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HVfanatic 22nd February 2008 08:24 PM

DD amp for ESLs...

I've seen that some of you built DD amps.

My starting point were the Acoustat amps.

1. Why do they use SRPP and not Ŗ-Followers? (Advantage / Disadvantage)

2. Are there any triodes around instead of TV tubes? Does this make sense? Are there triodes (not the 833!!!) that withstand 5KV?

3. Does anybody have good experience (sound/stability) with these Amps? Can somebody send me the schematics of his Amp???

Regs, Dirk

djmiddelkoop 22nd February 2008 09:12 PM

There has been discussion on this subject before..


I rebuild this amps nowadays with a tube frontend.
The only weak point, as mentioned before, is the HV transformer and sometimes the 6HB5 tubes.
The HV transformer can be replaced by a new and better one.
If a HV delay is built in, which kicks in the HV when all tubes are heated up, the 6HB5 tubes will be more stable.
There are triode replacements, but the disadvantage is the low gain.
The SRPP, altough not the best choice, works well in this arrangement.
If you like, I can provide a better schematic than in the link above.

HVfanatic 23rd February 2008 12:17 AM

Thanks a lot!

I already got the schematics. I also think, an all tube solution is superior. Did you only replace the op-amps? Or did you also replace the driver transistors?

I found Sashas rebuilt. Looks quite promising. My Idea would be, to use a triode differential stage (cascode like paravicini?) and a cathode follower as driver for the TV tubes.

What was your approach?

The acoustat looks strange to me in the elements C22/R32,

Is this some kind of phase alignment due to capacitive load???

How did you manage the task to prevent oszilation???

Would you prefer Sashas tubes? Or goto PL519???

Regs, Dirk

djmiddelkoop 24th February 2008 06:46 AM

Hi Dirk,

yes, the driver transistors are also replaced to make it an all tube circuit.
Sashas rebuilt is indeed good, his tubes are better and easier to get, and cheaper to. PL519 is indeed also very usable, I tried it to.

My approach is a differential cascode, driven by a current source.
No need for the cathode followers as the penthodes are relative easy to drive.
There is no oscillation, but I reduced the NFB to about 10dB overall.

Don't forget the filter at the input. I use a passive HF filter, adjustable with the original HF balance pot.

Regs, Dick

HVfanatic 20th March 2008 06:37 PM

Hi Dick,

Do you mean with "Cascode Differential Amp" that you have 2 stage differential??? Or that you use a Cascoded Differential Amp (1 stage???) Which tubes do you use???

I think of the following topology:

Output stage: PL519, mu follower like Allan Kimmel (upper Tube as Pentode, lower tube as "screen drive" Triode

Driver: Mosfet as follower instead of Cathode follower.

Input: Two stage differential Amp, cascoded with Fets for better Bandwith, lower Miller and more amplification. Both stages should get a bipolar current source.

Should the driver also operate with a current source in your opinion?

Where do you see the shortcomings in this design idea???

Regs, Dirk

djmiddelkoop 20th March 2008 07:20 PM

Hi Dirk,

With Cascode diff amp I mean one stage, i.e. 2 cascodes where each cascode is one side of the diff. Amp.
I use ECC88 tubes, which have more than sufficient gain .

I think your topology works fine, but you donít need the second diff amp.
The current source is not needed for the driver, as all stages are diff already.

Donít forget a delay for the HV and be very carefull, these voltages are deadly.

Regs, Dick

HVfanatic 20th March 2008 08:02 PM

Hi Dick,

I understand. But: If I look at your design, you use 3 tubes!???
Supposed the tube is a double triode, you would only need 2 tubes for the differential cascode. What is the third tube used for???

Regs, Dirk

P.S.: Funny: Paravicini uses the cascode solution as well, but with a cathode follower for driving the Pl509 in triode mode...

djmiddelkoop 21st March 2008 08:44 PM

Hi Dirk,

I use 2 ECC88 tubes for the diff cascode, and 4 6HB5 ( or any other suitable tube for this purpose ) for the diff HV amp.

If you drive a HV tube in triode you certainly need an extra buffer like a cathode follower to drive it properly.

Regs, Dick

HVfanatic 22nd March 2008 08:19 PM


I got it. You use the third tube for equalization circuitry??? What I read in other threads, that the HF needs damping. Looks strange to me, because an audiostatic has already compensation after the transformer... (?). Well, I made some thoughts about the circuit. Besides the HV, I also need heaters. These should be separate trannies - I guess I need 4 of them, center tapped, where the center tap is connected to the cathode. Wouldn't the capacitance between pimary and secondary harm me???? What type of transformer do I need to get the lowest coupling possible there???

I found another problem: If I want to use a pl519 sceen drive (triode like, lower tube), I only find data sheets to draw my loadline for less than 1000 Volts... Where can I get appropriate data? I'm not experienced with screendrive. Are there recommended values to set the grid voltage, and then fixing the bias with the screen (superimposing the signal voltage???) Or: Put the signal to the screen, no matter (within allowed ratings!) DC, and adjust bias with appropriate DC on the grid???

Regs, Dirk

Calvin 22nd March 2008 08:49 PM


I like the circuit topology of Neil McKean and similar, using a current source loaded FET-stage.
My idea is to use a JFET-input stage and a MOSFET-driver, coupling to a HV-Triode( >4kV possible) working into a (voltage controlled) current source made from cascaded MOSFETs. This should give the triode the highest possible load (linearity) without the need of a floating heater for the upper tube and maybe even the possibility to get rid of the global feedback. At the moment Iīm still a bit puzzled about the cascaded FET-current-source. Anyone schem suggestions?
The large amount of feedback and compensation is what I dislike about the Acoustat-amp.


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