new 5E3 build has problem

[dfoxdavies]

I have recently finished a build of a tube amp to the 5E3 circuit. It was not a branded kit, but built from components sourced from a variety of suppliers. It performs well for most of the time but occasionally it cuts out to a much lower volume, sometimes with distortion. It can often be "reset" by re-seating the input jack, sometimes it fixes itself after a period. I have replaced all the input jacks with Switchcraft product.The issue occurs on all four of the inputs, which I guess rules out the first tube, which is a 12AX7. I have tried substituting different power tubes, but the issue persists. I have monitored the power rail, but the voltage remains largely constant, no big dips during a low volume phase.

It performs without issue for long stretches but then displays the fault randomly. I understand that diagnosing issues is largely impossible at a distance. This is the first tube amp I have built and I'd like to know if anybody has any pointers or suggestions on where I should look to try and solve the issue?

Thanks for any input and help. Dave.

[xtian]

Intermittent issues are tough. You can only diagnose them while the amp is misbehaving.

This sounds like a mechanical issue (bad solder joint, bad ground connection, etc.). I would approach this by playing a signal into the input and probing every component and connection with a wooden chopstick.

[angelodp]

Run the amp while open and wait for the problems, then chopstick all areas. It may be an overheated joint gets funky when very hot. I reckon you have re-flowed most solder joints? Wait for the next round of re-flowing until you get the symptoms, then with the amp open you have a better shot at finding the culprit. Post some hi-rez pics so it might get spotted.

[Stevem]

When it working normal note the D.C. Voltage on the red wire feeding the output transformer and see if it has dropped down when it acts up.

If you rig up a way to drive a constant signal/ tone into the amp You can also trace out signal flow with your voltmeter set for low ac volts if it's not auto-ranging or under 100.
If you resolder any of the pins on the tube sockets be sure to have the tubes out so that things heat up fast and the solder flows better.
It's easy to have a poor connections on the pot lugs and and any ground wires to the pots or too the chassis especially.

[dfoxdavies]

Thanks for all your inputs. I have reflowed many of the solder joints, but I plan on removing the chassis and giving it a thorough going over. I suspect one of the output tube sockets has a problem as one of the tubes did not 'light up' when I replaced it. Had to reinstall it.

[angelodp]

You have heater voltage all around?

[dfoxdavies]

Well it's taken me a few days to get back to my workbench, so I apologise to anyone looking for an update.

I have found a poor joint in the cathode resistor for the second 12AX7. Where it is mounted across the 'bypass' capacitor one end was barely joined causing the resistor to be intermittently unconnected to ground. I have not experienced a repeat of the issues so far, although not extensively tested yet I am hopeful that this has fixed the problem.

Thanks for all the comments and the time you have taken to help me out.
Dave.