FrankCooter

[ATTACH=CONFIG]3320[/ATTACH] Up and running today! Here's a "beauty" shot of the interior. Lots of iron in a 6"x13" space. The large black transformers are the Electra-Print outputs. The silver center pair are 1:1 3k interstages made by a now defunct Florida company for an old "Sound Practices" article. I'll neaten things up later. If the amp is a "keeper" I'll convert the screw type terminal strips to soldered turret terminals. At this stage I like to be able to change things around. Already thinking about a "big brother" 845 version. More later.

My understanding of transformer design is strictly technician level, but I think your design parameters are unrealistic. No electrostatic amp that I'm aware of swings 800Vrms. Most Stax amps swing about 350Vrms. Even the Blue Hawaii "only" swings 1500p-p, which is a little more than 500rms. If you're looking for ruler flat 20Hz response, you probably shouldn't be using a transformer at all. 25Hz is probably about the practical limit.

Maybe you could detail your numbers a bit. I'm curious about how you arrived at these conclusions without any specific design information. The "stupidly huge" core is 2.5 x 3.0" with a 1.5" stack. It tests at -1db at 27hz. I will test for wave form anomolies, capacitive and otherwise, when the amp is operational. Based on many years experiance with Electra-Print designs, I don't expect to find anything signifigant. Preliminary listening tests were very positive. The amp is a test platform. If I'm missing anything, I'd appreciate enlightenment.

As usual, Doug has some interesting suggestions. I've never tried a CCS/Ultrapath configuration for a cathode circuit. I can see where it might be a signifigant improvement over a simple resistor/bypass cap. Will definitely try this. Not sure what the added complexity of parafeeding the d3A would accomplish. The additional plate choke or CCS plus capacitor would seem to make the interstage transformer redundant. Maybe I'm missing something here. Changing the interstage transformer to accomodate a 418A probably isn't practical. I've asked Jack Elliano about high level step up transformers before and he's always been very negative. In the practical world, higher output impedance usually results in restricting the bandwidth. One of Doug's ideas I'd like to try is his shunt regulated Shining Path/ Electric Avenue circuit on the output tubes. The late John Camille had an article about something similar years ago in "Sound Practices" magazine. He used a 211 to shunt regulate another 211 in the output position of an SET amp. I'd like to try something similar here. Audiocat kindly sent me a pair of his custom Stax jacks. They look great and they fit much better than the standard Amphenols. There's a thread over in the DIY section in the "other forum". At least on this front, I'll no longer be an embarassment to the big guys. Probably eventually substitute an 845 for the 3D21/7403. As many of you know, I'm a big fan of the Russian GM70 transmitting triode. Unfortunately, in order to keep the bandwidth up , the Electra-Print output transformers are a bit low in the primary for a GM70, 211, or 813. 845 should do fine however. I'll never be able to afford any of the originals, but the cheap Chinese are actually pretty good. If any of you have any experiance with any of the premium types, I'd appreciate hearing about it.

Since noboby shit on my schematics, I'll assume no major flaws, or at least a passive indifference. The original prototype has been running without issue for about a year now. Gain and voltage swing are both about 1k. Beyond the basics, since I had ad-hoc output transformers, I haven't bothered with lab measurements. But like an any tube based, single ended, zero feedback design, lab measurements aren't what it's all about. You either like what you hear or don't. I think it sounds pretty good. Holds it's own against an SRD-7 and a solid state exStatA. Hopefully, I'll get a chance in the not too distant future to compare it to some of the "big boys". One thing this amp isn't is cheap. I'm fortunate to have a big parts "junkbox". The only major expense was the output transformers, which ran about $260.00 with shipping. If everything was purchased new, the total parts content would have run near $1k. You could knock $200.00 off that by substituting a resistor or CCS for the d3A interstage transformers. At this amount of money, it needs to be a serious contender not to be a failure. The amp in the photo will be finished shortly. Once it's stable and I'm familiar with the sound, I'll start trying to "improve" the design. The basic topology and tube compliment will stay the same (except for a possible DHT output). I've already experimented with most of the likely tube candidates. I tried the 12gn7, 417a, 6c45pi, 7788, 418a, 8233, as well as the d3A for drivers. My favorite was the Western Electric 418A, but it just didn't have enough gain. The d3A came in second. The 7788 was also good. Not a fan of the 6c45pi. All pentodes were run as triodes. From the beginning, I assumed the output tube would be either an EL34 or a KT88. Not a lot of candidates that can take a 600v plate voltage, have decent gain, and are suitable for transformer loading. Tried the 6BX7 and the 6CK4, didn't like them. Settled on the KT88 over the EL34 because of the bigger power dissapation. I also am fortunate enough to own some original Genelex and Tung Sol. Somehow I missed the 7403/3D21. I'm using those now and liking them. Some people would be bothered by the plate caps, but I'm not. Spritzer asked about 807's. The 807 probably wouldn't be suitable due to it's relatively low screen rating and high rp. I'm anxious to try a DHT in the output position. The ability to do this may be the biggest single advantage of this type of amp. Originally, I'd hoped to use an 801 (an uprated 10y) for the output position. The 801 has a gain of 8, can take 600v on the plate, and has a managable 1.25A filament. Unfortunately, it's 4.5k rp precludes precludes transformer operation in this application. ( a 9k primary in a 1:1-1 configuration makes a total 36k secondary. Not buidable in the real world without a huge high frequency roll off.) The 300B is a possibility, but an 845, run at a relatively low 600v looks good on paper. I'm a big 845 fan anyway. The second major area of experimentation is the power supply. I'll substitute 816 mercury vapor rectifiers for the 6AU4 damper diodes. I also want to try shunt regulating the output tubes with mirror imaged power tubes. Any other ideas or suggestions would be appreciated!

Now for the power supply. Here is where I expose myself for the hacker I am. It might have been more elegant to pull the 180v off the power transformer center tap, but I didn't have room for a second filter network. The cascaded 6ea7 regulators each drop 200v at about 40ma. Wastes some power, but who cares. I'm open to alternatives, as long as they're tube based.

Playing with my new camera again. Here's a rough schematic of the amp. As you can see, it's brute simple. Plenty of opportunities to try different bias schemes, power regulation, and perhaps parafeed. Go ahead, rip it apart! I'll put up the power supply in the next post if this turns out OK.

I'll bring this amp, but you're right about the "Premier" thing. A true "Mayberry Premier" would require something much more indulgent, decadent, and "over the top". Feb. is still a long way off. I have some ideas. How big are your electrical mains?

Wow! My image post worked! Thanks for all the kind words. I've had a few distractions in the past months, but now I'm back on track and should complete this amp in a few weeks. A "Mayberry" public debut would be great, but first listen will go to my fellow "Santa Barbarian" Ken, after a barbecue and a few beers. I need to put a kudo in here for Jack Elliano of Electra-Print. For 15 years he's built transformers for me. Almost everything I've asked for has been a non-standard, custom design. He's never let me down no matter how crazy my requests, and his prices are dirt cheap considering the custom engineering involved. Originally, I used an old recycled pair of 35ma 1:1-1 high level phase splitting interstage transformers for the output of the amp. Although they subjectively worked well, I knew that running 600vdc with a 1kv output swing was way beyond their design parameters. He doesn't normally make this transformer anymore and now reccomends an autoformer for phase splitting duties. I called Jack and asked for a pair of purpose built transformers that could do 600vdc, have enough primary inductance to support a wide variety of tubes, generate a 1:2 step-up,and have a 20hz-30khz bandwidth. He laughed. This is an extrodinary challenging set of specs for a transformer. At first he wasn't sure it was possible, but after a couple of attempts we have purpose-built electrostatic amplifier output transformers. If this amp has any value at all it will be because of his work. Thanks Jack! On Spritzer's suggestion, I substituted a 3d21/7403 for the kt88 as output tube. In a no feedback, single ended amp, the quality of the tubes makes a big difference in the sonics. I'm fortunate enough to own a pair of original British Genelex kt88. They sound great, but the suppli8es are gone, and they now cost a fortune when and if you can find them. The reissue "Gold Lions", the best of the current production, while very nice, are a step down. Standard Chinese Shuguangs sucked. I've got a pair of 7403 on tap, but I've been running 1970 British STC 3d21a. You can buy them NOS all day on Ebay for $20.00 a pop. In this application, I can't tell them apart from the Genelax kt88.

Here's a picture of my electrostatic amp under construction. It's my first attempt to post a picture on a forum. Hope it works. I'll get better. Any advice would be appreciated. Right now I'm in the process of rebuilding it in a more professional form. More info. later.

Right now the D3a/ KT88 amp is my "daily driver"electrostatic amp. I recently completed an exSTata, which sounds pretty good in it's own right. Over the next week or so I'll be doing extensive comparisons. My first attempt at upgrading the D3a/KT88 design was the 813 based amp I brought to LA CanJam.This amp was not what I'd hoped it would be. I made a number of mistakes in the output transformer specs, and my SRPP driver stage was not up to the job. Since there are a few others interested in transformer coupled electrostatic amps, I think it's time to take a second shot. This time the approach will be a little more practical. The output transformers will be speced properly, and instead of an 813, I'll try an 801. The 801, while still a DHT, is much more "real world" than the 813. Whether this whole approach is a viable alternative to existing designs I don't know. Just have to build a few and see. I'm hoping to make this thread a sort of "miniblog" of the journey.