EL84/6BQ5 triode pp with no cathode biasing - diyAudio
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Old 14th May 2006, 08:18 AM   #1
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Default el84/6bq5 triode pp with no cathode biasing

Folks, I've had the good fortune to inherit a second hand amp that was dead but with good output trafos and psu tranny.

My grand plan is to make a push pull triode-connected el84 amp using these trafos.

Input stage is pretty much negotiable.

My aims are to avoid cathode biasing resistors and caps if at all possible. I'd prefer to hook up an additional tranny on the chassis and take a bias votage to the grids with the cathodes connected straight to the centre tap. Additionally, if I can reduce the number of caps in the signal path that'd be nice, but I don't really want to go to the extreme of interstage trannies.

Planning on running minimal or no output stage or overall feedback. This amp will be hooked up to single driver Fostex FE168 Sigma speakers with a pretty benign impedance load.

I'm still a relative newbie, just want to get the best results I can out of the psu, el84's (4 of) and output trannys I have at my disposal.

Won't be filling a room any bigger than 12ft x 18 ft by 8ft (w/l/h) and no desire to bang head. 1.5 watts should do me.

Any suggestions gratefully accepted.

TIA

Drew
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Old 14th May 2006, 11:14 AM   #2
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Hi DrewP

You are a lucky guy! Push pull amplifiers with EL84 are the center of discussion at diyaudio.

Jane has just posted a thread with DC coupled stages
Just another PP...
This one uses cathode resistors and capacitors...

gingertube is also active with his baby huey
EL84 Amp - Baby Huey
auto bias circuit - or just fit in SY string of leds: both have no resistor + capacitor to bias the EL84.

And then there is this last thread, with some more information on EL84 PP
EL84 PP Diy. Pls help me!!!

At glass-ware you will find some articles on biasing with the lm317 or lm337.
http://www.glass-ware.com/tubecircui...o_Biasing.html
http://www.glass-ware.com/tubecircui...Biasing_4.html

Good luck reading them all!

Erik
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Old 14th May 2006, 09:14 PM   #3
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True but all of these examples are using CCS, diodes or some similar arrangement between the cathodes of the el84's rather than an external negative bias voltage being supplied to the grids.

All those threads came up when I did a search.

Not to say that they're not good designs, I'd just prefer not to have anything under the cathodes if i could get away with it.

sorry, should have specified a desire to stay away from diodes etc as well. That said, the LM317 CCS could be interesting to play with.

many thanks,

Drew
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Old 14th May 2006, 10:34 PM   #4
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Hi DrewP

Yes, you are right, none of the schematics use fixed bias and of course this is possible: take any RC coupled design, tie the cathodes to ground and put 10V negative on the gird, after the capacitor of the RC coupling. EC8010 defended the use of fixed bias and a interstage trafo in the design of an EL84 to prevent problems with recovery - this is also buried in some recent thread.

Erik
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Old 15th May 2006, 02:36 AM   #5
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Drew,

IMO, the solution is pretty obvious. Build a variant of Poindexter's "Musical Machine" that uses EL84s as the "finals" instead of 6V6s.

Poindexter's Site

If you want vacuum rectification in the B- supply, emulate "El Cheapo". Sub. a 6AL5 for the 12AL5 and power its heater with the little Allied trafo's filament winding.

"El Cheapo" PSU Schematic
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Old 15th May 2006, 07:31 AM   #6
SY is offline SY  United States
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Erik, the LED string and the voltage regulator feeding the cathode are actually fixed bias, not cathode bias (in the sense of the classic "automatic bias"). They just apply that fixed bias to the cathodes rather than to the grids.
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Old 15th May 2006, 08:46 AM   #7
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Hi SY

Thanks for correcting me! So I learn a little more every day - and can say I am close to build my first fixed bias amplifier!

Erik
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Old 15th May 2006, 05:10 PM   #8
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Default Re: el84/6bq5 triode pp with no cathode biasing

Quote:
Originally posted by DrewP
Planning on running minimal or no output stage or overall feedback.
Hi Drew. Mind if I ask, have you had a bad experience using the OPT secondary to apply cathode feedback? My experience, in SE at any case, is it works very well with EL84s.
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Old 15th May 2006, 08:28 PM   #9
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No, no bad experience, just prefer to avoid if possible.

I'll be treating the amp to a pretty benign load and there's something about sending a correction from before to be processed through the amp now that my mind doesn't really like the idea of, no matter what the lovely sine waves tell me.
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Old 15th May 2006, 09:31 PM   #10
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Trust me Drew, I t'aint no sine wave guy though I don't object when they look better either. Cathode feedback stays within the stage, except arguably for those who consider the OPT another stage. I highly recommend giving it a spin while breadboarding, you might be surprised. I was.
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