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Old 3rd January 2009, 05:16 PM   #121
Fuling is offline Fuling  Sweden
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Found this huge toroid for ~20 bucks on Tradera (Ebay, sort of):

800VA with 230 + 33 + 2x11,7v secondaries. Current rating of each winding is unknown but judging by the wire gauge it should be more than enough for output stage B+ and heaters.
300VDC is a bit on the high side for my application but with >60V dropped across the cathode resistors it might turn out just right.

With this beast in the power supply and the 120W-rated 1650T output transformers it would really be a shame not to use 4 x PL519 per channel and get 50-60W class A...!
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Old 20th October 2013, 11:46 AM   #122
tinitus is offline tinitus  Europe
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maybe not a cathode follower
but what if you have a CT, and connect that to cathodes
any ideas ?
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Old 21st October 2013, 07:57 AM   #123
tinitus is offline tinitus  Europe
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dangerous stuff
and I have no clew what happens connecting OPT this way
better forget about it
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Old 21st October 2013, 03:49 PM   #124
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by tinitus View Post

dangerous stuff
and I have no clew what happens connecting OPT this way
better forget about it
Move the cathodes to the end of the primary winding and ground the center tap and it might work..
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Old 21st October 2013, 10:44 PM   #125
tinitus is offline tinitus  Europe
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... ground the center tap and it might work..
cool

any estimate how this affects OPT load impedance ?
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Old 8th October 2014, 07:28 PM   #126
yan24 is offline yan24  France
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Hi
Somes news about this projet

I realized recently an amplifier with EL34 trioded and 5W output
The driver is a 6SJ7

I use a bootstrap installation to provide enough swing to EL34
This is a diagram of the 60s
Listening to this amp is nice with wide band Fostex or Lowther

Yan

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Old 4th November 2016, 06:36 PM   #127
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Certainly not 20W, but I've been looking at this transformer coupled cathode follower thing for a low power amp (headphones or desktop speakers). Not sure if the 12AX7 would really be able to swing enough voltage (about 200V p-p) for a full watt output with only a 300V B+. Maybe a 12AT7 with a CCS around 5mA or a 6SL7 is a better choice.

Click the image to open in full size.

Any thoughts or suggestions are appreciated.
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Old 4th November 2016, 06:50 PM   #128
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Historical throwback time - this discussion made me remember a schematic.

About 25 years ago I had a collection of old Audio magazines - I remember this project, or something like it:
A Direct-Coupled Amplifier with Cathode Follower by Raymond H. Bates

I've always wanted to build it but getting primary resistance value (not impedance) was difficult back in the pre-internet days. And I was a poor college student

Last edited by kstagger; 4th November 2016 at 06:59 PM.
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Old 4th November 2016, 07:37 PM   #129
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Cool link, thanks! That's very similar to what I thinking about. And direct coupled.
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Old 17th November 2016, 03:43 PM   #130
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Here's another idea using two halves of a 6SN7 (choke loading the second) and a 6L6GC for a little more power:

Click the image to open in full size.

Using a VR (could also be a 0C3 or a resistor) to drop some volts above to keep the 6SN7 plates in the second stage well below the 300V max. They will swing above that with signal but that's typical of transformer coupled output stages, so I'm hoping it isn't a problem here.

I wasn't really optimistic about the ability of the 12AX7 to swing a full 200V ptp with 300V B+ in the previous schematic.
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