6B4G / 6S4S Biasing Question

wrenchone · 13th September 2010, 09:04 PM

I'm looking at doing a push-pull amp using 6S4S triodes and some Electra-Print output transformers. All of the circuits I've seen so far use cathode bias. Is theere any fundamental problem with using fixed bias if the bias source is low impedance? I was thinking of using source followers to drive the grids. I expect I'll get the usual snarky remarks about polluting the vacuum's precious bodily fluids with nasty silicon, but is there any real objection to using fixed bias with directly heated triodes? I'll be using some homebrew SMPS circuits to heat the filaments.

Wavebourn · 13th September 2010, 09:25 PM

I would use them in PP with 350V on anode and -70V negative bias on grids, 15K P-P load. Should sound very nice.

wrenchone · 13th September 2010, 10:19 PM

The transformers I have from Electra-Print are 3k units.

Wavebourn · 13th September 2010, 10:28 PM

Hmmm...

wrenchone · 13th September 2010, 11:53 PM

Well, the original owner of the transformers intended them for a P-P 2A3 application. Right out of the RCA data sheet you get a set of design values that run the 2A3s at -60V fixed bias, 300V, 40 mA/side, about 140-150 ma peak current, 3k plate to plate load, 15W output. There we go... With the source followers I could go AB2 if I so wanted. A answered my own question, so it seems.

Wavebourn · 14th September 2010, 12:03 AM

Right.

My aim was on a clean 10W in AB1 on the edge of max allowed voltage.

wrenchone · 14th September 2010, 12:17 AM

Maybe so, but I want to work with the transformers I have on hand, especially as I got the pair of Electra-Prints for 'prox 1/2 the price that Jack charges for one piece these days. My other option would be some 6k Fisher transformers (I seem to be awash in them these days, as I've been compusively grabbing Fisher tranformers on Fleabay).

Wavebourn · 14th September 2010, 12:28 AM

300V 40 MA may be fine for 6K load as well.

45 · 14th September 2010, 12:34 AM

The source follower is a very good idea. I am having great results in a 45's PP. The best driver so far....IMHO.

With those 3K OPT's I would suggest 250V/60mA, true 8 ohm speakers and it will be fine (will sound!!

). Using a classic 6SN7 cathode follower I remember I got "only" 14W at low THD. With the SF maybe it could be more (depending on how you make it....).

Cheers,
45

Wavebourn · 14th September 2010, 12:55 AM

With 3K OPT you will probably get "That vintage sound"

By the way, what are max grid current specs for either 2A3, or for 6С4С? It seems to me I have not seen them...

13th September 2010, 09:04 PM	#1
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	6B4G / 6S4S Biasing Question I'm looking at doing a push-pull amp using 6S4S triodes and some Electra-Print output transformers. All of the circuits I've seen so far use cathode bias. Is theere any fundamental problem with using fixed bias if the bias source is low impedance? I was thinking of using source followers to drive the grids. I expect I'll get the usual snarky remarks about polluting the vacuum's precious bodily fluids with nasty silicon, but is there any real objection to using fixed bias with directly heated triodes? I'll be using some homebrew SMPS circuits to heat the filaments.

13th September 2010, 09:25 PM	#2
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	I would use them in PP with 350V on anode and -70V negative bias on grids, 15K P-P load. Should sound very nice. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

13th September 2010, 10:28 PM	#4
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	Hmmm... __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

14th September 2010, 12:03 AM	#6
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	Right. My aim was on a clean 10W in AB1 on the edge of max allowed voltage. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

14th September 2010, 12:28 AM	#8
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	300V 40 MA may be fine for 6K load as well. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

13th September 2010, 10:19 PM	#3
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	The transformers I have from Electra-Print are 3k units.

13th September 2010, 11:53 PM	#5
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Well, the original owner of the transformers intended them for a P-P 2A3 application. Right out of the RCA data sheet you get a set of design values that run the 2A3s at -60V fixed bias, 300V, 40 mA/side, about 140-150 ma peak current, 3k plate to plate load, 15W output. There we go... With the source followers I could go AB2 if I so wanted. A answered my own question, so it seems.

14th September 2010, 12:17 AM	#7
wrenchone diyAudio Member Join Date: Feb 2004 Location: Silicon Valley	Maybe so, but I want to work with the transformers I have on hand, especially as I got the pair of Electra-Prints for 'prox 1/2 the price that Jack charges for one piece these days. My other option would be some 6k Fisher transformers (I seem to be awash in them these days, as I've been compusively grabbing Fisher tranformers on Fleabay).

14th September 2010, 12:34 AM	#9
45 diyAudio Member Join Date: Dec 2008 Location: United Kingdom	The source follower is a very good idea. I am having great results in a 45's PP. The best driver so far....IMHO. With those 3K OPT's I would suggest 250V/60mA, true 8 ohm speakers and it will be fine (will sound!! ). Using a classic 6SN7 cathode follower I remember I got "only" 14W at low THD. With the SF maybe it could be more (depending on how you make it....). Cheers, 45

14th September 2010, 12:55 AM	#10
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	With 3K OPT you will probably get "That vintage sound" By the way, what are max grid current specs for either 2A3, or for 6С4С? It seems to me I have not seen them... __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

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