70 SF Twin screen resistors

[frankeg]

One of the 470 ohm CC screen resistors on the power tube is cracked. I don't have any 470 ohm replacements but I do have four 3 watt 1k ohm flame retardent resistors available. Any issues with using these? Thanks in advance, fg

[Gtr_tech]

It'll work with 1Ks, but the tone will be better with 470s.

[frankeg]

Thanks Gtr tech,
I like the tone so I'll order the 470's, Appreciate the quick response...fg

[pdf64]

OK but still 2 schools of thought; 1 watt or 5 watt? I prefer 1 watt so as to retain a very crude 'fusing' action in case of screen grid shorts. Others prefer a 5 watt rating, to keep the resistor intact if there's a screen grid short, in the expectation that the primary fuse will blow.
Which ever, flameproof/retardent types are essential given the application (looks like you were there already).
The '470 or 1k' thing would only have an effect at very high output, even then it's debatable whether it's significant. Pete.

[Gtr_tech]

I'll use the 1w for 6Ls just because of the tendency to open up under a tube short condition.

[Enzo]

The only protection that resistor offers the tube is limiting the screen current during normal operation. ANything that will open that resistor is evidence the tube had already failed. There is no fusing action.

I agree that the 470s would be better than 1ks. But if you are in a hurry, two 1ks in parallel makes 500 ohms, which is darn close to 470.

[pdf64]

ANything that will open that resistor is evidence the tube had already failed.

Agreed.

There is no fusing action.

If the screen grid draws way too much current for more than a few seconds, then surely the resistor will overheat and become open circuit; how is that not a fusing action? Likely tube failure mode that could cause that is the screen grid wire moving (due heat / vibration) and shorting onto another element.
My thinking is that the affected tube hopefully may just shut down, without the primary fuse being tested, thereby minimising stress on the PT and leaving the amp operational to a degree.
Has my train of thought screwed up somewhere?
Thanks - Pete.

[frankeg]

Thanks everyone! Lots of discussion and good advice. I'm in no hurry so I'll wait for the 470's. The amazing thing is that the cracked CC is clearly visable and I've had the amp on several times. The only indication that something wasn't quite right was a slight bluish flash (really slight) when I switched the stand-by off. All of the CC screen resistors were measured "after" the find and all of them were out of tolerance. My lesson here is to check them right away in the future. I feel very lucky not to have damaged the amp.

[pdf64]

Did you check whether any voltage found its way to the screen grid via the cracked resistor? Open circuit to 9V can be different to open circuit to 500V!
Even if it was totally open, it would just make that tube non-operational, wouldn't damage anything. The damage potential would be mainly from arcing across the crack, resulting in flames, hence the desirability of flameproof/retardent for a replacement. And it's only 1 tube in 4 - you may not have noticed it!
Pete.

[frankeg]

Yes, I checked. Everything is good and nothing is burned including the cracked resistor. The resistor was not fully opened until I started fooling with it. As you mentioned, It may have been arcing which would account for the slight flash I noticed but I had the chassis tube side up each time. I'm awaiting the new 470 ohm resistors which I'll install before doing anything more with it. Thanks Pete...

[Enzo]

If the screen grid draws way too much current for more than a few seconds, then surely the resistor will overheat and become open circuit; how is that not a fusing action? Likely tube failure mode that could cause that is the screen grid wire moving (due heat / vibration) and shorting onto another element.

Well, it would take more than a few seconds for that resistor to open, it would probably take quite a while. They rarely pop like a fuse, they generally will get hot, and in the case of composition resistors, like the stock Fenders, they usually tend to climb up in value. They burn in the process. I find a lot more damaged ones than open ones really.

Yes, of course, if a resistor opens, it breaks the circuit like a fuse, however when someone says "fusing action," I tend to read into it that this burning up of the resistor will somehow be protective. In my view, just an opinion, calling something a fuse means it is protective of somethiing, not simply that it opens. I don't see the burnt up screen resistor as really protecting anything.

COnsider too, that with a 470 ohm resistor, and for convenience 470v, a dead short to ground would try to draw 1 amp from the supply. The resistance of the transformer secondary and the choke probably won't let you do that, but even if it did, that represents 470 watts of failure current from the tranny. Likely a lot less, and that won't be enough to break the mains fuse.

Transformers are just wires, and what kills them is heat. So I'd rather have the mains fuse pop quickly than have it hold on a while. SO when that tube fails, let it take out the mains fuse, the transformer will weather a momentary huge current spike better than it will handle a lower current draw that goes on a while.

[Prairie Dawg]

I've been working on a Bassman 50 and once thing I always do is replace the 470 ohm screen and 1.5k ohm grid resistors because they get heat soaked and drift located where they are. I have some neat small 2w Dales from Mouser that are 1/3 the size of the 470 ohm resistors.

On a whim I checked one of the two-it measured about 560k-but the other one measured an astounding 33k ohms.

[frankeg]

Enso said:
"Well, it would take more than a few seconds for that resistor to open, it would probably take quite a while. They rarely pop like a fuse, they generally will get hot, and in the case of composition resistors, like the stock Fenders, they usually tend to climb up in value. They burn in the process. I find a lot more damaged ones than open ones really".

This is exactly what seems to have happened to me! I think that the properties/construction of the carbon comp resistor allowed it to maintain some continuity even with the crack. Before I really opened it up with my probe lead, it was reading about 10k ohms which varied with the amount of pressure I applied to the probe. I wish I would have had the board side up the last time I turned it on. I'm curious to see if and how it flashed.

[frankeg]

Please check my math.
I have four JJ 6L6GC tubes installed in the twin. My plate voltage is 416 vdc. Using the transformer shunt method, I should be biasing between 80 and 100 ma. Has anyone here pushed these tubes? thanks