Yet another Dyna ST-70 Input Board

stokessd · 30th June 2010, 02:53 AM

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

Ty_Bower · 30th June 2010, 03:06 AM

Nice looking board. Proof-read your blog text, you've repeated yourself in some sections.

20to20 · 30th June 2010, 03:30 AM

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.

Original Burnedfingers · 2nd July 2010, 10:54 AM

Very nice looking board. Would love to try it

stokessd · 3rd July 2010, 12:31 AM

Quote:

Originally Posted by 6BG6GA

Very nice looking board. Would love to try it

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

20to20 · 3rd July 2010, 02:51 AM

Quote:

There is an AC balance potentiometer on the phase splitter

Is that the same as a hum balance? How do you set it?

stokessd · 3rd July 2010, 11:14 PM

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

30th June 2010, 02:53 AM	#1
stokessd diyAudio Member Join Date: Nov 2002 Location: Fort Wayne, IN	Yet another Dyna ST-70 Input Board 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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30th June 2010, 03:06 AM	#2
Ty_Bower diyAudio Member Join Date: Mar 2007 Location: Newark, DE	Nice looking board. Proof-read your blog text, you've repeated yourself in some sections.

30th June 2010, 03:30 AM	#3
20to20 diyAudio Member Join Date: Jun 2010 Location: W-S, NC	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.

2nd July 2010, 10:54 AM	#4
Original Burnedfingers diyAudio Member Join Date: Nov 2002	Very nice looking board. Would love to try it

3rd July 2010, 11:14 PM	#7
stokessd diyAudio Member Join Date: Nov 2002 Location: Fort Wayne, IN	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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