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Old 9th December 2004, 05:26 AM   #21
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You'll need a capacitor from the screen grid to ground.

Also, a 10K plate load resistor is a little low for the ECC83/12AX7
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Old 9th December 2004, 07:55 AM   #22
Yvesm is offline Yvesm  France
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Quote:
Originally posted by Mr. Triatic
ok, the transformers isnt that good... but they will be sufficient 8-)

here's my schematic so far:

input please!! 8-)

ps. i guess that B+ would be around 200-250V?
Hi Triatic !

The 832 is a dual tetrode where the cathodes are internally tied together as well as the screens.
It is difficult to use just an half because if you apply screen voltage without anode one, you'll burn out the screen !
The unused section could be "overbiased" (some -50V at its grid) to void that.
Moreover, ther is also an internal bypass cap between screens and cathode.
But data sheet says its value is some 65pF, forget it !

B+ is allowed to be as hi as 750V, but screens no more than 250V.
Rated anode dissipation is 15W per section.

It was designed to be used in PP, but both sections in parallel in an SE configuration should work.

As a (rough) starting point, I would suggest the following setting:
Va +400, Vg2 +250, IA 35mA (per section) this calls for some -25V grid bias (or a 350 ohms common cathode resistor for the 2 paralleled sections).
With your 9K tranny, this should produce some 7W PO in penthode mode.

In PP you could certainly obtain up to 20W PO.

As a stereo amp (that is one section per channel) it would be avantageous to increase the load up to 15K.

I agree than 12AX7's plate resistor should be 100 to 200K rather than 10K

Regard, Yves.
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Old 9th December 2004, 08:27 AM   #23
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Quote:
Originally posted by Yvesm
The 832 is a dual tetrode where the cathodes are internally tied together as well as the screens.
Oh dear, how could I have missed that! Yvesm is of course right, you'd probably have to go with parallel single-ended
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Old 9th December 2004, 10:21 AM   #24
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ok, then i have to use two tubes in order to get stereo, and parallell the anodes on each tube due to the SE-mode...?!
the ECC83 can of course be single because i use one half to each channel, i have to raise the 10kohm to 100-200 kohm.
you dont think this construction would be prone to oscillate? i have a feeling that it wont, but?! -i guess i have to try it out 8-)
i can arrange a B+ at 400V and filament supply from a transformer i have.. perfect!
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Old 9th December 2004, 03:10 PM   #25
Yvesm is offline Yvesm  France
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Hi Triatic !

Quote:
ok, then i have to use two tubes in order to get stereo, and parallell the anodes on each tube due to the SE-mode...?!
Not really.

It would be a very nice and cute to use one half of the 832 for each channel.
Just oversize the cathode and screen bypass caps, or even use Zeners ?

And at 35mA anode current, your tranny will be happier !

9K is not the optimum load, but they should do the job, specially if the loudspeakers are a bit above 4 Ohms !

Anyway, toobs are very tolerant to design "twists", and you should reach your " 2 to 3W " target easily.

Quote:
the ECC83 can of course be single because i use one half to each channel, i have to raise the 10kohm to 100-200 kohm.
This will give you some gain margin to apply feedback (no tetrode are highly linear).
Quote:
you dont think this construction would be prone to oscillate? i have a feeling that it wont, but?! -i guess i have to try it out 8-)
Yes ! You MUST try ... and keep us informed !

Cheers, Yves.
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Old 10th December 2004, 08:20 PM   #26
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Guys,

Is CV788 the same as 832A ??
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Old 10th December 2004, 09:29 PM   #27
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-> jamesjung21
Yep, it's the same tube 8-)

http://www.tubecollector.org/cv788.htm
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Old 10th December 2004, 09:58 PM   #28
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Hi Triatic,
You don't need to use two 832's for stereo provided that the cathode bypass capacitor is large enough to assure minimal cross-talk at the lowest frequency of interest.. There is precedent for this approach - just take a look at Bob Danielak's darling or dc darling to see what I mean. (Good sounding amplifier design incidentally.) IMHO I would ground the cathode and use fixed bias instead, with independent bias adjustment for each half of the tube... Make sure in either case that the screen is well bypassed right at the tube to prevent parasitic hf/vhf oscillation and a good low impedance supply seems warranted - perhaps using a gas tube as a regulator..

Kevin K
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Old 11th December 2004, 03:02 AM   #29
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9k ... probably best suited for EL84/6BQ5, EL95, 6V6, ELL80 (2 EL95s in 1 bottle), SV83, ECL86 (often called half a 12ax7 + EL84 in one bottle), ECL82/6BM8 (1/2 12AT7 + EL95 in one bottle), 6AQ5, Russian equivalents....

you need to consider how much current you can flow thru the OPT before it saturates as well.

dave
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Old 14th December 2004, 08:44 PM   #30
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I have made some changes according to your comments, what do you guys think of this?

( all changes are marked with "*" )
Attached Images
File Type: gif 832amp_rev.gif (5.7 KB, 529 views)
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