Screens glow

[MHProd]

I removed a PPIMV from a 50W Plexi type amp. The amp overdrives more than before, not in a good way.

I noticed the screens of V4 glow when I play (toaster style). I swapped the tubes and only V4 is affected. Tried another set of tubes, same thing.

Measured some voltage, everything is similar on both sides. B+ is 480V on the plates, 478V on the screens. Checked the 1K screen grid resistor and it looked fine. Changed it anyway and also replaced the PI cap going to V4.

Issue is still here. EL34s are biased around 35ma (around 60%).

What else should I check?

[pdf64]

The lead dress of the control grid pin 5 wiring can be critical.
Twisting them together and routing them close to the chassis, with good separation from both pre-amp circuit parts / wiring and the power tube plate OT leads, can help.
Have you got the grid stoppers (at least 5k6) mounted at the tube socket terminal?
Same for the 1k screen grid resistors, as they're grid stoppers too.
With short leads between the resistor body and relevant socket terminal.

Bear in mind that the symmetry of the signal may vary with signal level etc, so it's possible that one power tube is being driven a bit harder than the other, ie it was like this before but it was masked when the PPIMV wasn't maxxed out.
Try experimenting with various control settings and monitoring the issue, starting with all tones at minimum.

[Gaz]

It's totally normal for the screens to over dissipate with that plate voltage, however ultimately it's not a good thing.

[MHProd]

Yes I have 10K on pin5.

I wonder if I should change the socket as it really only does it on V4. I've had to tighten the metal parts in the pin holes as the tube was very loose.

I liked the amp's sound around 3/4 of the way with the PPIMV.

[Firestorm]

One output tube is being driven harder than the other. Likely it's the one fed signal from the non-inverting side of the PI (the side with the 100K plate R). With conventional 82K/100K plate resistors, the 100K is slightly hotter than the 82K side. If the values are off (like 100K reading 110 or 120 K, it will be even more out of whack. Since you probably don't have a 91K resistor on hand, an easy fix to slightly drop the voltage gain on both sides is to drop the value of the 220K grid load/grid return resistors. Try 200K or even 150K (I personally like the 150Ks on amps that are too easily overdriven).

[Littlewyan]

Measure the 220K grid leak resistors and the 82K/100K PI Anode resistors to make sure they're in tolerence. My amp has a 470VDC B+ and my EL34s don't glow red.

[Stevem]

Crappy sound, signal coming into the grid / pin 5.
Tubes red plating, bias coming into pin 5 / grid.
Something in common like the tube socket is not making a good connection with the tube pin itself?

[martin manning]

I wonder if I should change the socket as it really only does it on V4. I've had to tighten the metal parts in the pin holes as the tube was very loose.

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