old question - lost answer 6AS7 SE amp
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 5th April 2008, 12:05 AM #1 multi-volti   diyAudio Member     Join Date: May 2005 Location: Michigan old question - lost answer 6AS7 SE amp Hi again I had asked previously about 6AS7/6080 in SE, and someone was kind enough to send me a schematic for an amp using 5687 driver and small toroidal power xfmr as interstage and OPT. Now I can't find it . What I'm particularly interested in at the moment is the circuit parameters if I were to find or wind a 'real' OPT. I'm looking at Richard's stuff/Triode Guy's xfmr math 'lesson'. If plate resistance for a 6AS7 is 280 ohms, is it 140 for a pair in parallel? I would assume so, but am asking in case mu or something else changes with two paralleled 6AS7. I also assume plate current for 0ne is in the range of 80-120 ma (say 100) so I'll take 2 at 0.2A. Any links or confirmation of my musings above are appreciated. Thanks Murray __________________ Murray
 5th April 2008, 01:07 AM #2 dsavitsk   diyAudio Member     Join Date: Jan 2005 Location: Hartford Re: old question - lost answer 6AS7 SE amp For two in parallel, mu stays the same and Rp is cut in half. A nice trick for determining operating points is to look at the plate curves, and just double the current values on the left side for two in parallel. __________________ http://www.ecpaudio.com :: http://diy.ecpaudio.com
G
diyAudio Member

Join Date: Apr 2002
Location: Belleville, IL.
Re: old question - lost answer 6AS7 SE amp

Quote:
 Originally posted by multi-volti Hi again I had asked previously about 6AS7/6080 in SE, and someone was kind enough to send me a schematic for an amp using 5687 driver and small toroidal power xfmr as interstage and OPT. Now I can't find it . What I'm particularly interested in at the moment is the circuit parameters if I were to find or wind a 'real' OPT. I'm looking at Richard's stuff/Triode Guy's xfmr math 'lesson'. If plate resistance for a 6AS7 is 280 ohms, is it 140 for a pair in parallel? I would assume so, but am asking in case mu or something else changes with two paralleled 6AS7. I also assume plate current for 0ne is in the range of 80-120 ma (say 100) so I'll take 2 at 0.2A. Any links or confirmation of my musings above are appreciated. Thanks Murray
Here is a starting point. The plate voltage is 135v@80mA.
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Gavin

 5th April 2008, 03:00 PM #4 multi-volti   diyAudio Member     Join Date: May 2005 Location: Michigan Thank you. Both. __________________ Murray
 5th April 2008, 04:36 PM #5 G   diyAudio Member     Join Date: Apr 2002 Location: Belleville, IL. I want to add the fact that dsavitsk had a hand in that schematic I posted for you. It is untried but I'm comfortable with the operating point on the 6AS7. You should get about 2 watts out with those operating points without stressing the tube very much. The interstage is a Edcor as are the output transformers. Please post your impressions if you end up building a SE 6as7 amp. __________________ Gavin
 5th April 2008, 06:21 PM #6 multi-volti   diyAudio Member     Join Date: May 2005 Location: Michigan Can either of you take a guess at a plate resistance at that O.P.? I've seen 280 ohms, but at what O.P. I don't know. If that doesn't vary much, I admit I don't know that. Thank you __________________ Murray
Jeb-D.
diyAudio Member

Join Date: Nov 2005
Location: SoCal
Quote:
 For two in parallel, mu stays the same and Rp is cut in half.
Yes, and transconductance(gm) doubles.

 5th April 2008, 07:51 PM #9 woodturner-fran   diyAudio Member   Join Date: Jan 2006 Location: Ireland Email user here Shoog - he has one of those amps and very good it sounds too! Fran
dsavitsk
diyAudio Member

Join Date: Jan 2005
Location: Hartford
Quote:
 Originally posted by multi-volti OK, and is rp is function of operating point or the tube itself, pretty constant?
In general, Gm decreases (and thus rp increases) as current decreases. A quick look at the datasheets shows the 280 ohms to be at 120mA, so at 80mA rp will be higher. How much might be estimated from the plate curves, but I haven't seen this information published anywhere.

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