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I had a not so common problem with a new build I just completed that I thought I'd share in case someone else can benefit from my experience.
The amp passed the initial testing procedures fine. All the voltages checked out normal throughout the power supply and at all the tube sockets. The power tubes were idling with the expected cathode current. All ok so far.
Now the problem: when I plugged it into a speaker cabinet and tested it with a guitar there was hardly any sound.
I checked the obvious first. Changed all the tubes to see if a bad tube was the probem - no. Next checked for an obvious wiring mistake - none to be found. I figured the signal was being lost somewhere and realized it was in the first pre-amp stage since it could not overdrive the next stage when it's output was turned up full.
I checked the tip of the input jack for isolation from ground and found there was only 740R when there should have been 1M. This explained where my input signal was going. There must be another path to ground besides the 1M grid leak. The input goes straight to a switching circuit with a relay that I built on one of those boards with all the holes that are spaced for IC's (perf boards I think they're called). This switches a triode stage in and out of the chain. I tested the relay for internal shorts and it was fine. I tested for continuity on the board between points that should have been totally isolated and found a finite resistance between them. I concluded the board surface was conductive and that I had to rebuild the circuit on a new board.
I pulled the board out after cutting all the wires (not something I wanted to have to do on a new build). When I tested the connections on the board out of circuit there was no continuity anywhere. I re-checked the input jack tip in the amp, which was now definitely connected to nothing, and still found the 740R to ground. What????? This narrowed the problem down to to either the jack itself or the shielded cable connected to it which had the sheath connected to ground.
It turns out the way I'd soldered the hot wire of the shielded cable, it's insulation was also touching the lug in addition to the bare wire. The insulation consists of teflon followed by a thin layer of black material that the sheath sits on. It turns out this black layer is conductive. As a result, the hot wire was connected to the sheath (grounded) by a distance of a few millimetres along this layer of black material which was the 740R path to ground.
Although this may be a rare kind of problem to encounter I thought I'd post it in case it can save someone else tearing a new build apart or their hair out trying to troubleshoot a problem that makes no sense.
Greg
The amp passed the initial testing procedures fine. All the voltages checked out normal throughout the power supply and at all the tube sockets. The power tubes were idling with the expected cathode current. All ok so far.
Now the problem: when I plugged it into a speaker cabinet and tested it with a guitar there was hardly any sound.
I checked the obvious first. Changed all the tubes to see if a bad tube was the probem - no. Next checked for an obvious wiring mistake - none to be found. I figured the signal was being lost somewhere and realized it was in the first pre-amp stage since it could not overdrive the next stage when it's output was turned up full.
I checked the tip of the input jack for isolation from ground and found there was only 740R when there should have been 1M. This explained where my input signal was going. There must be another path to ground besides the 1M grid leak. The input goes straight to a switching circuit with a relay that I built on one of those boards with all the holes that are spaced for IC's (perf boards I think they're called). This switches a triode stage in and out of the chain. I tested the relay for internal shorts and it was fine. I tested for continuity on the board between points that should have been totally isolated and found a finite resistance between them. I concluded the board surface was conductive and that I had to rebuild the circuit on a new board.
I pulled the board out after cutting all the wires (not something I wanted to have to do on a new build). When I tested the connections on the board out of circuit there was no continuity anywhere. I re-checked the input jack tip in the amp, which was now definitely connected to nothing, and still found the 740R to ground. What????? This narrowed the problem down to to either the jack itself or the shielded cable connected to it which had the sheath connected to ground.
It turns out the way I'd soldered the hot wire of the shielded cable, it's insulation was also touching the lug in addition to the bare wire. The insulation consists of teflon followed by a thin layer of black material that the sheath sits on. It turns out this black layer is conductive. As a result, the hot wire was connected to the sheath (grounded) by a distance of a few millimetres along this layer of black material which was the 740R path to ground.
Although this may be a rare kind of problem to encounter I thought I'd post it in case it can save someone else tearing a new build apart or their hair out trying to troubleshoot a problem that makes no sense.
Greg
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