Gemini 2 bias changes while playing

[J M Fahey]

Please increase those screen resistors now.

How much?

Shortest path is experimental: try sucessively 1k > 2k2 > 4k7 until you stop seeing those flashing screens.

Which on close inspection may still get dull red but at least won´t move them into a molten metal puddle or, much more probable, crack under vibration.

[g1]

Is this the correct schematic?
https://music-electronics-forum.com/...1&d=1384986506

I assume you would lower screen voltage by increasing the screen resistance? I am at 470ohm currently.

Do you mean what is shown on the schematic? Or have you added individually at the tubes also?

[Helmholtz]

I'll check to be sure but believe it is an 8ohm tap and an 8ohm speaker. I assume you would lower screen voltage by increasing the screen resistance? I am at 470ohm currently. My gut told me this wasn't normal.. sounds like there are a few that agree.

Do we have a schematic?

(simul-posting )

[tdlunsfo]

G1 that is the correct schematic. Everything is stock. The stock screen resistor was a 470. Now it is 4.7k. See video. Much better.
https://youtu.be/yO3cLLK8MpA

[tdlunsfo]

Here it is dimmed with 4.7k screen resistor.
https://youtu.be/AqE5LSho9aw

[Helmholtz]

G1 that is the correct schematic. Everything is stock. The stock screen resistor was a 470. Now it is 4.7k. See video. Much better.
https://youtu.be/yO3cLLK8MpA

The 470 Ohm resistor shown in the schematic is no screen resistor. It's part of the screen supply. The 40µ cap prevents limiting of instantaneous screen dissipation peaks. Of course replacing it with 4.7k reduces screen supply voltage and thus helps.
I would have started with individual 470 Ohm screen resistors.

[tdlunsfo]

Oops sorry for the confusion. I am no tube amp master. I can go that route if you think it is worth it. I am a little unclear as the difference it would make.

[Helmholtz]

Oops sorry for the confusion. I am no tube amp master. I can go that route if you think it is worth it. I am a little unclear as the difference it would make.

Everything that lowers screen dissipation and prevents bright glow is a good measure. Individual screen resistors would be a more common design. I would expect differences in sound (dynamics) and max. undistorted output. Choose what you prefer.

[tdlunsfo]

I'll look into it. Thank you all for your help. I learned a lot on my first encounter with a power tube grid issue.

[tdlunsfo]

I tried helmholtz suggestion and went with 1k screen resistors and then lowered my screen supply resistor to 1.5k. Looks like some of the screen flashing came back so plan to put the screen supply resistor back to 4.7k. I couldn't hear any sonic difference but do not claim to have the most sensitive ears for these things. Amp stays clean till about 6 out of ten on the volume and based on my research this appears to be close to what others have said. I'm scratching my head on why this happened and didn't 50 years ago. I assume the rise in wall voltage coupled with new filter caps was too much for these screens to take or maybe the 1960's 7591A's where much more robust tubes?

[Justin Thomas]

Amp stays clean to 6 out of 10? Congrats ! You've gone as far as the company founder intended you to go. To be sure, this is and always was conceived as a JAZZ amp. It was never intended to do rock of any kind. I mean, it has an ACCORDION input on it, correct? (Actually you should try that one if you haven't yet.)

Ampegs came with written warnings to turn it down at the first sign of distortion. Advertisements of the time bashed R&R as the worst thing to ever hit muzak.

Any Ampeg tobe that even slightly resembles R&R is purely coincidental, & a bonus.

Justin

[g1]

I assume the rise in wall voltage coupled with new filter caps was too much for these screens to take or maybe the 1960's 7591A's where much more robust tubes?

Probably a combination of some of each.

[tdlunsfo]

I need to get that Ampeg book, sounds interesting.