Channel Problem with JTM 45/100 Build

[jckid649]

I recently built a JTM 45/100 and everything has been working perfect.

I let a friend borrow it to try it out and had been jumping the channels. While he had it he said after he pulled the jumper neither input of channel 1 was working. Both inputs of channel 2 work perfectly fine.

When you plug into either input of channel 1 you can turn the volume up and down and hear the change in volume from the hiss of the signal. But if you hit the strings on the guitar you hear nothing from it.

I can hear plugging and unplugging from the inputs of channel 1. Just nothing from the guitar.

Checked all my solder joints.

Did tap checks through everything in the preamp section and couldn't find anything loose or poorly soldered.

No stray pieces of solder or metal shavings or pieces of wire floating around grounding out anything. Any ideas?

[sunnydaze]

The normal and bright channels of the JTM45/59 bassman feed separate sides of the first triode. If one side of the tube is dead you would hear nothing from that channel. I spent a lot of time chasing a non-existent wiring problem once because of this. In your case, you can hear the hiss increase with the volume control, but no signal from the guitar is getting through - so it sounds like this is not the issue. There is some interaction between the bright and normal channels on these amps, so you may want to try subbing a different tube in the first spot.

You've also checked the connections. Assuming the tube is OK, it sounds like your normal input jack is somehow grounding the tip. Try plugging and checking the continuity to make the sure tip is not being grounded out. Sometimes the switching mechanism fails on the input jacks. If it fails open, you usually get hum, if it fails closed no signal.

Hope this helps.

Mike

[jckid649]

It crossed my mind that one side of v1 may be bad. Just went and swapped it out and its working fine. Thanks Mike.

[Structo]

Tubes are always the first suspect unless something else is obviously wrong.

Many, many guitarists try to troubleshoot a tube amp without verifying that the tubes are in good operating condition.

[jckid649]

Yeah everything was working fine and the tubes had not been used before installing in that amp. They were NOS, but had tested almost perfect. So I don't know what happened there. After reading Mike's response it confirmed something I had suspected, but just hadn't tested.