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Old 5th April 2008, 12:05 AM   #1
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Default 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
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Old 5th April 2008, 01:07 AM   #2
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Default 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.
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Old 5th April 2008, 04:13 AM   #3
G is offline G  United States
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Default 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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Old 5th April 2008, 03:00 PM   #4
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Thank you. Both.
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Old 5th April 2008, 04:36 PM   #5
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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.
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Old 5th April 2008, 06:21 PM   #6
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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
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Old 5th April 2008, 06:48 PM   #7
Jeb-D. is offline Jeb-D.  United States
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Quote:
For two in parallel, mu stays the same and Rp is cut in half.
Yes, and transconductance(gm) doubles.
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Old 5th April 2008, 07:40 PM   #8
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OK, and is rp is function of operating point or the tube itself, pretty constant?

I ask because I don't know in what context the 280 ohm rp number I found relates the the operating point of the example circuit recommended.

I'm making this more difficult than you may think necessary because I started designing an SE OPT for 6C33C-B years ago. I started to think that why not kill two birds with one heavier-than-necessary stone. One 6C33C-B with a 600:8 OPT or a 6AS7 at 1250:5...I thought why not a parallel pair of 6AS7's at 600:8, use the same OPT design (I have some laminations and bobbins already). Even the plate current for 2x6AS7 vs 1x6C33C-B are agreeable.

I want to work thru both cases and design for the which ever has the tougher freq. response req't, if there is even much difference.

I kind of skipped the tube O.P. theory years ago, hence the dummy questions, and I've already done magnetics design in a different field, enough to follow the instructions others have posted. I did all the ham & 2nd Class FCC radiotelephone & radar stuff years ago, so I'm not totally green...just on the stuff I've never applied before.

Two other sad projects I never finished were a bobbin winder with indexed layer winding (someday), and bought an ancient toroid winder on eBay that needs repair/conversion to electronics after removing it's medieval rack & wedge limit switches & broken mechanical wire counter. No promises when I'll finish those, but spreadsheets are easier to finsh!

Thank you all for helping and humoring me.

gack - I will never figure out this security krap - every time I reply I'm told I'm not logged in...home, work, various places :O(
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Old 5th April 2008, 07:51 PM   #9
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Email user here Shoog - he has one of those amps and very good it sounds too!

Fran
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Old 5th April 2008, 07:59 PM   #10
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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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