem, should've measured more thoroughly before putting the tubes in (thanks for the reminder Dana!)