Is this Mordor glow on my KT66s normal?

[ezietsman]

This is my recently completed 68 spec Plexi with KT66. I noticed the power tubes glowing brightly when chords and notes are played and the glows goes down when the playing stops.

Here's a short video:
Edit: Uploaded an upright version:
https://youtu.be/dOAzpXv0yPo

I have the 470R screen resistors, 5.6k grid stoppers. My plate voltage is around 480V and I've biased the two tubes so that the highest one is at 39mA, the other one is at 31mA or so. My Classictone choke had a short to the casing so I have a 5Watt 1k resistor in its place at the moment.

I have a Marshall RI Output transformer and a Classictone 18033 PT.

Can anybody tell me if this is normal or not?

[Roe]

plates looked good to me but screens might be a little worse

[ezietsman]

OK, so I just played it in a dark room. The plates (I'm assuming its the big metal box things) did not glow any shade of red I could see, I played for about 10 minutes so my eyes would become dark-adapted. Inside, when I do not play I could see one element having a faint blue glow, which would turn orange and go brighter when I played. The other element next to it (they both look like small pipes running vertically and parallel to each other) stayed orange and glows brighter when I played, as you see in the video. I could not see anything else weird. There was a hint of a very faint blue glow around the plates (when not playing), next to the glass, this could be the thing inside being reflected.

[danman]

The blue glow is normal in some tubes. It does appear that your screens are glowing as the amp is pushed hard. You could try upping the screen resistors to 1k and see if it helps.

[ezietsman]

I think I have this figured out now. I built basically a 68 spec 50 watt but used 470Rs on the screens instead of the 1k. But on the JTM45 there's a 1k from the screen supply that connects to the 470R, so each screen is seeing at least 1k5. On the 50 watt there's is only 1k per screen. I think I must up the 470Rs to 1k5 each. I think what is happening is that at idle everything is OK, the plates are at a higher voltage so the electrons go there, but when I push the amp the plate voltage drops below the screen voltage, or close enough and the screens see a massively increased current.

[coldengray]

What you're seeing is normal for this amp, at least in my experience.

I have 470R installed on the screens but also have the 1k "flying resistor" from the supply. My tubes pulse with orange glow when I push the amp hard.

My B+ is 460v.

[ezietsman]

What you're seeing is normal for this amp, at least in my experience.

I have 470R installed on the screens but also have the 1k "flying resistor" from the supply. My tubes pulse with orange glow when I push the amp hard.

My B+ is 460v.

OK, well I think to keep the screens from glowing that much, I'll install the 1k resistors anyway. This is not a JTM45 but really a 1987 with KT66 and different OT. It can't hurt right? I want to change the input gridstoppers to carbon films too, the CCs are a bit too hissy / noisy for my liking. This will also help me determine if all the noise I'm hearing is coming from them or if there is something else causing trouble too.

[coldengray]

I don't think increasing the screen resistors is going to stop the tubes from pulsing. Let me know what you find.

My grid stoppers are carbon film, no issues.

[ezietsman]

I don't think increasing the screen resistors is going to stop the tubes from pulsing. Let me know what you find.

My grid stoppers are carbon film, no issues.

Increasing those resistors will obviously decrease screen current and the KT66 max screen dissipation is only 3.5W so I may well be over that already. This is just a safety precaution.

[Haze13]

What is the voltage across the screen resistor?
I mean, you can find out what the the current goes to the screens, right?

[ezietsman]

What is the voltage across the screen resistor?
I mean, you can find out what the the current goes to the screens, right?

Yes, I can in theory, however, I have a really cheap multimeter, no true-rms or the like so my measurements may be suspect. I haven't had the amp out of the box yet, I'll make these measurements when I do.

[Haze13]

Better than nothing

[ezietsman]

OK I replaced the 470Rs with 1k 5 watt resistors. The screens still glow when I crank the amp and play, when I stop playing the glow dies down again. I imagine the current must be less now either way, I now have a standard 50 watt circuit, besides for the switching, the bias supply I modified for the KT66s, and the output transformer. I measured a 1mA screen current at idle (1V drop over the 1k resistors). The hottest valve is biased at 38mA, the other one is then at 31mA. I'm going to just assume this is all fine now.

I still need to get and install a replacement choke, still using the 1k resistor in that position.

[coldengray]

What you're seeing is normal.

[Haze13]

What is your B+ and pri of the OT?