I am trying to debug an amp that I helped Jason Garel build last year. The circuit is based on the DECWARE SE Stereo ZKit1, but point-to-point wired instead of using (buying) the circuit board and kit components. It is cathode biased with a shared cathode resistor for both channels (150 ohm) for the two EL84s. The driver (6DJ8 plate resistor loaded common cathode) is capacitor coupled to the grid of the EL84, with a 330K grid leak and 1K grid stopper resistor.
Problem Description: With the amp started, both output tubes bias up with 12.5V across the shared cathode resistor, i.e., 83mA total current. After a few minutes, I am seeing the voltage at the grid of one tube (at only one of the two sockets) rise from nominally 0V with respect to ground until it eventually after several minutes rise to the cathode voltage. Naturally, at this point the tube is conducting heavily with a bit of an orange glow on the plate. Overall voltage across the shared cathode resistor rises to about 20V, with a toasty 133 mA total current.
Swapped tube, and the problem followed the socket, not the tube, I checked all connections, everything reads A-OK in terms of value and layout. Tested the coupling cap, with the output tubes removed, and the capacitor passes no DC from the driver tube plate. Resoldered all the connections just to make sure there was not a bad solder joint.
Still, the problem is there. The tube sockets appear to be fine, new bright condition. The amp was setup in UL mode AND triode mode exhibiting the same problem. One thing of note, the voltage is a bit high for an EL84, i.e. 325V on the plate, but using Russian EL84s (New Sensor "Mullard"s). Under normal operating conditions, the plate dissipation would be 13W, which is a bit high. Oh, there is a 1K screen stopper installed, and pin 6 is not connected to anything (should be internally connected to cathode in the tube)
The puzzling thing is both channels are absolutely identical in layout, but only one socket has the problem, and the problem follows the socket, not the tube. Therefore, it is not a case of a mismatched tube.
Any ideas?
David
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