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Installed new OT, now amp doesn't work. Troubleshoot time.

willhutch

Silver Supporting Member
Messages
1,907
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?
 

RussB

low rent hobbyist
Messages
11,165
put the original OT in and try it. Looks like you killed the new OT

My tip for the day is that you need to make yourself a current limiter. It is just a 100 wat or so light bulb in series with the hot lead of an extension cord. There is info all over the web about these. What this gadget will do is show you if there's a short in your amp when you power it up. No one should be without one if they want to work on amps
 

willhutch

Silver Supporting Member
Messages
1,907
It appears to be a bad OT. I will get another OT and replace the socket as well. I was about to delete the thread since I have the problem solved, but figure I'll keep it as a lesson to others.

I believe this happened because of a piece of debris, a clipped off piece of wire, shorted two socket terminals.

Moral of story: keep your space clean, work slow, and be conscientious.

Sure wish a fuse would have just blown instead.
 

WesKuhnley

Member
Messages
2,241
I like to blow the chassis out with an air compressor (de-humidifier run in the line to keep condensation out) to make sure there's no garbage in the chassis before I fire something up. That, and use a variac (or a light bulb current limiter will work) to make sure everything is hunky-dory as the amp fires up.

Hope everything ends well for that amp...I've heard it's a killer!
 

dwvinky

Member
Messages
123
It appears to be a bad OT. I will get another OT and replace the socket as well. I was about to delete the thread since I have the problem solved, but figure I'll keep it as a lesson to others.

I believe this happened because of a piece of debris, a clipped off piece of wire, shorted two socket terminals.

Moral of story: keep your space clean, work slow, and be conscientious.

Sure wish a fuse would have just blown instead.

I've worked on electronics for 15 years and the fuse never blows. I've worked on plenty of stuff hit by lightning and the fuse is the only thing that wasnt fried. Fuses in amps only protect the power supply.
 

rockon1

Platinum Supporting Member
Messages
13,202
I've worked on electronics for 15 years and the fuse never blows. I've worked on plenty of stuff hit by lightning and the fuse is the only thing that wasnt fried. Fuses in amps only protect the power supply.
Is that a Murphy's law type of thing? I was just admiring the amount of fuse protection in my Peavey equipment. My Ultra Plus has five internal fuses. Are you telling me they are worthless? Bob
 




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