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Old 30th August 2013, 10:34 AM   #1
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Question Building a Baby Huey EL84 amp - might need some pointers

Hi guys, I'm planning a build of a Baby Huey EL84 amp, but have gotten stuck on a few things. I didn't want to clutter up the main thread with silly questions, so I decided to make my own. I hope that's okay.

Diyaudio Wiki link for the Baby Huey

1. My output transformers cannot do ultra-linear (three wired, no air gap, etc), so I will have to go with pentode mode instead. I'm not 100% positive on the wiring for this, but I have a guess. Picture right here. Am I thinking right?

2. Since the amp appears to be designed for ultra-linear operation, will I run into any sort of complications when switching over to pentode? Do I need to modify the global feedback or other things?

3. Resistor-wise... will I be fine using typical standard 0.6W metal film resistors (1%, rated for 350VDC)? Like these. Is that fine? I will of course also be using those 1 and 2 watt resistors as marked on the schematic.

4. The 5 volt supply... I don't have that. My transformer only has 240V and 6.3V secondaries. Can I use the 6.3V secondary (a parallel connection to the heater circuit, or something) instead? This is the real deal breaker for me, that's currently stopping the entire project. I'd really like some input on this. The subject is mentioned in the wiki link, but I don't really understand the explanation given there.

Thanks a lot for your time. By the way, I will make sure to post my build progress in this thread, for those curious.
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Old 30th August 2013, 11:53 AM   #2
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Hi Welcome,

There are no problem using this schematic without UL. Yves'original schematic was pentode mode only.

I did built one using 6P14P-EV and 300V HT and had a great times listening to it.

Cheers
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Old 10th April 2014, 03:11 AM   #3
fleap is offline fleap  United States
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Quote:
Originally Posted by Welcome View Post
By the way, I will make sure to post my build progress in this thread, for those curious.
Hello. Any progress on this build? I was looking at the Baby Huey also, but don't have output transformers with UL taps. I want to build a pentode el84 pp, but want a design that is tested and reliable. Thank you.

fleap
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Old 10th April 2014, 03:54 AM   #4
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UL Taps are not essential but you will probably require some small amount of global feedback without them.

Yves doesn't like Ultralinear and it was'nt used in his orginal design (as linked above).

John (jkeny) converted a Rogers Cadet III to the Baby Huey Scheme, it did not have UL taps either. He thought it a considerable improvement.

Cheers,
Ian
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Old 10th April 2014, 04:10 AM   #5
fleap is offline fleap  United States
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Thanks Ian!

Do you think your circuit will be a good mod for el84 heathkits and other vintage amps?

fleap
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Old 10th April 2014, 05:43 AM   #6
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Fleap,
Yes - the balanced shunt feedback from the EL84 anodes effectively reduces the internal rp of the output tubes. That means that the tubes then better drive the primary inductance of teh output tranny to improve bass response and better drives the leakage inductance and shunt capacitance of teh output tranny to improve the high frequency response. This circuit screws the best out of the output transformers an shifts the critical phase shifts further out making the amp more stable when global feedback is applied.

Characteristics are good stereo imaging and good PRATT (Pace Rhythm and Attack).

I have a few older vintage amps which I will be modifying to this scheme. Once I clear the decks of a couple of new builds.

One of the ECL86 Amps I rectified the 6V3 heater and tied the +ve side of the rectifed output to 0V to give about -7'ish Volts for the for the bottom of the diffamp "tail" CCS. This offset the heater supply by that -7V but did not seem to have any adverse affects. Just be a bit more careful than usual to twist the heater wires and run them well away from the audio circuitry.

Cheers,
Ian

Last edited by gingertube; 10th April 2014 at 06:04 AM.
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Old 10th April 2014, 07:15 AM   #7
Tesla88 is offline Tesla88  Italy
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Can i ask how much signal is required to get full output? I want to use this circuit but in some situations i don't have a signal near 0dB for the input.
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Old 11th April 2014, 02:23 AM   #8
fleap is offline fleap  United States
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Thanks Ian and everyone else for the replies.

So it's as easy as connecting the 150 ohm grid stoppers to the center tap of the output transformer?

I have a couple pairs of output transformers. One has 16/8/4 ohm secondaries. The other has 8/4 ohm only. Which tap would I use for the global feedback?

Thanks!

fleap
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Old 26th April 2014, 05:49 PM   #9
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Hi, I'll like to ask if You think that using El34's ,with adequate OPT, and full pentode mode operation, will retain the basic caracter of this design. Or is better to stick with the 6BQ5's...?
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Old 27th April 2014, 11:45 PM   #10
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The BH scheme trades output tube gm for reduced rp so it works best with higher gm tubes like:
6GW8, 6BQ5, EL34, KT77, KT88 etc.

I modified a PP EL34 Music Angel to this scheme.

Having said that I have also made several lovely versions with 6V6 which is a MUCH lower gm tube.

The "standard" EL84/6BQ5 can be driven to full power from the normal 2V'ish CD player output.

Cheers,
Ian
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