Glowing screen grids

[azatplayer]

I have this going on in a Concorde clone i did a while back.
Even at low, like 4, volume and half on the guitar , they will glow as i play.
Got 1K 7 watters on the screens and 1k5 on the signal grids as spec. Voltages are pretty close to equal with the plates.
Is this just a response by the tubes to 500Volts? Ive tried svet's, GT's, JJ's, winged C 6L6's. The only things that dont do it are my bigarse Valveart KT66's.
SOunds ok, just dont like seeing them do that.

[Roe]

voltages are over spec so it can happen. does it draw a lot of current?

[Colossal]

Azatplayer,

The load line is 4k8 in the Concorde and 5k8 in the K60. Have you measured the grid current with a big signal? Just wondering what it might be? Those screens have got to be drawing a good bit of current with a big swing. Do the screens pulse/glow in time with the huge power chords you are pumping through the amp? Sounds about right and I'll bet it sounds great Part of the performance of the amp; just the price of getting the tone! You could put a fat dropping resistor off B+2 to lower the screen voltage on the way to the screen resistors if it's bothering you.

[tubeswell]

Is the screen node regulated with a choke?

[azatplayer]

Its running around 485-490V which is about right. Screens are on the choke, and its bigarse.
Dave grid current rises up into the high 100's with big chords. Im biasing around 30ma and with medium kinda playing it rises to 75-90 but really grind it and it will see mid high 100's.
Thats really bothered me. So ive tripple checked my bias supply, bias supply voltage remains static, pins 1 and 8 are grounded well thru the bias checking circuit. There is a snarl over the top end that ive been trying to dial out, a nasty brightness, but it sounds pretty good, tho i have nothing to compare it to.

[pdf64]

What's the resistance of the choke?
My preference is to add resistance to get it at least 500ohms and use a large cap, eg 100uF at that node.
That arrangement serves to limit the screen grid current on heavy sustained overdrive (as the node voltage will drop) but retain the dynamics.

[azatplayer]

I've not strayed from formula, has the heyboer specs iron in it for this build. Really have it pretty close to a clone bar the use of different types of cap and resistor.

[Structo]

It's strange but I hear this complaint about some amps.

I think sometimes the choke alone doesn't drop the screen voltage enough.
I've been told that during play the inductance does act against the screen voltage and keep it down below the plate voltage but I'm not sure.

But one thing is for sure, running the voltage that high is pushing things, don't you think?

Most amps are running beam pentode tubes down around 420-450v.

I'm afraid to look at my power tubes when I'm cranking the volume myself.

[azatplayer]

The voltage lines up with the charts I have for the K50. The pt is what is specd for it too, 360 a side primary. Was KF following superlead voltages I suppose. I have been thinking about lowering it tho and adding some resistance to the screens. Just that I'm at spec.
I might pull the wire from the screen node to the screen resistors and swap in a 1K in its place, see if that puts the brakes on it.
Other than murdering the tubes, what effect on tone does having the screens draw too much current have? Could that account for the raspy top end bite?
Doesnt sound right up the top end. However when the amps wide open, it sounds amazing. Tho very loud..

[M Fowler]

The Choke should be 10hy 200mA not sure what the spec'd DCR is.

Don did you use the larger main filter caps a pair of 220uf/350v.

1k screen resistors.

Mark

[azatplayer]

Yep Mark

[M Fowler]

Well as usual your just running the crap out of your amps over there

Perhaps larger screen resistors or lower that B+

[azatplayer]

Hehe that would also explain the sore ears!

[pdf64]

See Andy Le Blanc's idea regarding screen grid supply -https://tubeamparchive.com/viewtopic.php?t=14770

[Gaz]

As you've probably gleaned from this thread - it's normal. Power pentodes draw a butt load of current. If you dropped 6L6s, beam tetrodes, in there you wouldn't see the screen glow because they'll be drawing way less screen current. Of course they can't dissipate as much power either, but they just aren't as masochistic as EL34s or the like.

I've had screens glow in a Plexi style circuit with only 420vdc B+ and 1k screen resistors. I think a non-master design with they typical power supply and power pentodes is just asking for screen abuse.