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If I want to make an SRPP amp using 8x EL34 to drive my electrostatics, will I be called a freak?

post #1 of 23
4/30/10 at 1:13am
Thread Starter 
Not very green, I know.
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post #2 of 23
4/30/10 at 1:37am
Have you considered Vhk limits in your plans?
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post #3 of 23
4/30/10 at 1:40am
Thread Starter 
hmm, if I use independent heater supplies for each tube, that shouldn't be a problem, no?
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post #4 of 23
4/30/10 at 2:14am
You could simplify things a whole lot by using output transformers instead of the srpp stack. Right now I use a two stage single ended design. D3a with an interstage transformer driving a triode connected KT-88. The KT-88 drives a 1:1+1 phase splitting transformer for the output. Very simple, and holds it's own quite well. Next step is to convert it to a DHT output. I'm kind of surprized others don't do this.
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post #5 of 23
4/30/10 at 2:59pm
First question, why SRPP? It's a horrible choice for anything except a fixed load (which electrostatics clearly aren't) and while you can "trick" the amplifier and minimize the effect of the load it's no more then a band-aid on a bullet wound. There are clearly better alternatives out there and some are very easy to build (WES or ESX for instance).

Now if you were to build a SRPP stack then 8 tubes are the absolute minimum and ideally having 5 filament supplies just for the output. You can get away with a custom transformer for this. I was working on my own SRPP amp which used a basic differential amp as the driver and then 4 individual SRPP stages with two 6S4A's each. The PCB's are close to completion but SS amps take up all my time now.

Quote:
Originally Posted by FrankCooter View Post
You could simplify things a whole lot by using output transformers instead of the srpp stack. Right now I use a two stage single ended design. D3a with an interstage transformer driving a triode connected KT-88. The KT-88 drives a 1:1+1 phase splitting transformer for the output. Very simple, and holds it's own quite well. Next step is to convert it to a DHT output. I'm kind of surprized others don't do this.
Is that the KT88 amp you had at CanJam? Beautiful to say the least and something I'd like to build one day.

I really want to play with single ended amps and output transformers but use SS instead of tubes. Call me crazy and all that...
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post #6 of 23
5/1/10 at 8:44am
Quote:
Originally Posted by spritzer View Post




Is that the KT88 amp you had at CanJam? Beautiful to say the least and something I'd like to build one day.

I really want to play with single ended amps and output transformers but use SS instead of tubes. Call me crazy and all that...
Yes, it is indeed the KT88 amp I had at CanJam. It developed a wiring issue in transportation that I was unable to fix until after the meet. Kind of too bad because it actually sounded better than the 813 based monster I had running.

The 813 amp wasn't one of my better efforts. I was off considerably in my transformer specs and I got too cute in the driver section. Next time will be better.

I dabble from time to time in the electrostatic world, but I think that somebody who really knew what they were doing could do something first rate based on a transformer topology.
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post #7 of 23
5/1/10 at 10:52am
I have some of the lesser known power tubes here that would be fun to adapt to something like this, such as Bendix 7403's, some 807's or the old SinglePower favorite, 3d21a.

I've been playing around with Lundahl transformers as stepup units (LL1630 and LL1627) but a push-pull choke makes a lot more sense then tube amp driving a Stax transformer.
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post #8 of 23
5/1/10 at 3:08pm
Quote:
Originally Posted by spritzer View Post
I've been playing around with Lundahl transformers as stepup units (LL1630 and LL1627) but a push-pull choke makes a lot more sense then tube amp driving a Stax transformer.
Rather than a P-P plate choke you can use the primary of a P-P output transformer with a power resistor across the secondary. Part sourcing becomes much easier, and the amp could do double duty driving speakers or dynamic headphones.
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post #9 of 23
5/1/10 at 6:23pm
Thread Starter 
well if I want to build a tube amp I want direct output to the stats.... If an big output trans have to used, then why bother with a tube amp design, might as well use LL AM box driven by good SS amp.
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post #10 of 23
5/1/10 at 6:54pm
Quote:
Originally Posted by AudioCats View Post
well if I want to build a tube amp I want direct output to the stats.... If an big output trans have to used, then why bother with a tube amp design, might as well use LL AM box driven by good SS amp.
In Spritzer's design (assuming I am interpreting his description of his design properly, which is where my recommendation would be the most appropriate) the choke/transformer is not between the plate of the tube and the stator: it simply provides a load for the tubes.

Frank cooter's design with the transformer used like a transformer (heh) offers some very compelling advantages & opens the door to a SET type amp. Not everyone likes the way push-pull output stages sound, this gets around it elegantly.

There are plenty of reasons to stick with a tube design when dealing with purely reactive loads. Considering the capacitance of a large electrostatic speaker they are all the more relevant and compelling.
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post #11 of 23
5/1/10 at 8:19pm
Thread Starter 
Forget chokes, might as well go CCS....

"single end then split" is a good idea if the build cost is not a concern. One amp, capable of driving speaker, dynamic phones and electrostatics. Cool. But will EL34 single end design give enough power? KT88 will probably become a mandatory.
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post #12 of 23
5/1/10 at 8:39pm
Quote:
Originally Posted by AudioCats View Post
Forget chokes, might as well go CCS....
CCS's dont go above B+
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post #13 of 23
5/1/10 at 8:47pm
Quote:
Originally Posted by AudioCats View Post
well if I want to build a tube amp I want direct output to the stats....
If you want the amp to be as pure as possible then why on earth would you want SRPP? That circuit will never be stable into such a varied load and distortion will probably be off the chart. It could also mean a fully AC-coupled circuit and all those capacitors will color the sound even further (though it will be a small issue compared to how the circuit is behaving). The main attraction of the SRPP stage is low cost and that is simply lost here and it hasn't got anything else going for it.

Quote:
Originally Posted by AudioCats View Post
If an big output trans have to used, then why bother with a tube amp design, might as well use LL AM box driven by good SS amp.
That isn't quite true since the output transformer is only used as phase splitter with a ratio of 1:1+1 so it is more linear then a transformer with a ratio of 25-35.

Quote:
Originally Posted by nikongod View Post
In Spritzer's design (assuming I am interpreting his description of his design properly, which is where my recommendation would be the most appropriate) the choke/transformer is not between the plate of the tube and the stator: it simply provides a load for the tubes.
Indeed, that's how you could build a pretty simple push-pull amp or even modify a stock speaker amp. The only 845 based electrostatic amp I know of uses a push-pull choke for the load.

Quote:
Originally Posted by nikongod View Post
Frank cooter's design with the transformer used like a transformer (heh) offers some very compelling advantages & opens the door to a SET type amp. Not everyone likes the way push-pull output stages sound, this gets around it elegantly.
A SET driving one of the most transparent transducers ever made would be a match made in heaven for many. Not that expensive to make either (at least in comparison to the T2/BH monsters we are working on).
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post #14 of 23
5/2/10 at 2:58pm
I wouldn't call you a freak. Possibly related to
KG but certainly not freaky.

If Spritzer's comment above about a SET amp is anything closely related to my own limited tinkerings with low volt dynamic amps, then I would agree with the sound assessments. My ears prefer SE to PP.
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post #15 of 23
5/2/10 at 3:33pm
I must say the idea of a fully transformer coupled D3a/7403 amp is growing on me. The 7403 is a beast with 40W plates and voltage handling of 4kV so it might be a truly impressive 'stat amp.
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