FrankCooter

Here's a view of the underside of the amp I brought to the LA meet. It's a simple 2 stage direct-coupled design with Electra-Print 7k:32 output transformers. First stage is a CCS loaded 6SN7 with LED bias. Second stage is a 6AH4 power triode. It was designed to produce about 12dB of gain and a 1.5W output. Power supply is solid -state rectified followed by dual-mono tube regulators. Filament supplies are DIY Hifisupply originally intended for DHT's. This allows for easy future conversion to DHT outputs if desired. Nothing fancy here. This was built for a friend and the emphasis was on safety, stability, and dependability. I'm honored that it would inspire Enrique to pursue a similar construction technique. When I get a little more time, I'll add some suggestions for construction practices. I have acouple of extra power supply boards for the KGSS if your interested. Probably could scrounge up a chassis as well.

From time to time I think of building a "poor man's" all DHT electrostatic amp. One idea is building an "Egmont" type circuit using Eimac 3C24's. The 3C24 is sort of like a DHT 6SN7 on steroids. Usually available on Ebay for about $25. The big problem, besides the exposed plate and grid connections, is the filament supply complexity. With a little creativity, you could probably do it with 2 6A supplies per channel. At the very least, it would be an interesting novelty, and might possibly be considerably more.

I've been following this thread myself. There's some interesting discussion going on, mostly about transformer design in relation to headphones. The design being discussed above is only going to work for something very sensitive, like a Grado. I imagine that each of those OPT are going to cost more than an RS-1. I wouldn't mind having a pair if the price were right. The Russian 4P1l and it's big brother the 6p21S are both excellent sounding tubes that are plentiful and dirt cheap.

Interesting project Remolon. Unfortunately I'll have to second all Kevin said about the resistively loaded 2A3 outputs. There are also factors of low gain, high plate current for decent linearity, and high Miller Capacitance at the input. Although some people do it, the 12AX7,with its high rp and low current cabability is a poor choice to drive a DHT. Cascaded 6SN7's would be the traditional way to go. Better yet would be a single high mu/high current pentode like a 7788 or D3A. The only reasonably sized traditional class A DHT that I'm aware of that can handle 600V on the plate with an appropriate current and gain level for a project like this is the 801A. It's a great tube and would be my first choice for a project like yours.Unfortunately it's not in current production and an NOS quad, when available, will probably set you back $400.00. If you're stuck with the 2A3 outputs and 300V B+, you can abandon the plate resistors in favor of chokes or output transformers.There's a lot to be said about not burning up half your B+ in waste heat. Any pp speaker transformer will work as long as the primary is suitable for your output tubes. The easiest thing to do would be to build a classical pp design like the Williamson and take your headphone output off the output transformer primaries via coupling capacitors. The 6P21S does indeed look interesting. I'm going to hop over to Ebay and pick some up. Don't get discouraged, you're not alone, For me at least, rarely does something work right the first time (and sometimes not the 10th).

Interesting project! I'm a fellow S. Californian. I've got wood and metal working capabilities as well as a garage full of tube related parts. Let me know if I can be of assistance. I have a quad of EL156 if you're interested in doing something really different.

Really appreciate all your time and effort maintaining the site! I'm not here a lot, but I didn't realize how much I'd miss it until it was gone. Thanks Todd!

Lots of interesting and informative work going on in a still relatively underexplored area of tube application. So far, at least on paper, the circuit that interests me most (and I understand the least) is the modified SRX circuit. Still don't fully understand the cross-coupling and the ultra-low voltages on the 12AT7 plates. If I build this, I'd like to with 6BL7 instead of ECC99 outputs, use a C- bias supply, and run the HV rails at 400V. Any reason not to up the 12AT7 stack to the full HV rail if the output bias is now handled by a C- supply? If it's not to much trouble, I'd appreciate you posting a schematic. I'm a bit puzzled why you didn't use a voltage divider off the negative HV rail to offset the operating point of the first stage of the BATE and the El34 amp. Seems like this would have been a good way to eliminate the input cap and perhaps enable direct coupling between the first and second stages. Also wonder why you went with the 12AX7 instad of the 12AT7.

I agree. Most people who would want to build even the simplest electrostatic amp would want to be more than just "board stuffers". Building any amp should be an educational experience. I imagine a potential builder of this type of project is someone who's already built a cmoy, a spud, or maybe restored a Dynaco. They'll be interested in at least a conceptual understanding of the circuit. So what's going on with the first two stages of the SRX? You really don't see very many cross-coupled circuits in tube audio. At first glance it looks like a Van Shoyoc differential amp, but unlike the Van Shoyoc circuit, the first tube pair are not cathode followers. So if the first tube pair are actually amplifiers, are the second pair simply some sort of active load, or are they amplifying too? Since the second pair have plate loads and the output signal is being pulled of the plate rather than the cathode, looks to me like some amplification is going on here too.

A reworked ESX would be a valuable addition to the stable of DIY electrostatic amps, but I think you're getting away from the original"Volksamp" concept, which, if I understood it right, was to build a small, cheap, easily understandable novice amp that didn't suck. Very few novice tube builders understand balanced inputs. Even fewer (including me ) are going to really understand what's going on with that cross-coupled first stage. Add features like fixed-bias, 4 gang volume pots, and pentode based CCS's, and you now have a very ambititious project. If you want balanced inputs ( I am not against balanced circuits per se despite whatever impressions I may have generated earlier), an ECC99 based BATE with a simplified power supply looks about as simple as you can get and still meet your design goals. On a humorous note, I've actually seriously considered an electrostatic "spud". It's technically conceiveable using something like the 6HV5. With a 6dB input step-up, you can generate a gain of 600 and a 600V swing with pretty decent linearity. The Rp is too high for for a transformer, but if you choke loaded it and cap coupled the output to an autoformer, it looks like it would work. Major downside is that the 6HV5 needs about 1200V on the plate to do this. Not something to try at home.

I don't buy into the whole "balanced is better" mania that pervades the headphone world , but I understand the necessity of appealing to the widest possible audience. From the standpoint of parts count, design complexity, physical size, and perhaps even ultimate performance you're paying a heavy price for those balanced inputs. Instead of two stages, you'll now probably need three. Instead of a single coupling cap in the signal path, you now have three. Global feedback will probably be required to linearize and stabilize the circuit. To support the feedback, you'll need higher gain (but less linear) tubes. Simple is not always cheap. Probably not practical in this situation, but the simplest balanced solution I can think of would be to use an input transformer with dual secondaries driving a pair of ECC99 voltage amps which are cap-coupled to a pair of ECC99 outputs. Very low parts count. Downside is cost. Mid-level transformers like Cinemag ( I think this is what Craig uses in the B.A.) run about $150.00 a pair. Edcors are dirt cheap, and if money is no object, then Lundahls with exotic cores. In answer to your question about feedback , I've not used it in any of my Egmont variants, but perhaps I should have. My first attempt , which used 12AX7's in differential pairs driving IXYS loaded 6P14P was unstable and blew the pass transistor of my primitive regulator. Went with the Williamson front end, added gate stoppers to the 10M90, stuck protection diodes in the regulator, had no more problems. Not a very scientific approach, but that's the way I work. I'm very interested in pushing this simple circuit to it's limits. One of these days I want to try it with 801A DHT's and 600V rails.

The Egmont is a great place to start an all tube electrostatic "Volksamp" type project. It's as simple and cheap as possible yet offers lots of possibilities for easy tweaks and upgrades. Right now I'm running a sort of Egmont/Williamson/Bate hybrid. Because I have no need for balanced inputs, the front end is a simple 6SN7 CCS loaded voltage amp directly connected to a 6SN7 concertina phase-splitter. The 6SN7GTA/B will take 450V on the plate, so it can be run directly off the positive rail. It's not sexy, but it's not chopped liver, and gets the job done in the simplest possible manner. Output is CCS loaded 6BL7 with individually adjustable "fixed" bias. The 6BL7 plates run 12mA/400V. Four tubes, three IXYS10M90, and two coupling caps make a channel. You could do the whole thing equally well, maybe better, with with ECC99's in all positions. The "Bate" power supply is a little intimidating for a novice. Perhaps the initial filter could be beefed up with a couple of cheap chokes and the regulator simplified to a core of a zener stabilized pass device. Maybe forgo the regulator entirely in favor of a simple, but well designed choke based filter. This is pretty much what I'm doing except I have mirrored 6EA7 tube regulators on the outputs instead of sand. I like the amp well enough well enough to build a case for it. It'll be a single chassis. Amp will be stacked on the power supply. Footprint is 5.25x13" uncased. Height is 10.5" This is about as small as I can go. Looks sort of like a miniature PC tower except the case is veneered zebrawood. I'm very interested in seeing how far you guys can run with "Volksamp" concept. There's not enough of this sort of thing out there.

Dug out the Egmont variant I made a while back and reinstalled the original 6BL7 outputs (I was using triode connected 7591's) and changed the the cathode bias to fixed bias. Preferred the 7591's by a small amount (an NOS quad will run $300.00 compared to $30.00 for a pair of 6BL7). Changing the bias scheme, although I can't measure the results, resulted in a signifigant subjective improvement. The amp seems punchier, more defined, and tighter in the bottom than with cathode bias. Definitely worth the small additional complexity. I used a battery stack as a neg. supply to get things started, but as a permanent solution (assuming you are building your own supply) I'll simply use a tapped voltage doubler off the neg. rail ( basically a mirrored image of the headphone bias supply).

At the risk of making a fool of myself I'll play "Devil's Advocate" a bit: 1) The 6BL7 has a Miller Capacitance of around 100mmf. A triode-connected 7189A (and I assume a 6P14P-EV) has about 20mmf. Is this a concern using a 12AX7 with a 330K plate load as a driver and a grid resistor of 680K? Wouldn't something like a 5687 driving a 47K grid resistor be a better match? 2) Looks like the 6BL7 is operating at about 10mA@400V. Again comparing a 7189A , the 6BL7 plate curves look substantially less linear at this point. 3) As the output tubes of an Egmont, I had examples of 6BL7 that with the same cathode resistance, varied by as much as 50V between plates. Typical was about 20V. Section to section matching was quite poor among 20+ samples. Does this matter? 4) RV1-4 look like they will be exposed to full neg. rail voltage. The only pots I'm aware of that can withstand this sort of voltage level are the big 2.5W types. Don't get me wrong, I like this circuit a lot. And I'm a big fan of the 6BL7, which I've used many times as a driver. I'm actually surprized that nobody makes a similar commercial amp. Tart it up a bit with some CCS plate loads and a fancy case and you've got a $3K. product.

I like it a lot. If you built it pt/pt , it's a weekend "junk-box" project within easy reach of a 2nd semester electronics student. Looks like you're going to have about 50W of heat coming off this board. Enclosure will be an issue, especially if you're trying to make it small. I assume some sort of simple but regulated power supply? More heat? I still like the CCS plate loads, at least on the first two stages. The IXYS devices are small, cheap. and easy to use, On the first two stages you wouldn't even need heat sinks.

Looks interesting. Why not substitute dirt cheap Russian 6N1P-EV for the 12AX7 in the first two stages? More linear, and if you used CCS plate loads probably wouldn't loose that much gain. Probably require less feedback as well. Any reason for not connecting the "tail" of the LTP to the neg. HV rail? Use Russian k40 PIO coupling caps, and if you used 6P14P output tubes you'd have an amp built mostly of Soviet era military surplus. Ton of this stuff around. You could call it the T-34 (a cheap, simple, but very effective WW2 tank).

I built an Egmont with 6BL7 output. Worked quite well. Maybe it was just my luck, but out of 20+ tubes no two had anywhere close to matching sections and no two samples matched each other in thier differences. I was quite surprized actually. Same with the 6BX7. One the other hand, I had no problem coming up with a pretty good quad of 6P14P out of a dozen samples on hand. I would love to see a professionally updated small/cheap all tube electrostatic amp along the lines of a "hot-rodded" Egmont.

6P14P-EV. Tough as a boot Russian version of a 7189. Guitar guys push the voltage ratings all the time. If you were interested in octals, lots of TV vert. amp types still around. My favorite is the 6AH4.

Great post Doug! Besides the slow warm-up, the main advantage of tube rectifiers was their lack of switching noise. With the advent of high voltage Schottky diodes, this problem has pretty much been eliminated. Most non-regulated tube power supplies are pretty bad. In order to do it right you need multiple high quality chokes that few manufacturers or consumers are willing to devote the space, weight and resources to. One the other hand, there are lots of terrible solid state power supplies used in tube amps. Think of 80's style amps with banks of 6550's and power supplies consisting of nothing more than generic rectifiers, a resistor or two, and massive banks of cheap electrolytic capacitors. With the exception of mercury vapor types, I've pretty much abandoned tube rectification, although I still like tube regulators. Still have a problem with the electrolytic capacitors though.

I've been running my LCD-2's with a couple of tube amps. The first is a single ended, direct coupled 56/46 with Electra-Print 5k:32 output transformers. Power supply is solid state rectification with tube regulation. Output power is about 1.5w. The second, which might be more interesting to you, is a small push-pull amp based on the Russian 4P1L directly heated pentode (triode connected). Output transformers are recycled from an old Fisher 500C. Input stage is simply a W.E. 418A driving an Electra-Print autoformer. Power supply is same as the first amp. Output power is about 3w.. Small push-pull amps are underexplored in the headphone world, and at least in my experiance, can work quite well.

Absolutely! I'll even supply a six-pack to enhance the experiance.

Very generous offer Marv. PM sent

Don't know anything about the LCD-3, but if you're at all interested, there's been some good deals to be had on used LCD-2's. I bought a 3 week old pair for substantially under the original price. I wasn't a big fan of the originals, but I'm liking the r2 a lot.

You have a point about about rp variation in triodes. This is a very real problem when using the high gm types such as a 417a or 7788, but not so much when using 6SL7/6SN7 types. After several attempts with the high gm types, I reverted to a 6SL7/6N7/6AH4 line up. I like a high/medium/low gain arrangement. Using a low mu/low rp tube in the third stage for gain instead of a follower gives you a buit in line stage with reasonably low Z out. Full function preamp in three tubes. Been using this for over 10 years with the original tubes. Now I'm curious to see what, if any, changes have occurred over the years.

I'm sure I'm missing something , but why use tubes at all in this circuit? Besides setting up the RIAA network, looks like all the ECC88 does is act as the upper half of the cascode and an output follower. If nothing else, a completely solid state circuit eliminates the need for a HV supply. On the other hand, if tubes are desirable, a three stage split RIAA design using triodes can easily be made dead quiet and can be implemented with no more components than shown here.

Wow! Seems I stepped in a pile of shit of my own making over on H.F. My intention was to counter the perceived impression that power in an electrostatic system could somehow be equated to a traditional low impedance speaker system. I realized at the time that I was oversimplifying (any electrical circuit that creates mechanical energy, charges and discharges a capacitor, or even passes a current at all, no matter how small, of course has a power component) . I apologize for any factual errors and appreciate K.G.'s explanation and correction. My understanding of electrostatic theory comes primarily from the Broskie articles mentioned above, Morgan Jones, and of course the forum posts here and on H.F. There is actually surprisingly little published information available. I'm sure that K.G. and Spritzer get tired of going over the same old ground, it would be really useful if there was some sort of "primer" available. I'm sure I'm not the only one that knows "fuck all" about electrostatic theory and operation. Once again, I apologize for posting incorrect information.