ST-70 DIYTube Driver Board Bias Problem

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Fri Jan 17, 2014 9:09 am

Bingo Boys! We have that "Push Pull Perfection" going on! I was first in line at Radio Shack to get a 10K 1/4W carbon Film Resistor. I ended up buying the 500 assortment pack! Ran a 10K across R3, for a 5K result. The amp biases perfectly, per Shannon's board instructions. 5K for R3 seems to put me close to mid point on the 10K wipers. At full counterclockwise, I am about 20ma or so. I biased at 40ma, and was able to balance easily to zero. EL34 tube temps are now all at 350f. My hats off to you folks for the help, and a fine product.

Mike
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby Shannon Parks » Fri Jan 17, 2014 6:15 pm

Excellent! Sounds like you've dialed it in. What was your B+ voltage, btw?

Hopefully Radio Shack makes it another year. I wouldn't be surprised if it went belly up in 2014.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Fri Jan 17, 2014 6:46 pm

I did not measure B+. I was not real sure if it was ok to measure it at pin 3 of the EL34's with the tubes removed. If so, I will measure it the next time I have the tubes out. I'm guessing that it is about 450v.
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Wed Feb 26, 2014 7:54 am

Shannon Parks wrote:You can monitor the bias voltage at J6 & J21 on the diytube ST70 board. I would just set the bias to a minimum (like you have them already w/ the 10K pots cranked), turn off the amp and wait 15 minutes, and then parallel another 10K resistor across R3 (you won't even bugger up your board that way). Get one at Radio Shack even, or have one of us mail you one. When you turn the amp on the bias will be lower so no surprises. Hopefully you'll be lower than 50mA so that you have an adjustment range, else we'll rethink the bottom voltage divider resistor, too (at R4).

Just calculate the tube dissipation at bias current times plate voltage. I wouldn't run over 28W.

Shannon


Shannon. I just wanted to clarify that you meant 28W. Is that correct? Is that not over the rated 25W? Also, you recommend 50ma for your driver board. Is there a reason for this, as opposed to 40ma? The amp sounds great at 50, but everyone else seems to bias lower!

Mike
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby Shannon Parks » Wed Feb 26, 2014 8:37 am

mazeeff wrote:Shannon. I just wanted to clarify that you meant 28W. Is that correct? Is that not over the rated 25W? Also, you recommend 50ma for your driver board. Is there a reason for this, as opposed to 40ma? The amp sounds great at 50, but everyone else seems to bias lower!

Mike


My rule of thumb for max dissipation is anode and screen dissipation ratings (25W+8W=35W) times 85% = 28W. That's just when I know I'm going to have problems. I think I started using this as my max as that's where Eico *normally* biased their amps. (lol)

I like the standard 50mA biasing. Less than that really raises the distortion. I'd rather burn up my tubes and have them sound their best.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Wed Feb 26, 2014 10:58 am

Shannon. Thanks for the moment of enlightenment! I did not realize that the screen dissipation was 8W. I also did not realize that the 50ma bias current is the sum of plate & screen current. How much of the 50ma is associated with screen current?

Mike
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby VuTran2003 » Mon Jul 25, 2016 10:11 pm

I can buy 1pcs ST-70 DIYTube Driver Board Bias Problem PCB
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