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 Post subject: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Fri 13, 2020 4:16 pm 
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Hi all,

I was thinking abaout adding a picofuse in series with the plate of output tubes to protect them from redplating due to eventual leackage in grid coupling caps or problems in the bias negative voltage components.

I've never seen this before implemented. Is there anything against this? Something I am not taking into account maybe?

For example. In a 7868 tube the max plate current allowed is 140mA. Using a 150ma picofuse may work fine I guess.

What do you think?


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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Fri 13, 2020 5:36 pm 
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Or, you could just use quality capacitors and forget about red plating.


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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Fri 13, 2020 9:10 pm 
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Location: Utica, NY 13502 (USA)
Removing plate voltage from an audio output tube without also removing the screen voltage will usually destroy the tube.

Dave


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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Fri 13, 2020 9:30 pm 
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You'd need to fuse the cathode for that idea to work.

While the max current rating of that tube could be 140ma, depending upon plate and screen voltages you might be able to make it red plate at a lower number than that. Likely only runs around 40ma or so cathode current at idle.

I'd rather use really good quality coupling caps, and a bulletproof bias supply. It wouldn't be difficult to build a protection circuit which would kill the B+ if the bias supply drastically changed.

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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Fri 13, 2020 9:46 pm 
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Dennis is spot on. Good coupling capacitors and if necessary, you could use a 1/4 or 1/8 watt resistor in the cathode circuit. All current goes through the cathode. And you can measure the voltage drop across the resistor to find the current (bias if you will) through the tube. A precision 1 ohm resistor or even a 10 ohm resistor won't upset the circuit.

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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Fri 13, 2020 11:39 pm 
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Location: Utica, NY 13502 (USA)
I had a red-platiing problem with 7868's in a Fisher amp. Even with the proper plate and screen voltages, a stable bias supply, good coupling caps, and tubes that tested good, one tube would begin to red-plate. The cause turned out to be grid-emission in that particular tube. Actually measuring the grid voltage during the condition confirmed the problem.

Dave


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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Sat 14, 2020 3:00 pm 
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Dave Doughty wrote:
I had a red-platiing problem with 7868's in a Fisher amp. Even with the proper plate and screen voltages, a stable bias supply, good coupling caps, and tubes that tested good, one tube would begin to red-plate. The cause turned out to be grid-emission in that particular tube. Actually measuring the grid voltage during the condition confirmed the problem.

Dave

No doubt, Dave. As I read this response ^^^ what came to mind was "Grid Emission" and you confirmed it. A quick and dirty way to confirm is to move the tube and see if the condition follows the tube or stays at the "location". I have a few 7591 tubes that have grid emission. I have them in a separate box, can't seem to throw them away (just like the Mullard GZ-34 metal base tube with one cathode open).

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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Sat 14, 2020 4:31 pm 
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Grid emission....7868.....yep!!
Added small "fuse resistors" in series with cathode.
I have worked S/S stereos where fuse resistors were all over the place.

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"Even if you don't understand Ohm's Law, you are still required to obey it."


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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Tue 17, 2020 7:59 am 
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Location: Powell River BC Canada
If you drop 0.140 Amp from the circuit, how high will the B+ on the filters
shoot up to, to say nothing about the high voltage winding of the power
transformer.

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 Post subject: Re: Use of picofuses in tube plate to avoid redplating?
PostPosted: Mar Sun 22, 2020 5:56 am 
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radiotechnician wrote:
If you drop 0.140 Amp from the circuit, how high will the B+ on the filters
shoot up to, to say nothing about the high voltage winding of the power
transformer.


This is a valid point you should seriously take into consideration. In most tube amps B+ is unregulated and the output stage draws about 80 to 90% of the total B+ current. When your fuse opens the decreased B+ load will lead to a considerable increase of B+ which might put a stress on the filter caps and other parts of your amp. Also, the increased B+ will overload the other tube of the push-pull and the other channel tubes too with potentially damaging effects, ending with the opening of all the 4 fuses and overheating/explosion of the main B+ filter cap. Not the best protection scheme, and very likely the reason why it's not used in commercial circuits. To be safe, at least the filter cap(s) should be highly overrated in working voltage. Also, as it has been already rightly pointed out: you should NEVER put a fuse in the plate circuit of a pentode/tetrode tube since if it opens the screen grid becomes the plate and will melt in a few seconds ,irreversibly destroying your tube.


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