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PostPosted: February 2nd, 2018, 1:34 pm 
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Joined: July 24th, 2015, 4:17 pm
Posts: 656
Location: Parkville, Maryland
Pelliott321 wrote:
Two questions
Please explain the feedback loop. Why the caps and why is it connected to the input tube's cathode?
If one did not need the headphone output what transformer would ne use or just loose it and use a cap


Feedback to the first stage cathode is not unusual, however; that's a strange one to say the least. The feedback loop sets the overall gain -- so why capacitive-couple the feedback loop which results in full gain below approximately 6-Hz. ????

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PostPosted: February 2nd, 2018, 1:43 pm 
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Joined: January 15th, 2015, 7:19 am
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Location: Baltimore MD
Dave I would be interested in more info about the 6C45P transformer coupled spud amp


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PostPosted: February 2nd, 2018, 2:55 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 211
Pelliott321 wrote:
Dave I would be interested in more info about the 6C45P transformer coupled spud amp


Paul,

Very simple circuit. But been a while since I put it together. I used a pair of custom wound Electroprint line output transformers, 3K:600ohm that I had from a preamp project. Just a simple grounded cathode circuit, with LED bias (though I might have replaced that with SiC diode bias, have to check). Running around 30 mA through the tube. Carbon comp grid stoppers (x2) cathode stoppers (x4) and plate stoppers. Used shunt regulators on each tube, consisting of a 0D3 voltage regulator fed by a DN2540 cascode current source (150V). Power supply uses tube rectifier to choke input supply, filter caps are all motor run (oil), with Russian teflon bypasses. Goldpoint attenuator for a volume control.

I will have to trace out what I did and write out a schematic.

David


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PostPosted: February 2nd, 2018, 4:34 pm 
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Joined: July 25th, 2015, 12:47 pm
Posts: 60
DC block cap and a filter.

Did anyone try this "Zen"? Tom?

http://sound.whsites.net/project36.htm


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PostPosted: February 2nd, 2018, 5:41 pm 
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Joined: February 28th, 2013, 3:31 pm
Posts: 869
Other than the Cherry Bomb I never built a class A power amp. Although the CB is a very interesting and unique design my other SS amps will blow it away. I don't use it on a daily basis.

For phono and line level stuff I use some of the modern Op Amps. Much simpler if you just follow a few rules. Some people think simple is boring but if I can get great results with less hassle I'm all for it.


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PostPosted: March 4th, 2018, 7:26 pm 
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Joined: July 25th, 2015, 12:47 pm
Posts: 60
Is there any poor man’s Telefunken ECC83 / 12AX7 Smooth Plate? These are $500.

Any suggestion for high-quality low gain 12AX7?

or

12AX7 subst:
1964 RCA 5751 Black Plate
JAN 5751 U.S. Made


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PostPosted: March 4th, 2018, 8:34 pm 
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Joined: February 28th, 2013, 1:19 pm
Posts: 211
Concerning 12AX7's I personally like the Gold Lion B759, but there are so many options out there, it is hard to say, and it probably depends on how it is used.

If you want a Telefunken copy (made on the same tooling), a vintage EI smooth plate is alot cheaper than $500, but not sure if that is as good as the best of modern production these days.

David


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PostPosted: March 5th, 2018, 8:15 am 
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Joined: February 28th, 2013, 10:38 am
Posts: 912
If you're not stuck with 9-pin miniature, 6SL7...


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PostPosted: March 5th, 2018, 8:53 pm 
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Joined: July 25th, 2015, 12:47 pm
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Thanks David and Roscoe.


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