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Old 8th November 2007, 08:17 AM   #331
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Quote:
Originally posted by LowRedMoon
I was thinking the same thing. Do you use carbon pots or attenuators?
Carbon 24mm pots. Mount 'em right there on the panel; another knob to twiddle.
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Old 9th November 2007, 02:26 AM   #332
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Adam,
To attempt to tame that high end hash you can try connecting a zobel network across the 8 Ohm secondary tap. Try a zobel consisting of a 10 Ohm 5 Watt resistor and a 2u2 50 or 100V film capacitor in series.
I found that helped on one of the versions I built.

Cheers,
Ian
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Old 9th November 2007, 08:17 PM   #333
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I'll try the Zobel network, Ian.

What is interesting to me is that I only noticed the high-end hash when using global feedback.

The 'Huey is now part of a video art exhibit that my friend is holding in Philadelphia. I won't have it back for a month, so further experiments are on hold for now. I'm looking for another project in the meantime. I'd really like to try an SET.

Adam
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Old 12th November 2007, 10:34 PM   #334
athos56 is offline athos56  United States
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Default Parts list

Quote:
Originally posted by dstockwell
Thanks for the reply Ian..

So are these schemes now correct.

Quote:
Originally posted by dstockwell
PSU...............

I made a parts list for the cheap minded people like me (~$350)! I used the above schematics, the total cost is without shipping. The parts are nothing I would call High end but they should work just fine. I wasn't sure if the type of LED was super important so I got what I thought would do the trick (it is red). The sheet has my 12b4a preamp on it as well.
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Old 13th November 2007, 07:09 PM   #335
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Default working backwards from schematic

This amp seems like it may make a great project for me to build. As long as I can get my wife's OK. I may have enough spare-parts so that I wont have to buy too much and keep costs down.

However, I have a problem in simply understanding a few of the basics, in particular how to work backwards from a schematic and part values. The Baby Huey uses B+ of 300 - 350V. Given, say B+ of 300V, how do I figure out what voltage is seen by the tube plates? How do I calculate the current? From tube datasheets, I can easily figure out what the operating voltage that for a given plate voltage and cathode current. But given simply a B+ value, how do I get these values?

The answers are probably trivially simple, involve simple Ohm's Law and datasheets. I have looked at various references such as Morgan Jones, but they tend to approach things from the bottom-up ie plate voltage and required current or operating point voltage, rather than backwards.

Thank-you,
Charlie
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Old 13th November 2007, 08:39 PM   #336
Sheldon is offline Sheldon  United States
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Default Re: working backwards from schematic

Quote:
Originally posted by cbutterworth
However, I have a problem in simply understanding a few of the basics, in particular how to work backwards from a schematic and part values. The Baby Huey uses B+ of 300 - 350V. Given, say B+ of 300V, how do I figure out what voltage is seen by the tube plates? How do I calculate the current? From tube datasheets, I can easily figure out what the operating voltage that for a given plate voltage and cathode current. But given simply a B+ value, how do I get these values?
It's easier to work back the other way. If you look at the bias block, which is a constant current sink, you can approximate the bias current through the EL84. The 547 transistor biases the 340. The emitter of the 547 is at 0V, so it's base must be one diode drop higher than that, or about 0.6V. Since its base is connected to the 340 emitter, that point must also be at 0.6V. So, by Ohms law, to generate 0.6V over the 16Ohm resistor will require about 38mA of current. For now, add a couple mA of current to bias the 547 and call it 40mA.

With pentodes you have an additional complication and that is the influence of the screen and its current. You'll notice from the data sheets that when the screen and plate voltage are similar, the screen current is about 10% of the plate current. So assume about 36mA of plate current and find the intersection of 36mA and 300V (or whatever B+ you want) on the plate curves. Then estimate the bias voltage from the bias curves (extrapolate between them if you need to). For a sanity check, look at the typical examples on the data sheets.

At this point your approximations will be pretty good. If you want you can take your derived values and recalculate the current through the 547 transistor (Bias voltage minus 1.2V across the 5.6K resistor) it won't be far from the original estimate). You can also subtract the bias voltage from the B+ to get actual plate voltage. Plug these new values in for bias current and plate voltage and repeat your work on the curve graph. Again, the final results won't be far from your original estimates. If the values from the second iteration are close to the first, you needn't iterate further.

Sheldon
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Old 13th November 2007, 10:50 PM   #337
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Sheldon,

I am relieved that working backwards is tricky rather than it being me missing something fundamental. I tried to do the same with the Aikido and it felt like I was trying to use one variable and a datasheet to solve two interconnected variables.

Now, I do not want Ginger to think that I am criticizing him - his design seems to have rave reviews and I would like to build it, but maybe designers should include the relevant data such as tube current, etc. when they present their schematic. This would easily allow others (ignoramuses - like me) a better understanding of their rationale. Well at least, I would be able to look at the tube datasheets and see where they got their figures - maybe the proverbial lightbulb would actually come on above my head.

Anyway, I will cost out the Baby Huey. I would like an amp for my office. I also costed El Cheapo and decided I couldn't afford it right now. I also saw the Piccolo PCL86 at Walton Audio. It looks as if it is minimal in terms of parts and can put out 4W. I'll likely not be driving very efficient speakers, although I may be able to budget some single-driver bookshelf speakers if I use cheaper drivers.

Either way, I am itching to build more tube stuff, but have to be cautious of the expense.

GINGER......How does the Baby Huey compare with an AKSA? I assume you're the "Ginger" from that forum - if not, Sorry!

Regards,
Charlie
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Old 13th November 2007, 11:13 PM   #338
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Charlie,
Yes I'm the "Ginger" from the Audiocircle AKSA Forum.

Power Amp listening is really subjective particularly when we start comparing Solid State Power Amps like the AKSA and Tube Power Amps.

I have 2 of Hugh Dean's AKSA Power Amps - the AKSA55 N+ (with a couple of extra mods which were covered in the Audiocircle forum) and also the Lifeforce 55 (the first issue of the Lifeforce).

Once in a while I spend an evening cycling power amps (as a sanity check) between the 2 solid state AKSA amps, a 6V6 tube rectified version and an EL84 SS rectified Baby Huey and an 845 SET. I use a Hugh Dean GK1 preamp with a few mods/upgrades (also covered in teh Audiocircle Forum) the most notable of which (for this discussion) is to switch its output from directly off the volume control (tube buffer bypassed) or from the tube buffer output.

With the Lifeforce 55 I use the tube buffered output of the GK-1 preamp (to "warm up" the sound a bit). With the 55N+ I generally use the direct output from the volume control (as the 55N= has a bit of tube like warmth of its own) - ditto for all the tube power amps.

For shear bottom end grunt the AKSA Amps win (as you would expect as they deliver >8 times the power into 4 Ohms). The EL84 Baby Huey is not far behind despite its 10W power rating. At the end of each of these sessions I have always found myself back on the EL84 Baby Huey. Then again if I'm listening to Dylan or JJ Cale or just about any Blues stuff I go for the 6V6 tube rectified (an choke input supply) Baby Huey, its just super relaxed and easy. There is just something about the tube amps which suits my taste - mind you I will never part with the AKSAs. They are my "reference" amplifiers. Once I get a tube amp sounding (to me) better than the AKSAs I know I'm just about done.

Cheers,
Ian
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Old 13th November 2007, 11:55 PM   #339
athos56 is offline athos56  United States
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Quote:
Originally posted by cbutterworth
Sheldon,

I am relieved that working backwards is tricky rather than it being me missing something fundamental. I tried to do the same with the Aikido and it felt like I was trying to use one variable and a datasheet to solve two interconnected variables.

Now, I do not want Ginger to think that I am criticizing him - his design seems to have rave reviews and I would like to build it, but maybe designers should include the relevant data such as tube current, etc. when they present their schematic. This would easily allow others (ignoramuses - like me) a better understanding of their rationale. Well at least, I would be able to look at the tube datasheets and see where they got their figures - maybe the proverbial lightbulb would actually come on above my head.

Anyway, I will cost out the Baby Huey. I would like an amp for my office. I also costed El Cheapo and decided I couldn't afford it right now. I also saw the Piccolo PCL86 at Walton Audio. It looks as if it is minimal in terms of parts and can put out 4W. I'll likely not be driving very efficient speakers, although I may be able to budget some single-driver bookshelf speakers if I use cheaper drivers.

Either way, I am itching to build more tube stuff, but have to be cautious of the expense.

GINGER......How does the Baby Huey compare with an AKSA? I assume you're the "Ginger" from that forum - if not, Sorry!

Regards,
Charlie
My parts list cost out to around $360, any lower would be bargin basement stuff IMO, thats if it isn't already. My first list was $650 with above average parts (ie $15 caps and neato Riken resistors)
I can send the Excel file if it would be handier than the other
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Old 14th November 2007, 12:04 AM   #340
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Ginger,

Good, I thought that Gingertube and Ginger were one and the same. I love my AKSA 55N (nonplussed) and do want to upgrade to the LifeForce someday. For this reason, I have been reluctant to try anoy power amp other than the AKSA becasause I feel a strong sense of loyality to Hugh. His customer service is also the best I have found.

Now for some music - New Age / electronica (Klaus Schulze, etc.) I expect that the AKSA will outperform most other amps that I could afford to own. However, some music (folk such as Dylan, Scottish such as Capercaile, etc.) while they sound good on the AKSA may sound better with a more intimate amp. Hence my interest in a tube amp - although I have no space for one in my main system, but I can use the idea of an office amp to allow me justification to build one.

Ginger, please read Sheldon's last post to this thread and see if he's correct in his method of estimating current draw. If I build the Baby Huey, I would really like to be able to use tube rectification, unless the cheapness of solid-state recitifcation is a big difference.

Athos, I would really appreciate your Excel file. Maybe we could begin a sticky thread containing people's parts lists.

Thanks,
Charlie
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