Quick Question on screen resistors

[RWood]

Is 1987 Plexi 1k 5w screen resistors overheating indicator of parasidic oscillations? It only happens when run wide open. I just threw it together PTP to see if I like it so probably wiring proximity issue although it is very quiet except for very faint 60hz (less than my bandmate's new Carvin)
EL34s have 482 on plates, -35 on grids, bias at 37mv.
Lost scope in tornado, so I can't check 'til I get one.
I seem to remember chopstick/capacitor testor but haven't found old post.

[Stevem]

What brand output tubes?
Are all the 1Ks burning up?
37 ma with 482 on the plate is getting pretty close to 70% dissipation at idle I would think without doing the math?
Try install 15k grid resistors to pin 5 of each output, if it stops the issue than you have a layout and or filter issue going on making for the oscillations more than likely!
Sometimes all you need to do is to shield the grid wires off of the input jacks and all will be well, you should only ground one end of this wire and the grounding point should be at the last preamp filter can.

[Phil_S]

...Sometimes all you need to do is to shield the grid wires off of the input jacks and all will be well, you should only ground one end of this wire and the grounding point should be at the last preamp filter can.

I'm not trying to be disagreeable. I want to introduce a slighly different point of view on the ground point. I've always been told to think of the shield on a cable as part of the chassis and not part of the circuit. For that reason, it should not be grounded on the circuit ground. The shield can be grounded at any convenient screw near one end or the other, not both ends. Just my 2¢. There are many opinions on what gets grounded where and every likes to swear by what works for them. I'm not saying anyone is wrong.

[matt h]

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[RWood]

Thanks for your responses. As I said I just threw it together to see if I liked it. Pretty much nailed it with only that one issue so I'll either redo PTP with a little more logic or get a board. The tubes were horizonal in center of the chassis (Carvin TS100 power amp that fried the pcb) and the EL34's grid wires ended up pretty long. I suspect that that is the issue and maybe shielding them will do the trick. No noise though.
I measured the OTs at 12K (8ohm into 8ohm tap) using turns ratios. I figured 8ohm into 16ohm tap at would reflect about 5-6k to primary, but tonally sounds same with only slight volume loss. Should I use the 16 tap with 8ohm cab?
The Carvin had one OT for each channel driving 2 EL34s each so I used one on this build, but I think 12K was for more linear Power amp. Would that be correct?

BTW the amp absolutely kills with 2000pf on brite channel, and 500pf on normal. No cap on normal channel seemed too Fendery to me. PPIMV also great!

[Gaz]

Phil, I've never heard of that before--but I totally dig what you're saying about shielded wires being thought of like an extension of the chassis. Cool perspective to keep in mind (especially in those cases where the two schools of thought are inclined to overlap).

I first heard it described in that way in Merlin's 1st book. I do it exclusively now and never have any weird ground issues anymore like I would occasionally using circuit ground.