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Dynaco ST70 low bias - help needed

M

Miller

New Member
Recently purchased a dynaco st70 and everything appears to work - sound seems decent, but measurements from unit aren’t aligning with manual. Quad cap, bias knobs and caps and selenium rectifier appear to have been replaced.

bias measurements are low also - ~0.36

images and measurements

85787D69-EBB3-465D-A1C6-D6F73BCC9FBC

85787D69-EBB3-465D-A1C6-D6F73BCC9FBC

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  • Date added

D40E1185-480E-4CBD-9A39-72A0CABC4304

D40E1185-480E-4CBD-9A39-72A0CABC4304

  • Media owner Miller
  • Date added

271C7189-5219-4671-8FD9-54561ED532B1

271C7189-5219-4671-8FD9-54561ED532B1

  • Media owner Miller
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Check the bias voltage going into the pots, The selenium rectifier was replaced with a diode, possibly they didn't adjust for the increase in bias voltage.
 
What rectifier tube is in here? Doesn't look like a 5AR4. If it has been subbed with something that drops more voltage or if its just weak, the bias voltage will also be down.
 
What's the voltage to chassis from the center of the rear of the board between the two resistors? 330-350 VDC would be nice.
 
If current through the EL34 is too low ( by voltage on biaset test poing or pin 8 ) then the negative voltage
on pin 5 should be reduced. ( another cause is that the EL34 is worn out ).
Replacing the Se rectifier with a Si diode results in a more negative voltage. the resistors
 
Rectifier looks like a 5V4G ? Can't quite tell, but if its a 5V4, thats definitely not a stout enough tube to get the job done. The 5AR4 is rated for nearly double the current capacity. If thats just worn out from being abused terribly it could certainly explain this.
 
gadget73 said:
Rectifier looks like a 5V4G ? Can't quite tell, but if its a 5V4, thats definitely not a stout enough tube to get the job done. The 5AR4 is rated for nearly double the current capacity. If thats just worn out from being abused terribly it could certainly explain this.
Click to expand...
I think your right. I had a couple 5V4G's that looked just like that. Get a 5AR4/GZ34. But not a JJ brand. Mullard, Tung Sol or TAD would be better.
 
Thanks all. Ordered a Mullard today. What should I expect with this - will it change sound quality ? Power output?
 
If the old rectifier was weak, with a new one, you might notice a little more power with less distortion but it all probably depends on your ears. ;)
 
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Replaced the rectifier with a Mullard. Running at ~ 1.40 volt bias on both sides now. Thanks for the help.

I’ve seen in some forums - less bias ~1.25v is better - should I try to get it up to 1.56 or just leave / reduce a bit.
 
The stock 1.56v is honestly not hard on the tubes, and running it lower increases distortion. Still, 1.4 is way closer to right than 0.36
 
Well I biased my VTA ST70 yesterday due to tube rolling at 400 MV per side. Sure sounds good to me and the transformer doesn’t get really hot.
 
One quick question here - I think the 6ca7s on this thing are weak as well. When I buy replacements to I get matched pairs or a match quad or does none of that matter?
 
2 pairs / 1 quad, same-same. If you end up with 2 pairs, just make sure you mark them somehow to keep those together in the channel. Quad has the advantage of not needing to pay much attention to it.
 
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