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Old 3rd May 2005, 04:39 PM   #1
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Default Tube driver for a SS power amplifier

Hello to everybody!

As I already wrote in a different thread, I need a tube driver for a SS power amplifier

I read almost all the threads about tubes driver, and I received a lot of helps.

I would like to explain my needs for the pilot and my experiments:

Needs:

- I need 40 volt rms at minimum THD+N (0,5%)
- Very low output impedence (max. 100 Ohm)
- A gain around 20
- The voltage for the tube stage is not a problem (300 - 500 volts)
- No feedback
- The simplicity and the "sound" of a triode stage (SRPP - Mu follower or "Gomez")
- No cathode follower ont the stage (100% feedback, I ear the differences)


Premises:

- In my hybrid amplifier, I try a lot of experiments, with the second stage always the same, only changing the first - tubes - stage.
- I can ear in my loudspeaker when the THD is around 1% so I need less;
- I can ear the differences from different tube stages and I like SRPP - Gomez - mu follower
- I can ear the differences (expecially in low frequency) beetween a stage with Z=2K
- I can ear the differences with feedback or not. And I don't want feedback.
- I like the sound also of hybrid SRPP or mu follower
- I don't want any electrolytic capacitor on the signal path!
- I DON'T WANT A High MICROPHONIC TUBE (like 6c45 or 5842 or pentodes, or similar)!!!

Experiment:

1) I tried a Gomez with ECC99, 440 volts, 18 mA each part. The result is a very good output Z (aroud 200 Ohm) a good sound, but still too distorsion 0,9% THD+N at 20 volt rms output. I increased the power to 500 volts, but only a little improvement on THD. I tried different polarization of the different section, I tried bypassing the two cathodes, I tried linking the two cathodes... but the result above was the best.

2) a tried a Parallel SRPP with ECC99, as above all different experiment, but worse behaviour in almost all parameters

3) I tried a mu follower with ECC99, but the THD spectrum is the worst!!

4) I tried the same three types, with E88CC at 300 volts, but very good sound, but still 1& THD+N at 20 volts, and I can ear.

5) I tried an Paralled SRPP of 6072A (12ay7), super linear tube, direct linked to a cathode follower of a parallel 6SN7. Tried all differend configuration. Powering at 440 volt, I have execellent results (the ones I need): 20 volts rms out at 0,2% THD+N; Z out around 100 Ohm - an execellent bass control (slew rate also very very good in consequence) BUT BUT, NOT the same lively, great, sweet sound of SRPP or triode although their weakness in the mesurement.


The question is: May I have a sound like the one I mentioned, better in all categories, without the problems of the different configuration and with the request I mentioned above?

The idea is to use a very good sounding and low Z triode, like (for me) 6922, ECC99, 6H30 (also if... too small gain), 6SN7, alone or paralled...

...Or using of a simple Hybrid stage (with mosfet or BJT as upper dispositive)

I really need help, with if possible (please!! (:-))) the schematic

Thank you very much!

G.M.

p.s. Sorry for my really bad English...
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Old 3rd May 2005, 07:17 PM   #2
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Well, first off.. just so you know... SRPP has feedback in it, triodes too, so you're kinda screwed.

Tim
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Old 4th May 2005, 05:07 PM   #3
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Thank you!

Sorry, my message come from a non-expertice. So any help is of course welcome!!

According to that, my aim is of course not to teach anything to anybody...

What I wanted to say in one point of my message was simply I don't like the sound coming from a cathode follower when a high swing of volt is needed as in drivers for final stages (SS or VT)

I also said I like the sound of SRPP


Reading my thread, have you any schematic helping me?

Thank you in advance.

G.M.
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Old 4th May 2005, 06:50 PM   #4
Sheldon is offline Sheldon  United States
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This one might apply:

http://www.tubecad.com/2004/blog0013.htm
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Old 8th May 2005, 10:07 PM   #5
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GM,

Maybe you need some more "sand". Gary Pimm's CCS loads completely isolate the gain triode from the load. Since the triode "sees" a GOhm range load, you get both the full mu and VERY low distortion. GP's circuits have a LOW O/P impedance, especially when the tube's plate current is relatively high.
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Old 9th May 2005, 03:16 AM   #6
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a new type of driver circuit:
http://www.diyaudio.com/forums/showt...107#post638107

Don
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Old 9th May 2005, 05:36 AM   #7
Yvesm is offline Yvesm  France
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Quote:
Needs:

- I need 40 volt rms at minimum THD+N (0,5%)
- Very low output impedence (max. 100 Ohm)
- A gain around 20
- The voltage for the tube stage is not a problem (300 - 500 volts)
- No feedback
- The simplicity and the "sound" of a triode stage (SRPP - Mu follower or "Gomez")
- No cathode follower ont the stage (100% feedback, I ear the differences)
40 volts rms, 100 Ohms >> 16 watts ??
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Old 9th May 2005, 07:22 AM   #8
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Quote:
40 volts rms, 100 Ohms >> 16 watts ??
I am getting more and more worried that Ohm's law has finally been leaked to the public. Please do not abuse it.
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Old 9th May 2005, 07:40 AM   #9
Yvesm is offline Yvesm  France
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Quote:
Originally posted by analog_sa


I am getting more and more worried that Ohm's law has finally been leaked to the public. Please do not abuse it.
Hi !
I recon I was somewhat "provocative", just because it is not the first time I read such things about "internal impedance" that I can't understand.
It seems that (for exemple) a SRPP built with a 12AX7 exhibits such numbers, that is: low internal impedance around 100 Ohms and hi level output near 40v rms, but is unable to provide both at the same time.

What load is it really able to "cleanly" drive ?

Simply bothered !

Yves.
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Old 9th May 2005, 07:51 AM   #10
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Hi Yves

The output impedance of a stage is just an impedance connected in series with a voltage source. If unloaded there is no power dissipated. In your SRPP example the output impedance is not really dependent on the voltage output. That is if there is no load. As soon as you load it things change dramatically.
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