• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL34 Baby Huey Amplifier

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#1,363
tristanc said:
As perfect is the enemy of good I'm intending on hitting 'submit' unless anyone sees any issues.

Latest layout is here: https://www.tristancollins.me/diyaudio/ibom.html
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Excellent work. Too bad I have my share of Baby Hueys.

Not an issue per sē, but personally I‘d prefer a terminal block (or solder tabs) near the front tubes, rather than long parallel traces all the way from the back that are prone to hum pickup. If you provide three wire termination you could have the option to use 12ax7 with either 6.3 or 12.6 V, Russian 6n2p-ev (6.3 v only), 7058 (a 12ax7 for automotive use with relatively flexible heater requirements, nominally 13V but will work well at 12.6 V). As you know NOS 12AX7 and even fresh 12AX7s are not inexpensive while the others are often available in great tubes for much less.
 
#1,364
On top of what Francois said, I'd avoid to bring the anode signal (we are talking about 800Vpp with 450V B+) on the other side of the board, but I'd keep those traces as short as possible as in original BH boards.

The jumper to install 6N2P is another good feature.

As for the phase splitter's CCS, a resistor from its bottom to the negative rail would permit to keep the voltage across the transistors low, so to avoid to heat them too much: -15V is enough at the bottom of that CCS.
 
#1,366
Hi guys, I have a question about the grid stoppers in BH amp. R12 and R13 on a KT88. Aren't 4.7 kOhm too high and too dampening? It should be better with 1 to 3.3 kOhm. What do you say?
 

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#1,371
Thanks - my understanding is that ‘coated’ in these charts doesn’t refer to solder mask but rather epoxy or other material intended for higher voltage use. Might be wrong.

I’ve no idea what material is being used by the cheap fab houses so better safe than sorry. Also, there’s creepage to consider. Slots etc. But I’ve ignored that…

I’ve probably been over conservative - but looking at the group buy boards I’d be nervous about some of the clearances. I’m sure it’s fine (no one reporting issues, as mentioned) but for the sake of safety I’ll err.
 
#1,375
gbowling said:
Hey Francois, yes that is my voltage using the MK2 power supply board with the toroidy 250/001 transformer and the 330Vac. And that was unloaded, meaning the output tubes were removed but the 12ax7 was installed.

That's a good idea on making a new chart with fully loaded voltages, I'll try to do that sometime this next week.

As for the 50-55ma bias point, I did not really hear much of a difference. So I backed off a bit and have settled (for now) on a bias point of 48ma. I've run them as low as 40ma but the amp seems better just a bit higher.

The amp sounds fantastic after quite a bit of listening. However, it does get really hot, so I have taken it out of service and have it back in the shop. I'm going to cut some more venting holes in my chassis and I have also ordered a fan. I'm going to mount a standard PC chassis fan on the bottom cover. It's sort of an odd place to put a fan, but I don't have room on the back and don't want it on the front.

With the feet I have on my chassis, it'll be ok on the bottom. I have ordered a 115Vac fan so I can connect it directly to the mains input and not load up my PSU with that. I'm also going to put a resistor in serial with it to turn it down to a fairly slow speed to keep it quiet. I guess I could mount a pot on the back to turn it up/down, but I think just a fixed resistor is ok. I'll have to size it when I get the fans.

The idea is to just provide a bit of airflow from the bottom up through some holes in the top, keeping the noise (and dust) to a minimum. I did also order the filter for the fan to help keep the dust out.

gabo
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Is it possible to use the same output transformer for both EL34 and KT88 tubes in Baby Huey? If yes, which one? What would be the best primary impedance?
 
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#1,378
rankot said:
Is it possible to use the same output transformer for both EL34 and KT88 tubes in Baby Huey? If yes, which one? What would be the best primary impedance?
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@rankot,

member Bfpca (aka Brian) built a successful Baby Huey amplifier (BH) using the 6.6k primary Toroidy. You could read more about his build around post #303.
https://www.diyaudio.com/community/threads/el34-baby-huey-amplifier.326920/page-16#post-5688378

He purchased the 6.6k Toroidy output transformers by accident while really intending to get the 4k transformers. He built the KT88 amp anyway with the 6.6k transformers (which would work better with EL34 tubes) but he found a bit lower power and needed to make adjustments in the bias supply to deal with the KT88 tubes and 6.6k primary transformers. That is explained in post 303. (the BH EL34 PCBs were “corrected” for this issue in Mk 2). He later built am EL84 BH amp (actually using 6P15P-EV tubes with regulated screen voltage (in stead of UL transformer) and liked the sound better than his KT88 BH amp with the 6.6k output transformers.

Bottom line is the 4k outputs in the Toroidy line will likely work better with either KT88 and EL34 output tubes. As Zintolo mentioned above 4.5-5k pick if you choose outside the Toroidy line.
 
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#1,379
Francois G said:
@rankot,

member Bfpca (aka Brian) built a successful Baby Huey amplifier (BH) using the 6.6k primary Toroidy. You could read more about his build around post #303.
https://www.diyaudio.com/community/threads/el34-baby-huey-amplifier.326920/page-16#post-5688378

He purchased the 6.6k Toroidy output transformers by accident while really intending to get the 4k transformers. He built the KT88 amp anyway with the 6.6k transformers (which would work better with EL34 tubes) but he found a bit lower power and needed to make adjustments in the bias supply to deal with the KT88 tubes and 6.6k primary transformers. That is explained in post 303. (the BH EL34 PCBs were “corrected” for this issue in Mk 2). He later built am EL84 BH amp (actually using 6P15P-EV tubes with regulated screen voltage (in stead of UL transformer) and liked the sound better than his KT88 BH amp with the 6.6k output transformers.

Bottom line is the 4k outputs in the Toroidy line will likely work better with either KT88 and EL34 output tubes. As Zintolo mentioned above 4.5-5k pick if you choose outside the Toroidy line.
Click to expand...
Thanks a lot! I have 4k OTFs locally made (also toroid, with 40% UL) and I will try with them. My first choice are EL34 / 6P3S-E, but I want to have an option to try KT77/88 in case I get some.
 
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