7591 glowing screen on Panaramic (Magnatone/Audio Guild)

[alchemy]

Finally got around to finishing up this old basket case of a Panaramic 2x7591 combo I've been messing with on and off for a while. Everything is sounding great except for one of the output tubes screen starts glowing when I have it cranked up and hit a chord. Bias is set at 33ma with 393v on the plates. Plate dissipation for the 7591 is 19 watts. Screen current is 4ma at idle and 13.5ma at full signal (max is something like 30ma). This was calculated by the voltage drop across the 2.2k screen resistors I just put on divided by the resistance. Both screens were glowing badly initially, but with the screen resistors only one is glowing now, and only at cranked, full output. Should I go higher with the screen resistors, or is this maybe a faulty tube?
The screen resistors deffinately helped, but the fact that there is still a noticeable surge of current through the screen when I really get on it tells me something is not right.
Any help is, as always, most appreciated...Cheers>>>

[alchemy]

I just stuck some 3k screen resistors on, just to see what would happen. More compression, less glowing, but still a little bit when I dig in. The screen voltage drops as the current goes up, and while I did not test them at the same time, I'm going to take a guess that the plate is also dropping and falling below the screen, turning the screen into the anode. Just a theory as I'm still learning this stuff! I'll test it out and report back....

[Chuck H]

The 7591 has a max G2 diss of about three watts. So you're over that. IIRC the wrong primary impedance can effect screen diss too and the old Magnatone amps sometimes have strange primary Z. It's likely too that when the amp was designed no one expected the user to OD the piss out of it. Perhaps check the primary Z and if that looks fine just reduce screen voltage until your not glowing the screens anymore. I wouldn't raise the screen resistors. in fact I might reduce them. Too much impedance in the screen circuit is bad too. I don't know whats best for 7591's at your voltage but most 7591 amps I've seen use low value screen R's (470-) or none at all. You might try increasing the value of the B+ dropping R for the screen node. If there is no dropping R and the screens and plates are on the same node you should add a dropping resistor and another filter cap. Then decrease the screen R's to 220 or so. You may want to then decrease the dropping resistor to the preamp to bring voltages back up there.

Chuck

EDIT: There's also the possibility of a circuit failure so check all the components for the screen supply. If, for example, the screen supply cap is open that would leave the screens floating at a higher than normal impedance.

[alchemy]

Sorry it took a minute to reply...Got sidetracked!!

I actually wanted to drop the plate voltage a bit as well, so I am thinking Zeners. I have not ever done this before however.

I have some 1N5346b 9volt 5 watt zeners laying around. Could I stack these in series on the HT CT to ground to get my plate voltage right, then IF I need to drop the screen voltage more, put a couple of the zeners in series with the screen supply? From what limited amount I have read this seems like it should work, but like I said I am new to Zeners. This particular Zener should have no problem dissipating the current (if put in series) correct?

As for primary Z, the manual shows 6600 ohms for 7591's in push pull, and the amp has a OT that appears to be out of a Fender Princeton. Looks like the fender tranny is 8000ohms primary so that shouldn't be too far off (i dont think!!!).

[alchemy]

I took some more measurements and did some more math, and I am a bit confused at this point. The screens are idleing with a 4.5v drop across the (now 1.5kohm) screen resistors. When I really crank it, the voltage drop goes up to 30v. So, 30vX30v/1500ohms=600mW. Well under the 3.3w dissipation limit of the 7591 screen. BUT, the current going through the resistor is (30v/1500ohms=)20mA. Seems like a lot, but the RCA tube manual lists 30mA as withing normal operating conditions for fixed bias operation, so it seems like it should be fine there. Am I doing the calculations wrong? I honestly can't see any reason why the screens should be glowing. I really want to get this amp going!!!

Ideas?

[alchemy]

So it looks like 2x7591 in push-pull should see a 6600ohm primary impedance. The output transformer on the amp is actually 8500 ohms. Could this be causing the problem?