_Steve
Well-Known Member
Was really trying to avoid posting for help but this has me totally stumped!
I've been firing up my (metro spec) JTM45 clone for the 1st time. After investigating a very low/weak output I tracked it down to the phase inverter (V3). The PI is only outputting ~1V pk-pk and only on the RIGHT-hand triode (pins 1-3). The left-hand triode isn't outputting anything.
I've tried isolating the PI by pulling all the other tubes (except the rectifier) and it still behaves the same with a test signal injected directly onto the left-hand grid. Ive also tried swapping out for a different 12AX7.
Ive checked and double checked the components and they seem to be correct. I've also measured the resistance of the grids & cathodes to ground and get exactly the values I expect based on the schematic.
Here is the schematic for reference:
Here are my voltages. Note how the plate of the left-hand triode (pins 6-8) is at ~ full B+, and the right-hand side is at a more expected voltage. I have double checked that the pin is wired to the correct node. Also the grid voltages are different between each triode and I would have expected them to be the same:
Here is a pic of the board. Note the NFB has been disconnected eliminate it as a cause:
Any clues in there? Something else I could check? I really hope im missing something really basic and stupid.
I've been firing up my (metro spec) JTM45 clone for the 1st time. After investigating a very low/weak output I tracked it down to the phase inverter (V3). The PI is only outputting ~1V pk-pk and only on the RIGHT-hand triode (pins 1-3). The left-hand triode isn't outputting anything.
I've tried isolating the PI by pulling all the other tubes (except the rectifier) and it still behaves the same with a test signal injected directly onto the left-hand grid. Ive also tried swapping out for a different 12AX7.
Ive checked and double checked the components and they seem to be correct. I've also measured the resistance of the grids & cathodes to ground and get exactly the values I expect based on the schematic.
Here is the schematic for reference:

Here are my voltages. Note how the plate of the left-hand triode (pins 6-8) is at ~ full B+, and the right-hand side is at a more expected voltage. I have double checked that the pin is wired to the correct node. Also the grid voltages are different between each triode and I would have expected them to be the same:

Here is a pic of the board. Note the NFB has been disconnected eliminate it as a cause:

Any clues in there? Something else I could check? I really hope im missing something really basic and stupid.