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TSE-II Checkout - No Bias Current in Left Channel

jerkbag

Member
2010-03-10 7:03 pm
Hi Folks,

Getting to the final stages of checkout of my first Tubelab build -- a TSE-II. Everything went well during first stages of checkout -- but I'm now at the point of biasing the 300b tubes, and I can't get current to flow on the left one.

Both 5842s are ~175V on the plates, B+ is ~410V before I start adjusting 300b bias. B- is ~275V. Once right tube is at ~50ma the B+ has drops to about 385V. When I turn up the left bias pot, no current shows on the left 300b, but if I keep turning it the B+ drops to about 370, and the bias current of the right tube will start to drop.

I switched the tubes, and the multimeter, but still have the same issue. Everything is hooked up with temporary connections.

Is it actually possible that no current is flowing through the left 300b? Or do you think I'm somehow measuring it wrong? If this situation is possible, any advice on how to troubleshoot would be much appreciated. I'm measuring current across R18 & R29 -- is there another way to measure to determine if the left 300b is actually drawing no current? The fact that the B+ drops when I turn the pot makes me think it is. . .

Thanks!
 
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jerkbag

Member
2010-03-10 7:03 pm
Must be a circuit connection error of some kind. Are you sure the measuring resistor is in series with the plate?

Thanks -- I'll double check today. Am I right in thinking it's not possible for current to be flowing in only one tube, as far as how the circuit is designed? (As I've switched tubes, and see B- and plate voltage on both 5842s). . . Or should I be looking for a specific connection / part failure? I suppose there could be an issue with the tube socket or another solder connection for that tube.

Are there other measurements I can take from the left tube to determine if it's operating?