Tube Voltage Regulator Flicker...

[sonata149]

I built an SE amplifier using 807s with the screen grid 250 volts fed from two valve regulators. On crescendo passages, the VRs' glow flickers - brightens and dims and even blocks out. At first I thought it was the bass but it also happens with rich female vocals (Mina - this evening).

Has any member had a similar experience with VRs? Any info and guidance would be greatly appreciated. Thanks - Joe A

[EC8010]

You need more quiescent current through your VR valve.

[kevinkr]

Had no idea that Mina was still around. My sister's preferred artist for a while..

Reduce the value of the dropping resistor feeding the regulator tubes, depending on type they can handle up to about 30mA or so quiescent. A slight amount of flicker is acceptable at high levels, but they should never extinguish.

[sonata149]

Thanks EC8010. As I'm not very technical, does that mean I should lower the value of the dropper resistor?

[EC8010]

Yes, as kevinkr says. If you can post a diagram, we can even tell you what value to reduce the resistor to.

[Miles Prower]

Originally posted by sonata149
I built an SE amplifier using 807s with the screen grid 250 volts fed from two valve regulators. On crescendo passages, the VRs' glow flickers - brightens and dims and even blocks out. At first I thought it was the bass but it also happens with rich female vocals (Mina - this evening).

I would not rule out the possibility that your regulator isn't completely stable. You might have a damped oscillation in there. You'll need to o'scope to see if this is happening.

[Johan Potgieter]

I agree with Miles.

In days of yore we had several occasions of less than perfect tube regulators. Also, take into account that with beam tubes the g2 current can vary appreciably between low-max. output. You do not mention whether the VR tubes are used as primary regulators or whether they form part of a higher current tube regulator. Whatever topology, the problem is the same. Make sure the regulator can cope.

[jlsem]

You need more quiescent current through your VR valve.

I agree with EC8010. In a Class A 807 amp the screen grid (@250V) will probably not draw the 5ma minimum current requirement of most gas regulator tubes.

John

[sonata149]

Hi Folks,

Thanks a lot for your feedback. Although it's getting a bit confusing for me (with damped oscillations), I'm starting to think seriously about it. I have dropped the resistance by about 500 ohms and I'm still getting the same results.

I'll post a diagram of that section later on as I'll be out. But I wanted to say thanks to you all for your help.

[kevinkr]

Depending on the value of that resistor you may need to drop to something like half its current value. As a minimum you would probably need >25mA flowing through these vr tubes if they are supplying the screens of both output tubes.

Got your email, am writing a long winded reply..