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Yet another Dyna ST-70 Input Board

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Even in the face of many great ST-70 input board designs, I foolishly went out and built another one. This input board uses a paralleled 12AT7 as the input voltage amplifier. This is capacitively coupled to a long tail phase splitter made up of another 12AT7 (it could be a 12BH7 etc). There is an AC balance potentiometer on the phase splitter, and each tube has it's own bias potentiometer.

Fitting four tubes and all the coupling caps into the space of a ST-70 input board is pretty tough. I also wanted to have room for big caps like the Sprague orange drops. So I put the coupling caps under the board.

For more info, I've put the schematic, layout and more pictures of it here: Sheldon’s World Blog Archive Does the world need another Dyna ST-70 Input Board?

Comments and critiques are always welcome,
Sheldon


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Someone ought to build a board with some 3 wire Molex connectors underneath so a person could simply connect a short 3 wire harness swapping pins 2,6,7 so one could choose to use 7199's or 6gh8's without a big clunky socket adapter. OH, I forgot, I already did that! Nevermind.
 
Very nice looking board. Would love to try it:D:D


Well, the schematics and board layout is in the blog post.

BTW, I've added another compensation network in parallel with the input tube plate resistor. The compensation network is a 15K resistor and a 220pF cap which squashes a small amount of square wave ringing (especially when driving a capacitive load)

20to20: Thanks for that idea, damn shame it's completely irrelevant to this board.

Sheldon
 
Not quite the same as a hum balance, but along the same lines. The AC balance adjusts the gain of one half of the phase splitter to that the output tubes (if not perfectly matched) can be adjusted for equal drive signals into both halves of the output transformer.

There's a couple ways to adjust it, the easiest is to put a constant sine wave into the amp (with dummy load on the output, obviously), and measure the AC voltage at each bias point. Adjust the AC balance to make them equal. More accurately, use a spectrum analyzer to adjust. The bias point measurement works damn well though.

Sheldon
 
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