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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

EL84 Amp - Baby Huey

That looks like a "go'er".
Coupling caps could be 100nF (3.3Hz -3dB point) instead of 220nF (1.5Hz -3dB point). If you have 220nF use them but the smaller 100nF value would be fine.

Run without global feedback unless the speakers really need it (leave out the 12K to 22K resistor). I've generally found I like that local feedback set resistor (15K to 33K) at 15K to 18K but feel free to experiment - Set the local feedback first without global feedback connected. As you increase this (local feedback) resistor you put more demand upon the diffamp front end. You might like to get the diffamp current up marginally (to 1.4 to 1,5 mA) by using 180K tail resistor or used a -320V rail and a 220K.
All that can be tweeked once up and running.

I have to (respectfully) disagree with Chris - that input resistor is in parallel with the 12AX7 Miller Capacitance and WILL affect the top end response. The main top end frequency limit in this basic BH incarnation will be the 12AX7 driving the EL84 Miller Capacitance. Thats why the "full bore" incarnations use a MOSFET Source Follower buffer between the 12AX7 and the EL84s.
Cheers,
Ian

Just for posterity (and for my own benefit, as I haven't started building this yet), here's the corrected schematic following gingertube's suggestions. Attached, below.
 

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Hi,

I am playing around with PSUD2 to model a full wave (or probably bridge) tube-rectified (6AX4) choke input power supply for the Baby Huey. What would be a reasonable value for the constant current load? Has anyone ever ventured into that kind of PSU design for the Baby? Thanks in advance, and best regards,

Rudolph
 
Hi Rongon, I like this circuit a lot - I am also a little allergic to solid state devices in tube amps :)

I have a Hammond 370HX on the shelf, and 2 Hammond 1609 10K trafoes, and also some Hammond chokes to choose from.

Have you started building the amp yet ?

And a question - where do we get the - 320v from ?


Best regards
Arthur.
 
Hi Arthur

Ah, life has been busy. I haven't gotten around to this project yet. It was actually meant for a sand-allergic friend who wanted to build a PP EL84 amp, but he didn't wind up building this one (unfortunately, and after all that figuring and thinking...).

In the meantime, I did build a fairly decent PP 6L6GC triode amp in a Dyna ST70 chassis. Yet another 11 watt per channel PP triode amp that burns up 100W+ from the wall. Just what the world needs, right?

I have a nice chassis and a set of Dyna ST35 iron I was thinking of using for this simple BH. I'd use a full wave diode bridge (4x UF4007 or similar) to get +320V and -320V from that transformer. The center tap is left disconnected. Since the -320V supply is only feeding the cathodes of the two 12AX7's, I anticipate only 4mA max draw on the negative rail, so that adds only an insignificant drain on the current capacity of the transformer.

Your Hammond 370HX is rated at 550VCT @ 200mA. That should work better than my xfmr, actually. You gonna go for it?

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I am not sure :)

At the moment I am in the middle of building at pair of Open Baffle speakers, inspired by Troels Gravesens OBL-10. When they are finished I probably will have to fiddle al lot with the filter, because I didnt follow his plans.

His are 3 way, mine will be 2-way with a 15" Eminence Detalite and a Tang Band 8" fullrange speaker. And I am also building a Fender Deluxe Reverb guitar amp and and ...

I think that the -320v will decide that I dont do this one, because I dont know an easy way to achieve that.

Best regards
Arthur.
 
I think that the -320v will decide that I dont do this one, because I dont know an easy way to achieve that.

There is a lower voltage solution, and better too. All you need is a bias tap from your power transformer, or use a little 25.2VCT power transformer like you can get at Radio Shack. From that, make a -15V to -24V or so DC supply. Make a solid-state constant current sink (CCS) and put that in the tails of the 12AX7's. There are many suggestions in the Baby Huey thread.

The other crazy thing you could do is use 1/2 of a 12AX7 or 12AT7 (12BZ7 maybe?) as a current sink for the 12AX7 long-tailed pair. But that too would require a negative DC rail, and more parts (including another tube socket and heater feed).

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How much does it help to elevate the heater zero potential ?

I had to do that because the second stage 6GU7 has its cathodes up at about +140V. The first stage cathodes are -60V from the heater supply "ground", while the second stage cathodes are +80V from the heater supply. That keeps both within the +/-100V heater-to-cathode limit. No problems so far.

It's usually a good idea to raise AC heaters up about +40V to reduce the chance of hum finding its way from the heater supply into the tubes' cathodes. That was mentioned in the old RCA tube manuals IIRC.

--
 
Thank you - I am not sure that it makes me more brave :)

I have just recently discovered CCS, and I am not at home here. But it is something that I have to learn about.

I have the books by Allan Wright and Morgan Jones, so I guess I should just read those chapters again.

The lowest tap on my transformer is a 100v, but of course I could add a small transformer as you suggest.

Thank you for trying to help me out.


Best regards
Arthur.
 
The lowest tap on my transformer is a 100v, but of course I could add a small transformer as you suggest.

Hey, where there's a will there's a way.

A 100V tap could be used with a voltage doubler to get about -250V for the 12AX7 cathode supply. That should work well enough, and would not be difficult. No need to buy another transformer, and the only silicone you'd need would be two diodes.

--
 
There is a will, and 2 diodes are probably ok :) And there are also 120v taps.

Thanks for helping me out. Sometimes obvious things are so far away.

Have just looked at Allen Wrights schematics. The power amps are balanced, but in a foreseeable (spelling doesn´t look right) future I would like to go balanced, because some of my friends have demonstrated how much speaker cables influence the sound, and you can rid of them with balanced mono blocks.

I didnt believe it for one second that cables has so much influence, but they do.


Best regards
Arthur.
 
Bad transformer?

Hello,

I’ve recently finished constructing a Baby Huey style amp for a gift. Essentially the same but RC biased output stage. The amp sounds good, not as good as the original version but good enough for a present!

However, there was quite a large amount of hum with a shorted input, and one side had more hum than the other. So a little testing and one channel was down at about -65dB, and the other -55dB.

My grounding scheme is a bar of tinned copper wire across the amp and grounded at the input sockets. A little playing around with ground wires, and splitting the ground bar into two channels improved the SNR to where I expected - about -75dB. But only on one channel, the other being at about -55dB.

With a sine wave input, the noisy channel was about 1dB higher than the clean channel. So I went through to check everything was the same between channels.

Swapped valves, checked all voltages, checked each input ccs - all OK.

Now I think there may be some interference from the volume pot (the build is in an old radio and isnt ideal) so I grounded the grid of the noisy channel which dropped the noise down to -65dB - better but not good enough.

The last part I haven’t swapped/checked is the output transformer (Hammond 1608) as it will be quite tricky. Do you think this may be an issue?

Attached is a live trace of the noisy channel.

The peaks cycle at 50Hz, which suggests heater hum, but I have tried a few approaches with heater wiring and makes no difference. Is there any tell tale interference with this picture?

I will keep trying different ground approaches for the bad channel tomorrow, but I exhausted all my ideas today. Will need to keep at it!

Cheers

Charlie
 

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If you haven't yet segregated power ground from signal ground until you reach IEC and chassis ground you might want to try that. You can also try lifting the OPT off of the chassis, just to make sure you aren't getting chassis transferred circulating currents into the OPT.

From the looks of the wave form your problem may well be airborne EMF, certainly the trace position of the peak is showing that. This will entail revolving core center leg direction to an out of phase position from the emitter, but do try the other things first.

Bud
 
One could suspect that the input socket on the faulty channel is not isolated from ground.

Maybe sounds stupid, but check again that you made the ground connections exactly the same way on both channels :)

There is one more thing you could try, for general noise - isolate and elevate the heater ground connection.

I have drawn it, but wonder why it has to be so difficult to attach pictures ??

But here goes: Isolate the ground connection from the heater trafo, so that it is floating. Balance it with 2 100 ohm 2w resistors, and connect this point between a 220K,2w and a 22K, 2w, parallelled by a 1 uF cap, between HT and ground.

Best regards
Arthur.
 
Thanks for the tips, I will certainly try them out.

The fault can't be input socket related as it shows with a grounded input grid. I do have segregated signal and chassis ground, but it may well still be a grounding issue.

My worry is that because of the different signal size at the output, it may be something else?

Note: The very sharp peaks in the picture are my soldering iron. But the main shape was there in the other channel until grounding was adjusted, so I am hoping it is that.

Heaters have an artificial centre tap with 2x 150R to ground, so I may play around with that too - thanks for the tip.

Charlie
 
OK, just got into work and played around with the heaters. I moved the centre tap to another point and it improved the SNR quite a lot. So I will play around with that again when I get the chance.

The noisy channel is actually lower in output than the clean, not the other way round as I said before. About 0.6dB. So need to figure out what is going on there too.
 
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6L6

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Hello all!

I have been researching this amp with interest, and am going to build one, provided I can use up some parts that I have on hand...

A few questions -

Feedback or no feedback? What's the general consensus?

What to do about the Blackgate capacitors? As they are practically unobtanium anymore, is there a good alternative?

Power rating of the unmarked resistors in the circuit? 1w? 2w?

PSU - The impetus to build this is to use up a tranformer and choke that have been sitting around unloved for far too long. A Hammond 272JX and 159T (600v C.T. and 2.5H)

I only have Macs, so I can't run PSUDII, and am slightly in the dark about how this will play out...

StereoBHPSU.jpg


I have a bunch of volts to throw away, I would like to do it with the combo of input cap and the choke instead of some big resistors. (Or I could use a 5R4, but I would rather use the 5U4) Yes, I also wish the choke were bigger in value, but this is the only one I have with the proper current rating.

CCS power will be from it's own 6.3v 1.2A transformer I have.

The output transformers are the only thing I need to buy, I'm thinking of these - http://www.edcorusa.com/p/424/cxpp25-6-7_6k (The single secondary has it's advantages in coupling) or the standard type - http://www.edcorusa.com/p/429/cxpp25-ms-8k Any experience with these or other suggestions?

Thanks in advance for your answers and comments!

-Jim
 
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