Replaced OT in my JTM45 build in order to convert amp to JTM50 specs.
Now the damn thing doesn't work. It was working fine before making this mod. I changed the OT, installed 1K screen grid resistors, and redid the connections of the new OT primary and secondary. I did not change filtering, yet.
When I fired up the amp for the first time, there was a spark between pins 3 and 4 (plate and screen grid) of one of the power tubes. I immediately shut down and saw that a little piece of debris appeared to have shorted the terminals. I removed it, cleaned up the socket and used DMM to determine that there was no continuity between the socket terminals. There was no arcing thereafter. However, a faint groaning sound emanated from the chassis (NOT the speaker) then stopped. It sounded kinda like letting the air out of a balloon while stretching the mouth.
Now the amp only makes low volume, distorted sound.
I replaced all tubes with no improvement. I double checked wiring, and all appears correct. I checked the OT for gross defects (shorts and opens) and found none. I confirmed continuity between chassis-mounted components and the circuit board. Confirmed bias voltage is correct to both tubes. Appropriate voltages present on all pins of tubes. Plate voltages are normal. Suspicious of the power tube sockets, I took various voltage and resistance measurements to compare the two sockets. It appears that one that arced measures the same as the other in all respects, leading me to think that the one that arced is OK.
There is one anomaly that I found. The DC resistance and voltage measurements between the two sides of the OT vary a lot:
When measured from OT primary center tap to socket or V5:
DC resitance is about 40 ohms.
DC voltage during operation is about 1.3 volts
When measured from OT primary center tap to socket or V4 the one that arced):
DC resitance is about 6 ohms.
DC voltage during operation is about 0.22 volts
With these values, the two tubes still draw approximately the same current, around 32 milliamps.
Does this info lead to any speculation? What testing should I try now? What was that funky sound I heard? The OT burning out?
Now the damn thing doesn't work. It was working fine before making this mod. I changed the OT, installed 1K screen grid resistors, and redid the connections of the new OT primary and secondary. I did not change filtering, yet.
When I fired up the amp for the first time, there was a spark between pins 3 and 4 (plate and screen grid) of one of the power tubes. I immediately shut down and saw that a little piece of debris appeared to have shorted the terminals. I removed it, cleaned up the socket and used DMM to determine that there was no continuity between the socket terminals. There was no arcing thereafter. However, a faint groaning sound emanated from the chassis (NOT the speaker) then stopped. It sounded kinda like letting the air out of a balloon while stretching the mouth.
Now the amp only makes low volume, distorted sound.
I replaced all tubes with no improvement. I double checked wiring, and all appears correct. I checked the OT for gross defects (shorts and opens) and found none. I confirmed continuity between chassis-mounted components and the circuit board. Confirmed bias voltage is correct to both tubes. Appropriate voltages present on all pins of tubes. Plate voltages are normal. Suspicious of the power tube sockets, I took various voltage and resistance measurements to compare the two sockets. It appears that one that arced measures the same as the other in all respects, leading me to think that the one that arced is OK.
There is one anomaly that I found. The DC resistance and voltage measurements between the two sides of the OT vary a lot:
When measured from OT primary center tap to socket or V5:
DC resitance is about 40 ohms.
DC voltage during operation is about 1.3 volts
When measured from OT primary center tap to socket or V4 the one that arced):
DC resitance is about 6 ohms.
DC voltage during operation is about 0.22 volts
With these values, the two tubes still draw approximately the same current, around 32 milliamps.
Does this info lead to any speculation? What testing should I try now? What was that funky sound I heard? The OT burning out?
