I've been building an amp - my own design, but using the usual building blocks, and I've hit a problem that I really can't find the solution to. I'm hoping it's something simple, but I'm at a loss...
I put the circuit together, and it was okay (sounded great!) apart from what I suspect was parasitic oscillation on the EL34s - the screen grids were glowing blue, although the bias current at idle seemed to be fine. So I tidied up all the wiring, made sure that there was plenty of space between the wires, etc.
Since then, I can't get anything out of the first preamp tube. Voltage is getting to the pins, the cathode resistor is correct with reference to ground, and I know it's not the tube, as swapping the first two 12AX7s doesn't alter the results - the V1 plate readings are in the region of 360V, whereas the plate for V2a is about 240V. i get the same readings without the tube in V1 regardless of whether I test the top or bottom of the socket, so the socket itself seems to be okay.
The signal can go from "input -> V1a -> volume -> V2a", or "input -> V1a -> gain -> V1b -> volume -> V2a" depending on how a relay switch is set. The relay is working as expected.
V1 and V2 are separated on the HT line by a 10k 5W resistor, and each tube has its own side of a dual 50uF + 50uF cap can. The plate resistor for V1a is 390k, for V1b it's 100k and for V2a it's another 100k...
Can anyone offer any insights into why V1a isn't conducting anything at all - I'm not seeing any output wave on my scope if I connect after the decoupling cap on V1a?
Topic: Preamp problem (Read 2578 times)