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Old 28th April 2006, 04:12 PM   #131
Giaime is offline Giaime  Italy
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Thanks for the experience Ian!

I confess I'm now working on a BIGGER one... PP of EL36
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Old 11th May 2006, 02:33 AM   #132
ctaudio is offline ctaudio  United States
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Default Help with more tuning.

I finally finished building both channels of the Baby Huey. I must say I am totally amazed by how good this amp sounds (especially female vocals and strings). Ive done a good bit of AB comparison with my Mark Levinson 331 in the last few weeks. The Baby Huey brings a new whole level of clarity to the music. In certain busy passages, the Baby Huey renders each instrument clearly while the ML may sound confused. Instead of a blur, I can now follow each instrument. Lyrics that was not clear is now much more clear.

In my current build, I replaced the 16ohm resisters in the bias circuit with a trim pot and carefully match the bias currents. I also removed the global negative feedback. But what made the most difference for me was the removal of the 4700uF bypass caps. The improvement was nothing short of dramatic. Taking the cap out and the mid and highs become crystal clear with much more details and dimensionality. The sound stage becomes much wider yet very focused.

This might have something to do with the budget parts I am using. I wont go all out yet. Ive ordered some polypropylene 1uF (Wilma) and some low ESR 470uF and 1000uF (Xicor) to try.

My main reason for choosing the DIY route is to understand how each of the circuit elements and components affect the sound so I can tune the amp to the way I like it. Id welcome any suggestions on what else I might try to get the most out of this already great amp.
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Old 11th May 2006, 04:09 AM   #133
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Default More Tuning

ctaudio,
Glad its up and running and you like it.

The bias Current Source bypass caps I used was a parallel combination of:
4700uF/50 V ordinary commecrial grade electrolytic
470uF/50V Blackgate Standard
1uF Polypheylene Sulphide (PPS) - polypropylene would be good too.

I was interested to note that you think its better without the 4700uF - I'll give that a try myself tonight.

For further tuning you might like to try adjusting the value of the 16K "cross coupling" resistro in the 220K/47K shunt feedback networks. This resistor sets the amount of local shunt feedback.

I have tried values beetween 12K and 47K. 16K is about the minimum I liked, there is a "sweet spot" with values around 16K to 20K. After that you need to go up to about 39K,43K or even 47K to notice any real difference.

As you increase that resistor you are effectively applying more feedback to the EL84s AND placing more demands on the diff amp front end so sound is changing from the sound of the EL84 toward the sound of the 12AX7 diff amp as you increase that resistor.

What Output Trannies did you use?

I looked for a picture of the prototype to post but all I could find was this one. The "Baby Huey" is the middle amp on the left.
The one in front of it is the ECL86 version from which it was derived. Others are 845SET (up the top), a pair of Parallel Push Pull EL34 Triode Mode Mono-blocks and a - shock horror - Solid State AKSA 55 Nivarna Plus (which holds it own with this lot reasonably well).

Cheers,
Ian
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Old 11th May 2006, 05:24 AM   #134
ctaudio is offline ctaudio  United States
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I did try varying the 16K shunt feedback resistor between 16K and 20K in 1K increments. I didn't notice much difference though. But that was before I removed the 4700uF. I'll try a wider resistance range later.

When I remove the 4700uF, I did notice a little less bass. But I find the increase in detail more than compensate for the lower bass. (Even my wife, whos not into audio at all, could immediately tell the added details.) I'm getting some low ESR 1000uF to see if I can find a better balance.

One thing I found about the global feedback is that as I apply the feedback, the base gets tighter, but I loose air. For my taste, I'd prefer an airier sound.

In an attempt to just get off my butt building this amp, I bought a power transformer that gives me only 150mA. With 38mA bias per tube, the transformer gets rather hot. Ive tried reducing the bias current to 30mA, but that doesn't produce enough power to drive my B&W 804 speakers. I have a few CD I like that are recording at a rather low level. I'm now back at 38mA per tube with a B+ of ~300V. I have a RCRC in the PSU of 330ohm/2, 220uF, 220ohm and 100uF. A good amount of power is currently wasted in the dropping resistors. I may try increasing the B+ and reducing the bias current. This is not really tuning, but to make up for a mistake in my transformer purchase. I could just buy another transformer, but I find playing with the PSU educational and a lot more fun.

I'm using Hammond 1608 as output transformers. Did you say you were going to try the 125E?
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Old 11th May 2006, 05:47 AM   #135
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My prototype is currently running some 8K Raa with 40% Ultralinear tap fully potted transformers which are 1970s vintage and despite being grain oriented steel core are 3 to 4 times the physical size of the Hammond 1608.

The transformers I have on order to try are the Hammond 1650E. These are the same impedance spec and UL taps etc. as the 1608 but are rated at 15W and the core size is about the same as the usual Hammond 25W. They do seem to have a different winding structure so who knows until I try them. I thought they might give me a bit more bass or at least cleaner bass (be less susceptible to core saturation).

I used the 1608 in the 6V6 version I built for niece Kristy.

Your observation on global feedback is exactly what I have found. I have a Pentode Mode EL34 Amp which has a "Baby Huey" ish mod. I can't get enough damping factor from that mod alone and have added some global feedback - any more than about 5 or 6 dB of global feedback kills the "air" stone dead.

Cheers,
Ian
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Old 11th May 2006, 05:54 AM   #136
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Hi Gingertube

I have also made a start with your 'baby huey', but for the moment just one channel is built and it is not quiet, yet. It's a prototype channel, in which I can easily changes things - takes more time to build, but can be easiy change things afterwards. Sorry I can't post a picture.

I recognize the parallel push pull EL34 amplifier - it is a project from Menno van der Veen/Plitron, using a single ECC82 as grounded cathode + concertina phase splitter. How does the baby huey compare to this EL34 PPP? I don't know much about tubed circuits, but I never liked MvdV designs that much - that's why I am asking.

Erik
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Old 11th May 2006, 06:12 AM   #137
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ErikdeBest,
Yep thats them.
With the 12AU7 ditched and replaced with an ECC99 and the EL34s in triode mode they are pretty good. Ultralinear Mode did'nt hack it (and was unstable unless the screen resistors were changed to 1K) and Pentode Mode was a complete waste of time. They have been sitting on the workshop shelf for the last 12 months not being used - until I get round to a redesign/rebuild.

The "EL84 "Baby Huey" is my favourite Amp and the one I use most of the time. The 845 SET is VERY MARGINALLY better at some music so long as its not turned up to loud - it just doesn't have the flexibility and all round gorgeousness across all music selections and all volume settings of the "Baby Huey" and it cost a hell of a lot more. Also the 845 SET's 880V rails probably makes it unsuitable for newbies.

The whole idea of the "Baby Huey" was something simple, elegant, cheap, suitable for newbies and above all gorgeous. I'm glad others are finding it as stunning as I do. One CAUTION I would add is that its a reasonably low damping factor amp and may not suit difficult speakers - it almost certainly won't suit electrostatics (although I haven't tried it with electrostatics).

Cheers,
Ian
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Old 11th May 2006, 11:52 AM   #138
SY is offline SY  United States
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You might be surprised. I drove my ESLs with a p-p EL84 pentode amp and, within its volume limitations, it did exceptionally well.
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Old 11th May 2006, 04:36 PM   #139
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Hi Gingertube

thanks for the reply - and I was sure you would have made some modifications on that EL34 PPP. Menno made such nice trafos, but the amps developed by him :S I can't do better, but at least I don't publish. I once asked, here at diyaudio, about opinions for his 6AS7 push pull design - reply was not very favorable...

About the baby huey. I am using a pair of fullrange in open baffle, so a low damping factor is no problem, maybe even better. Let's see.

Erik
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Old 11th May 2006, 11:45 PM   #140
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Default Damping Factor of Baby Huey

I did some measurements last night with 2 different shunt feedback values.
1) With 20K for the cross connect resistor
Vout Open Circuit (Voc) = 7.09 V RMS
Vout 3.5 Ohm load (VL) = 4.46 V RMS
Zout = [(Voc - VL)/VL] * 3.5 = 2.0 Ohms (4 Ohm tap) - at 5.7 Watts out
At full power (approx 7 watts with 280V rail)
Signal at 20K 220K 47K node = 40 V pk-pk
Signal at diffamp anodes = 15 V pk-pk
that is 55 V pk-pk generated across the 220K => 250uA pk-pk
With idle of 500uA each side of the diff amp thats OK ( up to 625 and down to 375 uA).

2) With 47K for the cross connect
Vout Open Circuit Voc = 4.67 V RMS(same input as above)
Vout 3.5 Ohm Laod VL = 3.38 V RMS
Zout = 1.3 Ohms (4 Ohm tap)
At full power (as above)
Signal at 47K 220K 47K node = 72 V pk-pk
Signal at diffamp anodes = 15 V pk-pk
That is 87 V pk-pk across 220K => 395 uA pk-pk (diffamp each side up to 700uA and down to 300uA).

With the 47K it was just starting to sound a little "dry" - a bit of 3rd harmonic distortion from the diffamp maybe.

Note that:
Increase the feedback set resistor by 2.35 times
Increased the feedback voltage at 220K 47K Rfeedback node by 1.8 times
Decreased Zout by 1.5 times
Decreased Gain by 1.5 times

That amounts to an extra 3.5dB of feedback

Sorry I did'nt get to try the 4700uF deletion yet to see how it sounds in my amp.

Cheers,
Ian
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