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Old 14th November 2007, 05:09 PM   #1
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Default Variable cathode/plate resistors?

I am building a number of small testing blocks for amplifier design and would like to know how safe it is to use pots for the cathode resistors and plate resistors for small signal dual triodes.


I am planning on building one or two phase inverter blocks and one or two preamp blocks, each one with variable cathode resistors (0-100k) and adjustable plate resistors (0-1m.) I would like to use standard pots, but I don't know what the ratings on them are. I would assume anything 1/4-1/2 watt should be fine , corerect?


As another option, I could use the little trimmer pots that are used for - bias adjustment in a lot of amps.
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Old 14th November 2007, 06:07 PM   #2
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Power dissipation will obviously depend upon the particular working points. One thing the pots are not likely to enjoy is a large voltage drop. It will likely cause them to become noisy soon.
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Old 14th November 2007, 11:59 PM   #3
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what about wirewound pots?
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Old 15th November 2007, 02:20 AM   #4
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Wirewould pots/rheostats are more likely to tolerate DC current through the wiper, but I don't think they're available in the high resistance values you're after.
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Old 16th November 2007, 06:40 AM   #5
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hmm...any other ideas then?
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Old 16th November 2007, 06:43 AM   #6
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hmm...any other ideas then?
How about arrays of fixed resistors?
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Old 16th November 2007, 06:30 PM   #7
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Default Re: Variable cathode/plate resistors?

Quote:
Originally posted by ThSpeakerDude88
I am planning on building one or two phase inverter blocks and one or two preamp blocks, each one with variable cathode resistors (0-100k) and adjustable plate resistors (0-1m.) I would like to use standard pots, but I don't know what the ratings on them are.
Most pots are rated for 0.25W, and in all likelyhood will be inadequate. I would avoid a scheme like this since these pots have a good deal of stray capacitance that will compromise high frequency performance.
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Old 19th November 2007, 03:49 PM   #8
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well, I don't exactly need perfect performance as this will just to be for testing amplifiers, however I do realize that pots have less than adequate dissipation capabilities. I am however able to find 1/2 watt pots.

Is there any other way than pots?

BTW, I did use a 50k 1/2 watt pot for the B+3 seciton of my power supply since the current demand will be low.


I am using LM317's with passing transistors for 6.3v regulation, and a 6 pos rotary sw to change between unregulated AC, regulated AC, reg. DC, and ac cath. reference and ground reference.

While I am at it , is there any way to have the a variable b+ supply?
I am using a 272FX into a selectable SS or tube (5ar4) rectifier. Output is approx~ 406v w/ tube and 430 w/ SS. I am using a 5H choke for b+2, which will be the most likley used tap for amplifiers for the main B+.

Also, I am using a 50k pot for negative bias voltage. I will only need around 30v at most for it, so would it be better to use something like a LM317 set up as an adjustable regulator to adjust negative bias? This would provide me with better B- regulation as well.
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Old 19th November 2007, 04:25 PM   #9
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Is there any other way than pots?
I find the idea of an array of resistors pretty good. Could be worth a try!

Quote:
I will only need around 30v at most for it, so would it be better to use something like a LM317 set up as an adjustable regulator to adjust negative bias? This would provide me with better B- regulation as well.
As has been pointed out in the other recent thread were you posted, one can't regulate just one of the voltages. Example: say you have a 6AS7 tube with the grid fed by a regulated -30V. If mains climb, so will the B+ on the plate, but the grid remains at -30V. Take a set of curves, and see what happens with constant grid voltage, but varying plate voltage.

Hope you can understand my somewhat clumsy answer, Erik
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