Your thinking is correct with regard to R510. The bias current is so low that anything less than 470K would probably take the power tube to cut-off, and anything an order of magnitude higher than VR2 would be OK with respect to the voltage divider. So I'm thinking your 270K value is good. I would expect R19 to be about the same.
Perhaps Sangram will post the values.
Perhaps Sangram will post the values.
Hi
R510 is 100K, as stated in first post 🙂
The bias circuit values are R501:1K, R502:10K, R503:33K, C512/504/503:100u/160V, transistor is specced as a 2N5401. IC1, as per your Q, is the heater supply regulator, a 78H05 mounted on a huge heatsink. Will be happy to answer any more questions.
Also, Captn Dave, you have a new private message 🙂
R510 is 100K, as stated in first post 🙂
The bias circuit values are R501:1K, R502:10K, R503:33K, C512/504/503:100u/160V, transistor is specced as a 2N5401. IC1, as per your Q, is the heater supply regulator, a 78H05 mounted on a huge heatsink. Will be happy to answer any more questions.
Also, Captn Dave, you have a new private message 🙂
I don't really know much about these things so I apologize in advance if this is a stupid question but I don't understand where the source of the short has been addressed. If this is set up to be sent back to the customer in running condition isn't there a likelihood of it happening again? I was also wondering, Is there a way for a grid short possibly pull enough current through the bias string to kill the pot?
Thanks very much
Thanks very much
^^Good question.
The replacement pot has been running for a few hours now, it has shown no signs of distress and the bias on that tube is rock-stable. I can only conclude that the pot was faulty to start with, and something caused catastrophic failure of the pot. The tube seems unharmed, as the rest of the circuit.
The protection probably works pretty well, as I understand it there are three possible fault conditions with a pot: short, open-circuit and wiper lift, and the biasing circuit copes equally well with all three conditions, putting the tube at cut-off.
In fact the user reported that one channel suddenly went weak and bias adjustment was not working (no change in bias for pot rotation), after which the flurry started.
To be honest, I don't really know why that pot would short out except if it was marginally built in the first place. It seems to be the only explanation, given that the replacement is basically able to handle only half the current of the original...
Maybe things will change after a few hours. I hope not, though 🙂
The replacement pot has been running for a few hours now, it has shown no signs of distress and the bias on that tube is rock-stable. I can only conclude that the pot was faulty to start with, and something caused catastrophic failure of the pot. The tube seems unharmed, as the rest of the circuit.
The protection probably works pretty well, as I understand it there are three possible fault conditions with a pot: short, open-circuit and wiper lift, and the biasing circuit copes equally well with all three conditions, putting the tube at cut-off.
In fact the user reported that one channel suddenly went weak and bias adjustment was not working (no change in bias for pot rotation), after which the flurry started.
To be honest, I don't really know why that pot would short out except if it was marginally built in the first place. It seems to be the only explanation, given that the replacement is basically able to handle only half the current of the original...
Maybe things will change after a few hours. I hope not, though 🙂
The grid resistor would have acted as a current limit resistor to the pot circuit in the event of a grid short. Considering the circuit topology, I would consider the pot to be the problem, not a symptom of a tube problem. That said, I'd cut the pot open to find out what happened.
A short from side to side of a pot is rare due to mechanical construction, and would require some mechanical short (wire fragment? etc) to get in the pot. There should not be enough potential across the pot to carbon track it.
Cut it open and tell us what you find.
A short from side to side of a pot is rare due to mechanical construction, and would require some mechanical short (wire fragment? etc) to get in the pot. There should not be enough potential across the pot to carbon track it.
Cut it open and tell us what you find.
I did that three days ago. The wiper seemed fine, but the pot basically measured 0 ohm from every terminal to every other terminal. Then I removed the wiper totally, so as to disconnect terminal 2, and it shows 0 ohm across the resistive element (1 to 3). Very strange, for a 1 watt wirewound pot carrying a few mA of current. Which is why I was totally befuddled to start with, as it did not seem to be a candidate for failure.
Though I've never seen the inside of one of these things before, the element itself looks fine. I'll pull out my camera in the morning and post some pictures of the innards of the pot. Till then 🙂
Though I've never seen the inside of one of these things before, the element itself looks fine. I'll pull out my camera in the morning and post some pictures of the innards of the pot. Till then 🙂
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