ST-70 DIYTube Driver Board Bias Problem

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ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Thu Jan 16, 2014 1:35 pm

I am a new member here. My name is Mike. I recently acquired a 1967 ST-70, with both the DIYTube driver board, and the SDS Labs cap board with solid state rectification. The unit seems to work fine, but I can not get it to bias properly. I am a EE who recently retired after 30 years at Collins Radio, but never worked on tubes until now. When I first got the unit, both 10K bias pots were full counterclockwise. The bias all four Svetlana EL34's was above 60ma. rotating clockwise only increased the bias voltage, so I stopped, and shut the unit down. Resistance checks at J6 and J21 read 10K-14K as I rotate the bias pots. The bias pots are both dated 1967. All four bias resistors measured 10.5 ohms. I am a little lost on where to look next. Any help would be appreciated. BTW, I tried to use the search function, with no luck.

Mike
Last edited by mazeeff on Thu Jan 16, 2014 2:54 pm, edited 1 time in total.
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby 20to20 » Thu Jan 16, 2014 2:30 pm

Mike, welcome.

Does your unit happen to still have the old selenium rectifier in the biasing supply circuit? If so, it's probably the culprit.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Thu Jan 16, 2014 2:37 pm

No. It is my understanding that the SDS labs board replaces the old selenium rectifier, and the old one has been replaced with a bolt and nut.
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby 20to20 » Thu Jan 16, 2014 2:55 pm

mazeeff wrote:No. It is my understanding that the SDS labs board replaces the old selenium rectifier, and the old one has been replaced with a bolt and nut.


OK, rules out the simple thing. That was the problem on my stock unit. Same symptom.

I've not used Svets in mine so I can't point the finger at them but from tube to tube some have different biasing characteristics and those may present a higher current demand than others. If the original pots and the other 2 stock biasing supply resistors for each are still close to normal value then it seems to point to tubes or the fact the B+ voltages are probably higher than the stock voltages since it's SS rectified now. It could be a combination of too high B+ and high draw tubes. Check the other resistors for normalcy... unless those are not part of the circuit any more. Flying blind about the SDS board.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Thu Jan 16, 2014 5:03 pm

I guess that I am surprised that a higher B+ and the Svetlana tubes would cause the bias adjustment to rail off like that. In a stock configuration, what range of bias current do you get over the entire range of the pot? I am fully counterclockwise and getting 60ma+. The SDS board provides the same bias supply voltage circuit that is shown in the diytube driver board schematic. The two 10K resistors used with the 10K pot have been tested, and are good.
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby Shannon Parks » Thu Jan 16, 2014 6:04 pm

Mike, the original bias adjustment range was tweaked for a B+ much lower than what you have since you are going the SS route instead of the 5AR4 (which has a 20V to 25V voltage drop). You'll need to change R3 to 6.8K or even 4.7K. A lot of people using the 5AR4 and having high AC mains (like 125VAC) need to do the 6.8K mod normally. This should fix it for you.
http://www.quadesl.com/pdf/st70_capboard.pdf

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Thu Jan 16, 2014 6:33 pm

Thanks Shannon, and everyone else. I will order the part, and look forward to listening to your board!

Mike
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Thu Jan 16, 2014 6:43 pm

Shannon. Just so that I am clear. Is it R3 or R4 that I should reduce? R3 looks like it connects to D1, whereas R4 connects to ground. I would have thought that R4 would be the one to reduce, since we are trying to reduce voltage. Perhaps, I am confused!
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby 20to20 » Thu Jan 16, 2014 8:23 pm

mazeeff wrote:Shannon. Just so that I am clear. Is it R3 or R4 that I should reduce? R3 looks like it connects to D1, whereas R4 connects to ground. I would have thought that R4 would be the one to reduce, since we are trying to reduce voltage. Perhaps, I am confused!



You are going to "reduce" the voltage relative to 0V. It will be going more (-). That is what is needed to reduce the current through the tube. Reducing the first resistor gets you closer to the (-) transformer output voltage after the bias rectifier.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Thu Jan 16, 2014 8:28 pm

Got it. Thanks,
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby 20to20 » Thu Jan 16, 2014 8:39 pm

mazeeff wrote:Got it. Thanks,


Cool.

You could get precice with it by using some clip leads to jumper in a 10K pot first, then dial in the exact value you need to center the main adjustment pots.

Just make sure the clips don't pop off during the hot run or else you will lose the biasing voltage and you'll get a runaway tube current. Be VERY carefull if you try that.

A safe way to do it would be to leave the 10K original in place then parallel jumper a 50K pot to it or a range of fixed values untill the original pots are centered then read what you have for total R.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Fri Jan 17, 2014 4:06 am

Thanks 20. I will try your "safe way"! It will take me a few days to get the parts, and I'll report back on the progress. Is there a way that I can test the bias voltage swing on pin 5 of the EL34's, with tubes removed, and power on? The Diytube manual mentions -35v on this pin, but does not describe the range. I have only owned the amp for two days. It is a lot of fun to troubleshoot these things. I bought it from a guy that restores radios and guitar amps. He indicated that he had restored it a few years back, and he gave me a package with all the re-build documentation, including the Diytube board manual. He must have been scratching his head, as to why he was never able to get it to bias properly. The amp sounded great at 60ma, but I would guess that tube life would be shortened. Is that correct? I measured that surface temp of the EL34's, and they were slightly over 500f.
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby Shannon Parks » Fri Jan 17, 2014 5:26 am

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.

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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Fri Jan 17, 2014 5:56 am

Thanks Shannon. I have a Radio Shack less than 2 miles away!!! Forgot about them. They have the 10K 1/4w in stock. I will report back after the change.
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Re: ST-70 DIYTube Driver Board Bias Problem

Postby mazeeff » Fri Jan 17, 2014 6:26 am

This may sound dumb, but is it ok to measure the plate voltage at pin 3 of the EL34's with every tube removed, including the three on your driver board?
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