How much performance difference w/ this config?

tomman888 · 22nd September 2011, 06:28 PM

Hi- I've got an EL34 ultralinear hifi amplifier here which I'd like to ut the bias down on for heat reasons.....amp maker calls for 35mA per tube (two per channel) but I've been running it at 30mA per. Sound has remained pretty much the same AFAIK.....

Part of the reason why I'm running it 'lean' now is because I use the 4 ohm taps, so the output xfrmr ratio is more, and it runs a high plate voltage of ~580V....tubes are a matched quad of Ruby tubes EL34B-HT....yes they are Chinese origin but have been more consistent than some more expensive tubes tried in this amp(Genelex kt77).

Thanks in advance.

nightanole · 22nd September 2011, 06:46 PM

well an el34 is 25watt on the plates, apply the 80% rule and your looking at 20 watts, which is what the 35ma bias will get you. Once you get down to 30ma you are at 17.4 watts and the tubes should outlast you...

Depending on the circuit you could just yank out a tube or 2 unless you need that much output.

Weber Bias Calculator

HollowState · 22nd September 2011, 06:58 PM

If running your EL-34s at 30 ma still sounds good to you, then so be it. But I would not run them any leaner because crossover distortion will begin to increase. I've read that the audioable perception of distortion begins around 1 to 2 percent. So apparently you haven't reached that, or maybe are just at it. Of course this will be dependent on the quality of the output transformer as to how lean you can actually go. Also, the system B+ will increase somewhat and possibly alter the operating point of an earlier stage. But I think that's more tolerable then the crossover distortion issue.

tomman888 · 22nd September 2011, 07:07 PM

The output transformers are used in another amp rated at almost double this one (so they should be OK) If I set it at 35mA one of the tubes starts to 'blush' in a dark room if i look close...this is unsettling to me.

Thought the rule was 70% ?

nightanole · 22nd September 2011, 08:27 PM

Alot of amps run the tubes at or above recommended plate dissipation. 70% is fine if you dont really know your amp, or how much you will beat on it.

plate glow at 70% bias...

tomman888 · 22nd September 2011, 09:16 PM

yes I likely recieved a bum tube. What else is new, when you can't spring for NOS tubes @ $75 per tube?

Still sounds nice.

I tested with a tone-generator, a 15kHz sine wave into 4 ohm loads doesn't have x-over distortion, so I guess i'm fine...right?

nightanole · 22nd September 2011, 10:21 PM

Ive had luck with the winged "C". For some reason people buy new amps with them in it, and then just sell the set on ebay. So my last new batch was less then $75 shipped for 4.

22nd September 2011, 06:28 PM	#1
tomman888 diyAudio Member Join Date: Nov 2006	How much performance difference w/ this config? Hi- I've got an EL34 ultralinear hifi amplifier here which I'd like to ut the bias down on for heat reasons.....amp maker calls for 35mA per tube (two per channel) but I've been running it at 30mA per. Sound has remained pretty much the same AFAIK..... Part of the reason why I'm running it 'lean' now is because I use the 4 ohm taps, so the output xfrmr ratio is more, and it runs a high plate voltage of ~580V....tubes are a matched quad of Ruby tubes EL34B-HT....yes they are Chinese origin but have been more consistent than some more expensive tubes tried in this amp(Genelex kt77). Thanks in advance.

22nd September 2011, 06:58 PM	#3
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	If running your EL-34s at 30 ma still sounds good to you, then so be it. But I would not run them any leaner because crossover distortion will begin to increase. I've read that the audioable perception of distortion begins around 1 to 2 percent. So apparently you haven't reached that, or maybe are just at it. Of course this will be dependent on the quality of the output transformer as to how lean you can actually go. Also, the system B+ will increase somewhat and possibly alter the operating point of an earlier stage. But I think that's more tolerable then the crossover distortion issue. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

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22nd September 2011, 06:46 PM	#2
nightanole diyAudio Member Join Date: Feb 2009	well an el34 is 25watt on the plates, apply the 80% rule and your looking at 20 watts, which is what the 35ma bias will get you. Once you get down to 30ma you are at 17.4 watts and the tubes should outlast you... Depending on the circuit you could just yank out a tube or 2 unless you need that much output. Weber Bias Calculator

22nd September 2011, 07:07 PM	#4
tomman888 diyAudio Member Join Date: Nov 2006	The output transformers are used in another amp rated at almost double this one (so they should be OK) If I set it at 35mA one of the tubes starts to 'blush' in a dark room if i look close...this is unsettling to me. Thought the rule was 70% ?

22nd September 2011, 08:27 PM	#5
nightanole diyAudio Member Join Date: Feb 2009	Alot of amps run the tubes at or above recommended plate dissipation. 70% is fine if you dont really know your amp, or how much you will beat on it. plate glow at 70% bias...

22nd September 2011, 09:16 PM	#6
tomman888 diyAudio Member Join Date: Nov 2006	yes I likely recieved a bum tube. What else is new, when you can't spring for NOS tubes @ $75 per tube? Still sounds nice. I tested with a tone-generator, a 15kHz sine wave into 4 ohm loads doesn't have x-over distortion, so I guess i'm fine...right?

22nd September 2011, 10:21 PM	#7
nightanole diyAudio Member Join Date: Feb 2009	Ive had luck with the winged "C". For some reason people buy new amps with them in it, and then just sell the set on ebay. So my last new batch was less then $75 shipped for 4.

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