DHT's, fixed bias, sharred filament supply

audiowize · 22nd October 2008, 05:27 AM

If running a stereo single ended amp with fixed bias, is it really a problem to have the filament supply shared? I could maybe see an issue when it comes to hum pots, but other than that, is it really an issue?
-Paul

kmaier · 22nd October 2008, 07:10 AM

Depends on how you share it. I've seen some budget setups where a single AC filament supply was used for a pair of outputs with a common self-bias resistor and bypass cap via the center-tap. Granted it works, but wherever the hum levels wind up, there you are... no ability to adjust.

I would suggest using a DC supply as AC filaments will still yield some hum with fixed bias. In particular which DHT are you planning on using? I have found that NOS 45 in ST glass tend to balance well (using a padded hum pot) with AC, better than most 2A3 (dual-paralleled triodes) DHTs. A single plate 2A3 is a nightmare with AC filament, pretty much the same for a 300B due to the center-tapped filament setup.

Regards, KM

dsavitsk · 22nd October 2008, 08:32 AM

I did it using a single voltage supply and a separate CCS for each filament.

http://www.ecp.cc/less-pressivo-plus-plus.html

22nd October 2008, 05:27 AM	#1
audiowize diyAudio Member Join Date: Aug 2005	DHT's, fixed bias, sharred filament supply If running a stereo single ended amp with fixed bias, is it really a problem to have the filament supply shared? I could maybe see an issue when it comes to hum pots, but other than that, is it really an issue? -Paul

22nd October 2008, 08:32 AM	#3
dsavitsk diyAudio Member Join Date: Jan 2005 Location: Chicago	I did it using a single voltage supply and a separate CCS for each filament. http://www.ecp.cc/less-pressivo-plus-plus.html __________________ http://www.ecpaudio.com

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22nd October 2008, 07:10 AM	#2
kmaier diyAudio Member Join Date: Jul 2007	Depends on how you share it. I've seen some budget setups where a single AC filament supply was used for a pair of outputs with a common self-bias resistor and bypass cap via the center-tap. Granted it works, but wherever the hum levels wind up, there you are... no ability to adjust. I would suggest using a DC supply as AC filaments will still yield some hum with fixed bias. In particular which DHT are you planning on using? I have found that NOS 45 in ST glass tend to balance well (using a padded hum pot) with AC, better than most 2A3 (dual-paralleled triodes) DHTs. A single plate 2A3 is a nightmare with AC filament, pretty much the same for a 300B due to the center-tapped filament setup. Regards, KM

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