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Old 29th March 2001, 09:47 AM   #11
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Grey:

The tube amplifier project sat in limbo for years and only got started after I rediscovered the power supply transformer, which runs a bit 'hot' at over 500 volts. Power supply caps are these huge 100 uF @2200 volt plastic film caps apparently used in defibrillators; an informal DA check suggests they must be polypropylene. Found 'em locally at an electronics distributor for $25 apiece. I might actually end up with a tube amplifier with no electrolytics!

It'll be push-pull with 6CA7s and maybe 40 watts channel using Hammond output transformers. Hopefully there will be room left on the chassis for the driver circuits!

I'm using the late Norman Crowhurst's book on tube amplifier design for general inspiration and would like to keep the thing fully push-pull, but haven't clearly understood enough to even get a vague schematic drawn up. But I want to attempt something a little more sophisticated than a Dynaco Stereo 70 clone.

So it's partially built, but waiting for me to find money for the output transformers and inspiration for the circuit design. What primary impedance do you recommend?

The main purpose of the project is to learn more about tube circuits and see how they compare in listening quality to a good solid state design. I don't expect a clear winner to emerge even if I manage to do a good design job on the tube amp.

Meanwhile, I got twitchy after reading Sloan's and Self's books on solid state amplifier design and decided to update my Leach amp, and delve more into diff amps and output stage designs, to see what I could tweak and get away with.

I can say with some frustration that the 4.5 boards test below the residuals of my geriatric Heathkit test equipment
(about .04%); I can't begin to think of affording an Audio Precision test system which seems to be what I need to do serious research.

I may have to start a test equipment thread...

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Old 29th March 2001, 05:14 PM   #12
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Damon,
1) How much current can the power transformer deliver?
1a) If it can deliver more than, say, 400mA would you consider 2 pairs of output tubes?
The tube amp I built uses all film caps in the second leg of the PI for the main power supply, to the tune of about 1000 uF. To a solid state person 1000 uF sounds laughable...but the laughter turns embarrassed and dies when you calculate Joules of storage--considering that the rail is 575V. (I'm using Solens. No one around here has surplus film caps. 2200V? Sheesh! You're not going to pop those with any reasonable rail...)
2) How many of those caps do you have on hand? (I'm jealous, by the way.)
Hammond transformers will do just fine. Like everything else, there are people who love them and people who will make fun of you. Ignore them. Old saying: The dogs bark, but the caravan moves on.
Regarding impedance of the primary: There is a single optimal number for every circuit. It will vary depending on the rail voltage (The plate resistance will change.) and a thousand other variables. *Don't* get obsessed over it. Think of it in this way--SS amps regularly face output impedances that vary by a factor of 2 to 1, right? They still function just fine. Same thing is true for tubes. Get a transformer with a primary impedance in the general range you need and you'll be just fine. My RCA tube manual shows plate-to-plate load for a single pair of 6CA7/EL34s to be 6500 ohms. (Class AB, 450V rail...) I've got a Hammond catalog around here somewhere, but I'll have to dig for it. Before commiting to a particular output transformer, consider question 1a above. If you use two pairs of tubes in parallel, you'll have more power (natch), but also lower the effective plate load (think of resistors in parallel). Think it over and get back with me. We'll worry about a transformer then.
I won't get into the sound differences between SS and tubes just yet--not in the mood to start a riot. But it sounds as though you've got an open mind and the curiosity to go along with it. That's all you'll need.
Oh, one thing...need some particulars on the 500V from the power supply.
3) Is that VAC off the transformer? Rectified DC off the caps? (Presumed unloaded, yes?)
I can help with the front end, or you can find a front end on the web. With tubes, you have a *tremendous* amount of latitude as far as mix and match. Build a functioning rear, leave it open on the front end of the DC blocking caps leading to the grids, and build three or four front ends. You can plug and unplug front ends until you find something you like. Since tubes (to a first approximation--we're ignoring grid current) draw no current (like FETs and MOSFETs), all you need to worry about is voltage swing. From what I can tell, you'll need a clean 80-100V p-p. Piece of cake.
My front end is comprised of a 6SN7 (wonderful tube, by the way) differential, also used as a phase splitter, and two paralleled 6SN7s, also differential, for the driver stage. (What Self & Slone call the VA. If you've read The Big Thread elsewhere, you will have gathered that I'm unimpressed with Slone. Self seems to be a somewhat more reasonable fellow, although I still disagree with him on a number of points.)
As far as a clear winner between SS and tube--they're both winners. Use them for what each is good at. Almost always use SS for low end (damping factor). Above that, it's optional, depending on the strengths and weaknesses of a particular circuit.
Back to you, sir...

Grey
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Old 30th March 2001, 12:30 AM   #13
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Hi Guys,

One thing to remember when looking for "relative" HV caps is that you can make them out of standard polypropylene snubber caps.

I do a lot of this for Tesla Coil work. My 25kVDC 0.1 uF cap is made from hundreds of Wima FKP-1 PP units.

This actually turns out to be cost efficient for the very HV stuff (cf. commercial Maxwell pulse caps) and would likely also be viable for the 600VDC range, just parallel 'em till you've got enough uF.

I say this because high capacitance / high VDC caps ain't cheap or easy to come by .....
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Old 30th March 2001, 01:06 AM   #14
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Mark,
Actually, I'd been considering getting copper or tin foil (aluminum is, of course, easier to come by but harder to attach leads to) and rolling my own caps. (Some claim tin sounds better than aluminum, but I have no opinion, not having experimented with this, myself.) The math involved is trivial. Anyone know of sources for polystyrene/propylene in mil thicknesses? I seem to recall running across some Teflon somewhere (which would be better still), but I can't remember where. It's going to need to be pretty thin, and I'll need to get info on dialectric strength.
Or roll a small one and test to destruction...
Solen makes 600V (or 630V, I can't remember which) caps, which is sufficient for me, but they're metallized polypropylene, and I'd rather have foil if possible. Yes, I know that there are several other lines of caps out there, but caps are so easy to make--and so (comparatively) expensive to buy--that they cry out to be done on the kitchen counter.

Grey
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Old 30th March 2001, 01:52 AM   #15
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Grey:

I think Supertronics in Tukwila, WA still has some of
those caps. (425) 251-8484

Any idea where I can get very small value polystyrenes,
say 10 pF? I'd rather use these than silver mica, if I
can get them. They can be low voltage types. I'm also
looking for a US distributor of Wima and Roderstein film caps, especially small 'box' caps.

The voltage was DC, measured with no load; I'm figuring
around 525 volts at 250 ma I think, but with a load and maybe with a choke input on the rectifier, maybe I can pull it down to something that won't fry the tubes. Chassis space limits me to four output tube sockets, which are already installed. This project is going to be a modest one in the first incarnation.

Given that polystyrene is rapidly disappearing, one can only hope that Teflon caps will come down in price as polypropylenes have done. I can't afford to pay such high
prices for 'audiophile' caps! I'll have to make do with Solen, Dayton, and whatever surplus stuff I can find.
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Old 30th March 2001, 02:56 AM   #16
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Grey:

Personally, I think both Self's and Sloan's books on solid state amplifier design are 'must' reading for anyone interested in design and building projects. I wouldn't take
everything they say as Gospel , but they're both very good
for getting up to speed on the subject and to jump off from in doing one's one work. Certainly there are a lot of ways to design amplifiers and Sloan in particular gives a wide choice of design examples to build.

My caveat is that they assume one has a good foundation in semiconductor and amplifier theory, and I don't! I've got
a couple of college-level texts that I'm using to approach
from the other side, but it seems I need to make an acquaintence with Mr. Nyquist, too. My math skills are poor.

The other books are Electronic Principles by Malvino, and
Electronic Fundamentals by Floyd. I'm particularly interested in diff amps and current source designs.

The one thing I did find surprising is that the most popular
design topology seems to have not changed very much since
the late 70's. Leach was one of the early designers to use
a mirror image design with dual diff amp inputs, a pretty
typical VA stage, and a triple Darlington output. There are refinements in design, but 99% of them still use global negative feedback, for example. I always found that configuration visually appealing and it's worked well for me in my audio system.

I have thought about a pure Class A design, but it's really not practical for me. I'll continue to struggle with Class B designs for now.
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Old 30th March 2001, 03:25 AM   #17
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Damon,
I wouldn't think you'd want to go so far as a choke input to the power supply; your rail would drop more than you think. I'm willing to bet that, under load, the voltage would drop to a quite reasonable value. At any rate, the 6CA7 is rated for something like 800V plate voltage. As long as you keep an eye on your plate dissipation, you'll have no problems.
Oh, a good site for tube spec sheets, in case you don't have proper info on hand, is http://www.triodeel.com. Interesting site. Schematics (including guitar amps, if you're interested in such things), but also raw data on tubes. While you're there, double check me on that 800V, as I'm quoting from memory, and my brain ain't what it used to be. But then, it wasn't what it was, even back then...
Antique Electronics (www.tubesandmore.com) carries WIMA, along with a wide variety of other interesting parts; odd, but useful things like copper braid to shield wires with. Their prices are quite reasonable, especially in comparison with what some others are charging.
10 pF film caps are a tough one. I, myself, would love to find such a critter. For the time being, I am using silver-mica caps in that capacitance range. This is one of the reasons that I'm considering doing my own caps--10 pF? No problem. A little fiddle factor should do it. Now, if I can only remember where I saw that blasted Teflon...

Grey
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Old 30th March 2001, 07:52 AM   #18
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Grey,

You can certainly "roll your own", in fact the Tesla world is full of this and I did just this for my "small coil".

The process would be a lot simpler for low V (less than 5kV for me). At HV you need to vacuum the cap and back-fill with mineral oil and the process is a pain in the $%&*.

Home brew CANNOT compete with the commercial caps due to dust, impurities in the dielectric, etc.

Maxwell sell their PP film, but for the cost you might as well buy good quality commercial caps.

I've been there .... I would advise against the path ;-)

cheers, mark
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Old 30th March 2001, 09:37 AM   #19
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Default leach amplifier

i have build a pair of leach monoblocks and I am very content with it. Sounds better than an accuphase I had several years ago for testing, worthwhile 3000$. But a hifi is always a chain, so an amplifier can coop with one set of speakers but doesn't with another. Anyway the class of the leach apm is in the upper price region 3000$ upwards.
nevertheless i have a problem: after switching of there occurs a heavy hum with a delay of 30 sec. the pos and neg branch of the powersupply ( 2 times 30mF )discharge with different speed if a load (speaker)is connected. If the ratio between pos and neg is 1 : 2 ( pos discharges faster) the hum occurs. As this happens on both of the completely separated blocks, I think this is not a wiring mistake.

Especially the n-type-output transistors mj 15003 vary in a high degree, so i have in one 100mA in the second 50 mA bias current ( measure the voltage over the emitter-resistors ). If you use a delay circuit with a relay in the output, as commercial amps do, you will have some problems less.

Good luck with your project, the amp is really good .

hans p
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Old 30th March 2001, 10:34 AM   #20
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Mark,
Is the dust only a problem in terms of voltage? There are several orders of magnitude between 2000V and, say, 75V for solid state. Even a tube rail ca. 5-600V would be less of a problem, I would think. I am assuming that the failure mode is for the dust to either puncture or stress the dialectric, leading to arcing. Or is there another factor?
Of course, if the film is so expensive as to make it uneconomic, then the point is moot. This elusive Teflon I'm remembering didn't strike me as all that expensive, all things considered. I don't remember specific figures, but I do remember a gut feeling that it didn't sound like a bad deal.

Grey
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