ST-70 Bias Issue

[lthursdayl]

Hey All,

So I have got a question.

Earlier today I was checking my tubes4hifi st-70's bias voltage which I do every few months. Currently, I am running 6550 power tubes which Bob suggests biasing at .500v per tube. I built this amp a little over a year ago and the bias voltage has never been an issue, until today. Today when I went to measure the voltage the first tube I checked (the rear left tube when looking at the front of the amp) was running at ~.3v which was low, but not all that unusual. However, when I went to change the voltage, via the tube specific pot, there was not enough turn in the pot to correct the voltage to .5. When I went to check the tube in front of it it (front left tube), it was running even lower, and I again ran into the same issue, their was not enough turn in the pot to correct the voltage.

Then, when checking the tubes on the right side, they were both running at about ~1.6v, and again their was not enough turn in either of the pots to bring them down to .5. I played around with the pots for a while seeing if there was a combination of pot settings that could correct the voltage issue, but nothing corrected it. I was only able to transfer the bias issue from the two right tubes to the left tubes.

I then changed out the rectifier tube, because I read that it could be failing and causing the issue, but this did not fix the issue. I then read that for this amp, with the power off, each tube should have 10 ohms of resistance, which they all do.

Not sure how to move forward in correcting this issue, and was hoping for some guidance. I am thinking my power tubes may be failing, but not sure if this is actually the issue.

Thank you!

 

[rayma]

the first tube I checked (the rear left tube when looking at the front of the amp) was running at ~.3v which was low, but not all that unusual. However, when I went to change the voltage, via the tube specific pot, there was not enough turn in the pot to correct the voltage to .5. When I went to check the tube in front of it it (front left tube), it was running even lower, and I again ran into the same issue, their was not enough turn in the pot to correct the voltage. Then, when checking the tubes on the right side, they were both running at about ~1.6v, and again their was not enough turn in either of the pots to bring them down to .5.

The tubes should be ok. Also the cathode resistors have the correct 10R values.
Remove the rectifier and power on the amp. Measure the bias supply range
as the pot is varied for each output tube, probing at the bottom of each grid resistor
(not at the tube pin). Are these ranges all the same?

If they all have the same range, measure the DC voltage drop across each grid resistor.
This drop should be very small, in the mV range. If you get say 100mV or more,
this indicates a leaky coupling capacitor at that grid.

It's normal to have some bias adjustment interaction with unregulated HV and bias supplies.

 

[nigelwright7557]

How many hours are on the tubes ?
Could be the cathode emissions starting to fade and causing low current if the tubes are old.

Check coupling capacitor leakage first.

Another approach is to put all the bias pots in the middle before starting to adjust them.

 

[lthursdayl]

The tubes should be ok. Also the cathode resistors have the correct 10R values.
Remove the rectifier and power on the amp. Measure the bias supply range
as the pot is varied for each output tube, probing at the bottom of each grid resistor
(not at the tube pin). Are these ranges all the same?

If they all have the same range, measure the DC voltage drop across each grid resistor.
This drop should be very small, in the mV range. If you get say 100mV or more,
this indicates a leaky coupling capacitor at that grid.

It's normal to have some bias adjustment interaction with unregulated HV and bias supplies.

Thanks for this suggestion. I will try this later today and see what I find.

 

[lthursdayl]

How many hours are on the tubes ?
Could be the cathode emissions starting to fade and causing low current if the tubes are old.

The amp is about a year old, and I run it a few hours just about every day, but this does not seem to be enough hours for the tubes to be failing?

Check coupling capacitor leakage first.

Another approach is to put all the bias pots in the middle before starting to adjust them.

I will also try this today and see what I find. Thank you!