Metropoulos Forum

Ok, so I just finished completing a JMP 2203 clone using Metro instructions. I got a little impatient and went ahead and fired it up without testing it. Well I heard a lot of crackling coming from the amp itself and not the speakers, and the power tubes were glowing bright orange. So I turned it off and started testing.

I've got a voltage reading of 160-180v coming from the IEC socket, should be 120. All the ac voltages on the tube sockets are at about 4.1, and I'm not getting any reading at all on pin 5 of the power tubes.

So what could be the problem?

Other than possible meter problems, the high AC at the IEC has to be your location's electricity. All voltages will be high until that is figured out. No negative voltage on pin 5 obviously means that something in the bias supply is not wired correct. Don't put any tubes back in until you fix the issues.

Check your wall socket and see what it measures.

I'm betting that the meter is crap and the high voltage your meter is seeing at the socket has no bearing on the fault.

Ok, ignoring the high ac voltages, what could be causing the problem for having no reading on pin 5 of the powertubes? And also, most of my other dc readings on my preamp tubes seem too high. But I realize they'll go down with powertubes installed so I'm not really worried about that.

If someone could point out what wires I need to check for the bias supply I'd much appreciate it.

Major Sparky wrote:...what could be causing the problem for having no reading on pin 5 of the powertubes?

Below is a picture of the bias voltage supply line. Check for DC voltage at the circled point and then trace/check the points moving to the left. After the diode (above and to the left of it), the voltage should be negative.

Make sure the blue bias caps and the bias diode are correctly oriented also (caps positive sides facing "down" to ground and the diode line facing the same direction).

Got a new meter and the high voltages are gone. Every pin is good now expect for pin 5. I started tracing the bias supply and it's good at that circle there and past the diode. But once I get to the other side of the 15K resistor I get nothing. I have about -20dc before it, but 0 after it.

I checked the 15K resistor and it's fine.


EDIT: Found my problem. I had accidentally wired the negative feedback wire wrong. Everything's fine now.

Major Sparky wrote:Got a new meter and the high voltages are gone. Every pin is good now expect for pin 5. I started tracing the bias supply and it's good at that circle there and past the diode. But once I get to the other side of the 15K resistor I get nothing. I have about -20dc before it, but 0 after it.

Are your bias caps the right way round?