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Old 25th January 2008, 12:54 PM   #1
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Default 6as7g Se

Ciao,

I have a lot of 6N13S (6as7G) russian tubes, and a lot of 6N6P double triode; with these tubes I'm now testing few SE circuits and ideas.
Currently I'm thinking about THIS idea.
The topology will be a parafeed output stage, driven by a single 6N6P (CCS loaded).
To increase the output power I'd like to parallel two or more 6AS7G tubes; i already tried it in another prototype, but the result was not so good.
So, my idea is to place a dedicate CCS on top of each output tube, and then "parallel" them through the parafeed coupling capacitor to the output trafo.
Would it work?
I can provide a single negative voltage for the bias, or do I have to provide a separate bias for each tube?
I could even designe a servo bias circuit, that will keep all the output tubes at the same operating voltage.
I'm not sure on the result, because the output tubes are known to not to be very balanced, so I don't know what will happen "paralleling" them through the capacitors.
Your comments?

Ciao,
Giovanni
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Old 25th January 2008, 02:06 PM   #2
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6AS7s are notoriously difficult to match - I think you should add some cathode resistance to each (bypassed) to help equalize the operating points. The data sheet DOES say not to use fixed bas, after all...
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Old 25th January 2008, 06:59 PM   #3
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The idea to use three separate CCS, one for each output triode, was indeed to help the matching: having (almost) the same current flowing in each triode, the operating point "should" be pratically the same...
And if, as said, I add a servo bias that keeps the anode voltage at the same level I "should" have three triodes working at the same voltage, with the same current.
Slight difference in operating curves should be irrelevant at this point, I think (I hope).
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Old 25th January 2008, 07:55 PM   #4
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Yes, but operating point is voltage -and- current - only one is controlled. My suggestion wasn't thought through - it won't help as current is fixed.

Try it and see. Adding separate bias controls will be easiest, and may not be necessary.
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Old 16th June 2008, 04:48 AM   #5
Akita is offline Akita  Malaysia
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Im planning to build a 6as7g SE amp too. Liliput Schematic look like a best choice to me.

Im thinking to parallel the two triode inside the same tube to get a bit more power.

what value/component should I change? what is the primary OPT value will be(625:8)?

Please advice.
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Old 16th June 2008, 05:10 AM   #6
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I see two potential trouble spots in your schematic..first your SS B+ will 'instantly' hit your cold tubes...might reek havoc, and you don't have a bleeder resistor on the B+.
Consider going tube rectifyed as it is gentler on your tubes.
____________________________________Rick..........
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Old 16th June 2008, 07:01 AM   #7
Akita is offline Akita  Malaysia
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Thank,

I will add a stand-by switch before the rectifier. On what I see on other circuit, a bleeder resistor with 1M resistor from B+ to gnd should work. ......right?
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Old 16th June 2008, 01:54 PM   #8
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I cannot download your link for some reason, but I think I get the idea.
The idea is excellent, I've had the same in my head for years, just haven't taken the time yet.
I've never had problems paralelling 6AS7s, but I've had to live with different idle currents thru each section. I really don't think it is such a big deal that some say. Just think of all those OTLs out there, do you really think they all have matched tubes? When the power is needed they will actually work together the way they are intended to do.
Btw, regarding 6AS7 ruggedness; for a couple of months I have thought my amp to be extra hot, but never took the time to open it up and check. Well, I finally did the other day, and the tubes were running at 28watts/triode!!! Thats 56watts per bulb! After readjusting the bias back to 12watts/triode, they still sound great. can't believe how rugged these tubes are. I have no idea how long they were run at 28watts, but it must be at least a couple of months. I've had the same tubes for years, and don't plan on changing them till they absolutely dont work.
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Old 16th June 2008, 03:19 PM   #9
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The 6as7g is a fairly none linear tube. This will effect the sound in SE, but in PP the none linearities will cancel each other out.
My gut feeling is that there are better tubes for SE than the 6AS7.

Shoog
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Old 16th June 2008, 11:45 PM   #10
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I don't think the Lilliput's driver can drive the 6as7/6080 to full power.

I am using plate choke loaded 12at7 for my 6080 parafeed amp.

There is something about a parafeed 6080 amp that I like, got tight bass and nice highs.
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