6HS8 tubes for volume control - Page 2

smoking-amp · 23rd February 2007, 12:10 AM

Well, I don't think there will be any easy way to do this. I would increase the 33K plate load resistors to look more like current sinks (maybe 330K) and connect them to say +400 Volts. Then capacitor couple each plate thru a series resistor to the cathode of a 2AV2 or 1V2 HV diode maybe. The diode plates connect to the +HV. The diodes then need something to draw some DC bias current, so will need high value resistors down to 0 Volts. (adjustable to get correct DC operating point) Will have to experiment with different resistors and currents to get good linearization (and hopefully not fry the diodes or 6HS8). One will really need a spectrum analyzer to monitor the distortion spectrum too (use sound card and FFT analyzer software). This will be quite a research project I'm afraid. Ideally one would try this on a circuit simulator first to get ballpark values, but good luck trying to find a simulator model for the 6HS8!

Another approach could be to use something like 6AL5 diodes in series with the 6HS8 plates to the existing 33K load resistors. (maybe a bunch of them in series) Output would come off the 6AL5 cathode to 6HS8 plate junction. Will require changing the 33K Ohm resistors to adjust amount of correction. A simpler circuit but less flexibility in adjusting. (Keep in mind the cathode to filament breakdown voltages of the diodes in all of these schemes too. Need separate filament power for them.)

edit: Oops, needs inductors across the diodes to keep DC gain variation currents out, then needs added constant DC biasing for the diodes. Worse than the other approach!

In theory one should be able to find a linearizing solution, but the amount of experimental work involved with the linearizer approach is a major obstacle.

Don

jkeny · 23rd February 2007, 12:37 AM

OK Don,

it's not easy - thanks for trying anyway. I'll probably guild the circuit shown and compare to existing 250K pot to judge it's sonic merits or demerits, as the case may be.

Offer stands for anyone who wants to experiment with these - will send one for the cost of postage.

Shoog, being an experimentalist - are you interested?

John

jkeny · 23rd February 2007, 10:38 AM

Ah well no takers

23rd February 2007, 12:10 AM	#11
smoking-amp diyAudio Member Join Date: Dec 2001 Location: Hickory, NC	Well, I don't think there will be any easy way to do this. I would increase the 33K plate load resistors to look more like current sinks (maybe 330K) and connect them to say +400 Volts. Then capacitor couple each plate thru a series resistor to the cathode of a 2AV2 or 1V2 HV diode maybe. The diode plates connect to the +HV. The diodes then need something to draw some DC bias current, so will need high value resistors down to 0 Volts. (adjustable to get correct DC operating point) Will have to experiment with different resistors and currents to get good linearization (and hopefully not fry the diodes or 6HS8). One will really need a spectrum analyzer to monitor the distortion spectrum too (use sound card and FFT analyzer software). This will be quite a research project I'm afraid. Ideally one would try this on a circuit simulator first to get ballpark values, but good luck trying to find a simulator model for the 6HS8! Another approach could be to use something like 6AL5 diodes in series with the 6HS8 plates to the existing 33K load resistors. (maybe a bunch of them in series) Output would come off the 6AL5 cathode to 6HS8 plate junction. Will require changing the 33K Ohm resistors to adjust amount of correction. A simpler circuit but less flexibility in adjusting. (Keep in mind the cathode to filament breakdown voltages of the diodes in all of these schemes too. Need separate filament power for them.) edit: Oops, needs inductors across the diodes to keep DC gain variation currents out, then needs added constant DC biasing for the diodes. Worse than the other approach! In theory one should be able to find a linearizing solution, but the amount of experimental work involved with the linearizer approach is a major obstacle. Don __________________ Ohms Law V = I R

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23rd February 2007, 12:37 AM	#12
jkeny Banned Join Date: Feb 2007 Location: Dublin	OK Don, it's not easy - thanks for trying anyway. I'll probably guild the circuit shown and compare to existing 250K pot to judge it's sonic merits or demerits, as the case may be. Offer stands for anyone who wants to experiment with these - will send one for the cost of postage. Shoog, being an experimentalist - are you interested? John

23rd February 2007, 10:38 AM	#13
jkeny Banned Join Date: Feb 2007 Location: Dublin	Ah well no takers

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