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Dynakit ST70 - No Output - No Bias

jefferylhenr

jefferylhenr

AK Subscriber
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I have repaired and restored just about everything under the sun solid state but have minimal exposure to tubes. I did have a set of Dynaco ST-70’s and PAS preamp in the 80’s but other than setting the bias it was trouble free.

Fast forward to today and this one has no sound out right or left and zero bias voltage at the front test points. All the heater Filaments light and there is no excessive line draw or dim bulb issues. The bias resistors test fine. Everything visually looks good and the last service sticker is dated 2005 from a Berkeley, Ca shop.

Could this be something as simple as a rectifier tube? I am just starting my research and other than what I mention have done no other testing. Ie I haven’t tested pin voltages. But since both sides are dead assume the only single source of failure is power. I don’t have a tube tester.

Should I just buy a rectifier tube and plug it in? Which one is recommended? Anything else?

I will post some top and bottom shots, but this one looks totally different than the two I had in the 1980’s. So guessing it was seriously upgraded. The fellow I got it from built it as a kit. There is basically nothing on the bottom to fail?
 
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jefferylhenr said:
I have repaired and restored just about everything under the sun solid state but have minimal exposure to tubes. I did have a set of Dynaco ST-70’s and PAS preamp in the 80’s but other than setting the bias it was trouble free.

Fast forward to today and this one has no sound out right or left and zero bias voltage at the front test points. All the heater Filaments light and there is no excessive line draw or dim bulb issues. The bias resistors test fine. Everything visually looks good and the last service sticker is dated 2005 from a Berkeley, Ca shop.

Could this be something as simple as a rectifier tube? I am just starting my research and other than what I mention have done no other testing. Ie I haven’t tested pin voltages. But since both sides are dead assume the only single source of failure is power. I don’t have a tube tester.

Should I just buy a rectifier tube and plug it in? Which one is recommended? Anything else?

I will post some top and bottom shots, but this one looks totally different than the two I had in the 1980’s. So guessing it was seriously upgraded. The fellow I got it from built it as a kit. There is basically nothing on the bottom to fail?
Click to expand...

What's the B+ voltage? :idea:
 
yeah, check the HV supply but it sounds like you have none. No HV means no current flow so nothing at the bias test points. Usually thats a dead rectifier tube but a loose connection isn't impossible.
 
HV DC output from the rectifier is on pin 8 of the 5AR4 socket.

8 on the EL34 socket is the cathode, if there is no current flow you get no voltage there. Would need to check 3 or 4 for high voltage DC. Thats the plate and screen, but honestly if you get zero volts at the cathode and the tube is glowing, I'd wager you get nothing at 3 or 4 either.

if you have zero DC at the 5AR4, its the 5AR4. The bias supply is working so that winding isn't open or disconnected from ground.
 
Really, try another rectifier if you have one. Even a temporary 5U4GB --- just to see if you get a nominal B+.
 
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