plate voltage and cathode resistor for octal outputs - diyAudio
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Old 25th April 2012, 02:56 PM   #1
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Default plate voltage and cathode resistor for octal outputs

I have seen 500ohms all the way to less than 300 ohms for class A SE outputs for the EL34,6550,6V6,6L6,5881, etc family.

I assume this question may have allready been asked, but for a standard output stage, what combo of plate voltage, output impedance, and cathode resistor would work for all the above?

Oh and include the 6W6 too.

This would be for a standard pentode output stage for guitar amp.

on a side note, is there a good tone control circuit that is low loss and single pot? thinking the moonlight tone control.

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Old 25th April 2012, 03:18 PM   #2
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Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
The cathode bias not only fixes the plate and screen current, also sets the maximum input voltage at the grids pin, so, increasing a bit the cathode bias ensures not grid current at a more high signal voltage. Check the manufacturer data sheet for the bias needed for each tube in particular.

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Old 25th April 2012, 04:00 PM   #3
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what i am looking for is 1 plate voltage, cathode resistor and output impedance that would work for all of those tubes, so i can swap them for different "sounds"
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Old 25th April 2012, 04:25 PM   #4
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1 plate voltage, cathode resistor and output impedance that would work for all of those tubes.......Oh and include the 6W6 too.
The maximum screen voltage for the 6W6 is 150 volts. Unless you are looking at a screen voltage regulator the plate voltage can't be much over 150 volts. The 6550 won't "rock" on 150 won't even sing the blues, but it might cry or wimper a bit.

You want a "one amplifier for every tube" deal? Use one of those Edcor "tinker box" power transformers that have multiple output taps so that you can switch between different B+ voltages. You can even use a seperate transformer for heaters with a variac on the B+ transformer.

Use a switch with several cathode resistors, then use a 5 or 6K OPT with multiple output taps and a switch for the speaker to the 4, 8 or 16 ohm taps.

I used to build "Turbo Champs" with additional switches for tube or SS rectifiers, different cathode bypass capacitors, triode or pentode connection, and switchable feedback.

These techniques can be used to extract the widest range of tones from each tube, but the user has to understand the limits of each combination.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 25th April 2012, 04:49 PM   #5
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You need to consider the screen voltage. Screen grid voltage dictates your idle current flow from plate to cathode. You then use your cathode resistor to set the control grid DC offset (idle bias). So for different B+ and different screen voltages (in different amps, let's say) you will likely come across different cathode bias resistor values and wattage.

I'm currently designing (well, it's designed, just needs to be started) a high voltage (500V B+) SE pentode so I'm in this zone.
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