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Old 22nd March 2008, 09:40 PM   #11
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Hi,

the SRPP advantage is, that it is push-pull (less power wastage...)
The Fet proposal is nice, but limited voltage brings it down. I only know designs that have Fets in series with resistance voltage dividers. So you can not couple in a good manner. This means: Low bandwith. Do you have any idea (schematic!) how to build a current source you can modulate with Fets that does not suffer the described problems? Otherwise, I think we have to stick to a HV pentode as current source and build something like Alan Kimmels "power mu stage"...
Somebody outside that knows where to get / how to build a very low capacitance heater transformer???

Regs, Dirk

@Calvin: For voltages not exceedind 1300V a Fet solution looks appropriate...
@Dick: How can you solve the heater trouble with a single transformer? Wouldn't a strong capacitive coupling mean a "low load" for the lower tube??? - loss in amplification of the SRPP stage???
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Old 23rd March 2008, 01:44 PM   #12
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Hi Dirk,

Don’t worry to much about the heater transformer.
Don’t forget, de heater does not have to be connected to the cathode, as long as the potential differance does not exceed the limits !
I connected the heater with the cathode through a 10k 2W carbon resistor.
Look for a transformer with 5kV isolation between primary and secondary and you are done.
For each “upper” tube you need such a transformer.
When both “lower” 6HB5 share the same connection for the cathode , their heaters can be powered from the same transformer.

For 2 PL519 in SRPP penthode connected, powered from 5kV, use a G2-kathode voltage of 50V for both tubes.
Connect 100 ohm 2W carbon to the G2 grid.
Bias them at 12mA with adjustable negative G1 voltage on “bottom” Pl519. Use a pot between 0 and –50V.

The EQ I use is passive right after the input.

The original Acoustat X has indeed a lot of feedback and compensation.
I use no compensation and only 10dB of feedback. Works great.

I have seen a cascaded Mosfet HV amp, but it also dissipated a lot of heat and when demonstrated it smoked and died. Tubes are a lot easier to work with and they can withstand some abuse.

Regs, Dick
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Old 23rd March 2008, 07:01 PM   #13
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Hi Dick,

thanks a lot for your help. Now it's time for me to draw a first schematic which I will post here (takes some time... ) I think, I will draw 2 or 3 solutions, which I will post. I will highly appreciate your valuable opinion (and all others as well ;-))

Did you make distortion measurement with your amp? Would be very interesting...

Regs, Dirk

P.S.: Do you still have contact with sasha? I saw, he tried triodes instead of pentodes... Got some feedback?
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Old 23rd March 2008, 07:02 PM   #14
Calvin is offline Calvin  Germany
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Hi,

well SRPP is not necessarily pushpull. At least it ceases beeing balanced pushpull when the load impedance is not right. And in our case we have a widely varying impedance. So SRPP is imho not a good way to go for ESLs. Thats why I´d prefer a Kimmel-style current source loaded Triode-stage. I know about the voltage vise limitations of FETs and the risks of not properly cascaded ones. And Yes, it´d be quite easy to implement the ccs with a Pentode, but I like the idea of less heaters and higher linearity because of higher FET-ccs-impedance.

jauu
Calvin
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Old 23rd March 2008, 07:06 PM   #15
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Quote:
Originally posted by djmiddelkoop
Hi Dirk,

Don’t worry to much about the heater transformer.
Don’t forget, de heater does not have to be connected to the cathode, as long as the potential differance does not exceed the limits !
I connected the heater with the cathode through a 10k 2W carbon resistor.
Look for a transformer with 5kV isolation between primary and secondary and you are done.
For each “upper” tube you need such a transformer.
When both “lower” 6HB5 share the same connection for the cathode , their heaters can be powered from the same transformer.

For 2 PL519 in SRPP penthode connected, powered from 5kV, use a G2-kathode voltage of 50V for both tubes.
Connect 100 ohm 2W carbon to the G2 grid.
Bias them at 12mA with adjustable negative G1 voltage on “bottom” Pl519. Use a pot between 0 and –50V.

The EQ I use is passive right after the input.

The original Acoustat X has indeed a lot of feedback and compensation.
I use no compensation and only 10dB of feedback. Works great.

I have seen a cascaded Mosfet HV amp, but it also dissipated a lot of heat and when demonstrated it smoked and died. Tubes are a lot easier to work with and they can withstand some abuse.

Regs, Dick

But Dick, the capacitance from sec to prim of that upper tube heater transformer can easily be several 100pF. That *is* a load on the amp that takes away usefull output current, isn't it?

Wouldn't it be a good idea to select xformers also on low capacitive coupling? I know that for instance in test amplifiers for high-voltage the upper heater xformer is a special one with the prim and sec at separate legs of the core.

Jan Didden
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Old 23rd March 2008, 10:17 PM   #16
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Hi Jan,
thanks for your comments.
Quote:
But Dick, the capacitance from sec to prim of that upper tube heater transformer can easily be several 100pF
Yes, easily.
Quote:
That *is* a load on the amp that takes away usefull output current, isn't it?
Yes, right again.
That's why I connect the heater to the cathode via a 10k resistor to isolate this capacitance. The value of 10k can easily be made 100k, it's just chosen because I had these resistors in stock.
Quote:
Wouldn't it be a good idea to select xformers also on low capacitive coupling?
Yes, a good idea.

The tubes Sasha uses are a better choice than the stock 6HB5.
I didn't make distortion measurements, yet.

Regs, Dick
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Old 24th March 2008, 09:12 AM   #17
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Quote:
Originally posted by djmiddelkoop
[snip]That's why I connect the heater to the cathode via a 10k resistor to isolate this capacitance. The value of 10k can easily be made 100k, it's just chosen because I had these resistors in stock.[snip]Regs, Dick

Dick,

That seems a very smart thing to do. I haven't seen it done before. I have been looking at the circuits of the Fluke 5215A where they use very special, very low cap transformers for the upper final tube. Can it be that a simple resistor takes care of this??

Jan Didden
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Old 24th March 2008, 12:35 PM   #18
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Well, yes.
The stray capacitance of the heater xformer is in parallel of the load, which is a capacitor to ( the ESL ).
By connecting this stray capacitance with a large series resistor it does not load the amp. stage anymore, while the voltage potential of the heater still follows the cathode, hence the heater-cathode voltage limit is not exceeded.
Anyway, it seems to work fine in the several HV amps I've made.

Regs,
Dick
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Old 24th March 2008, 12:54 PM   #19
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Quote:
Originally posted by djmiddelkoop
Well, yes.
The stray capacitance of the heater xformer is in parallel of the load, which is a capacitor to ( the ESL ).
By connecting this stray capacitance with a large series resistor it does not load the amp. stage anymore, while the voltage potential of the heater still follows the cathode, hence the heater-cathode voltage limit is not exceeded.
Anyway, it seems to work fine in the several HV amps I've made.

Regs,
Dick


OK. Not to want splitting hairs, just trying to make sure I get it:


There now is a lpf from cathode to heater. So, with increasing frequency, I would expect to see an increasing voltage difference between the upper tube cathode and its heater. Depending on the allowed cathode-heater voltage, one must take care that the lpf (10k resistor and the xformer cap) does not cause this max to be exceeded at max frequency and max output level.
Am I correct so far?

Jan Didden
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Old 24th March 2008, 03:31 PM   #20
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Hi,

first I want to comment on Jan: I guess, that Dicks solution will exceed the heater cathode rating at HF, but luckily level will be low and nothing goes wrong...

So, I did some drawings:

A: "Dick pentode amp"
B: "Dirk screen drive beta follower amp"
C: "Alan Kimmel design mu stage"

What do you think of it? What's wrong? Should I replace the CF with fets? Should I even go for fets instead of the upper ECC88???

Better ideas???

For the transformers:I guess I take "X" types... Jan: Where can I get this stuff???

Regs, Dirk
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