So, here we go again with some head scratching I got this Lucid amp in for service a couple of days ago and it's got an issue I can't seem to figure out. Its a simple circuit with 2 x 12AX7 and 2 x 6L6. I havent compared the circuit to anything yet, but it sounds like a Fender/early Mesa kind of thing with some additional controls. Anyway, it's got this ticking/popping/clicking sound that keeps comming randomly, mostly when the amp is warmed up. To me it sounds like something is discharging, but I'm just getting started at this so I could really need some help to get me going in the right direction. The popping is not very loud, you won't notice it much when playing, but it is very irritating none the less. I have traced it to appear before the power tubes, but I can't quite track it down. It's so brief that my scope (or my eyes?) doesn't pick it up everytime so it's time consuming to trace. I have tried to replace the tubes by the way. I could, of course, replace all caps but I'm wondering if this typically could be filter caps or other caps in the circuit? I have attatched a picture of my scope with a click that I managed to freeze It was measured at the input of one of the output tubes. Does anobody have any pointers here?
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There's two times that I have had issues like this and it was with 3 amps of 2 different brands.
The first amp had a bad OT that would arc between windings now and then, the other 2 amps where both circuit board reissue Fender Supereverb amps that would build up a charge in the board and that would arc over to the input grid of the PI.
In your amp you could have a bad output tube socket, a signal coupling cap or a bad power supply filter.
Proving out the coupling caps is easy as all you need to do is to lift the down stream end out of the circuit and see if the popping stops, unfortunately the power supply filters will need to be jumped out one by one to prove them out.
Good luck and please report back.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Thank you @Stevem This amp is built on a turret board, so I guess i'll start by swapping filter capacitors, that won't hurt anyway. There is one large 32+32 cap and the rest is tiny radial 22uf caps. It's easy to try new caps. Then i'll move on to the coupling caps it the filter caps don't do the trick.
Change the trigger settings on your scope will allow you to see the spikes more consistently. Set trigger to negative edge and set the trigger level to a negative level that's large enough to only trigger the scope on the negative noise spike. Wont fix your problem but you will be able to see it every time it happens.
I had a similar thing when the outer case of my standby switch was not grounded to the chassis. Charge building up for whatever reason at the high voltage switch. It may not be your problem on your amp but check your Power/Standby switches nonetheless. Maybe run it in the pitch dark and see if anything is arcing during the pop may help too. I had this happen internally on a multi layer PCB once also and I could see it but there was no fixing it. Hopefully this is not the case for you.
Thank you for all your thoughts on this matter I have poked around a bit more today and it seems like the pop gets a little more subtle when pulling V1 out, after pulling V2 (phase inverter) it is gone, so there must be something going on in the first half of the circuit, right?
Edit: I've replaced all filter capacitors without any luck getting rid of the clicking.
Well since you have narrowed it down now the the preamp section I would now take my voltmeter set for low D.C. Volts and a clip lead and hook it up at the down stream end of the plate coupling caps and the do the same on the tone stack caps that have high D.C. Voltage on one end and then look for the meter to read / show that intermittent D.C. Voltage pop.
A good coupling cap will be passing far less then a steady .250 volts of D.C. Down stream .
When you find it you would have found the bad cap, but it could be a bad preamp tube socket also.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
I'm really pulling my hair out on this one. I fiddled a bit with caps and rectifiers and I somehow managed to almost cook the power transformer in some way, I don't think I did anything wrong, but It started to make noises and smelling kind of funny (like a burned transformer ). I immediately turned it off and checked everything. After some cooling, I turned it back on and it seems like it is okay. But could this be a pointer to a bad power transformer? Could that cause these pops? Also, I poked a bit more (after getting the trigger settings right) with my scope and it seems like the pop is also present at the input jack... could it be as easy as a bad jack? The jack is also the main ground for everything except the first power section (theres two ground points in this amp, one ground point on one screw of the power transformer and the other at the sleeve of the input jacks).
Well hard wire that input jack ground to ground and see if it clears up the popping, that aside all those daisy chained grounds from the power supply on back to the the first gain stage is not the most noiseless way to do that in terms of 120 HZ hum!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
I had like two minutes yesterday ang did some more testing. What I noticed is that this spike is very much readable at the ground/chassis, so I guess we can rule out leaky/bad coupling caps? I also replaced a tube socket (V1) but that did not do anything for the popping. Just to summarise - this popping sound/spike is readable at a lot of spots in the circuit/signal path with my scope, including at ground and all the way from the hot end of the input, all the way to the secondary winding on the output transformer. This popping start after the amp has been on for about 10 minutes = heat is causing something..? Changing tubes and filter capacitors did not do anything. Changing coupling capacitors did not do anything. Does this start to sound expensive = transformers? or is it worth trying to replace the rest of the tube sockets also? Am I missing something..?
Have you tried this? Pull all the tubes and see if the sound is present. Without tubes, I'd expect to hear/detect nothing. Then put tubes in one at a time, starting with V1. If you get the noise with only V1, then you know it originates there and not further downstream. You can confine your search to that part of the circuit. If no, then insert V2. You hear noise? Then it is around V2, etc. I just skimmed the thread. Sorry, if this was already suggested, missed it.
So I tried pulling tubes one at a time, and remove all tubes and insert one at a time. What I noticed is that the popping is not impacted by pulling tubes, however the popping is not hearable when pulling V2 (there is only 2 pre amp tubes in here) but it is still readable on the scope (I'm monitoring ground/chassis). When pulling V1 and leaving V2 in, the popping is present in the speakers but it is less loud than when both V1 and V2 are present. The scope readings is about the same in both cases (also with the power tubes pulled). When reading the scope the popping is varying from time to time in amplitude. I'll upload an image, but it seems like theres a power discharge happening somewhere.
The popping is not present before the amp gets heated up, I left it on for like 30 minutes without tubes in and there was no popping/discharge read at my scope. After inserting tubes and leaving it on for about fifteen minutes more the popping started. After the popping started it did not matter if I pulled the tubes out again, the popping kept on popping.
Based upon the latest information, I'm even more clueless. I'll try to write down the schematics, at least a flow chart so that you can see the logic in this circuit. I'll be back in a little while
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I got this Lucid amp in for service a couple of days ago and it's got an issue I can't seem to figure out. Its a simple circuit with 2 x 12AX7 and 2 x 6L6. I havent compared the circuit to anything yet, but it sounds like a Fender/early Mesa kind of thing with some additional controls. Anyway, it's got this ticking/popping/clicking sound that keeps comming randomly, mostly when the amp is warmed up. To me it sounds like something is discharging, but I'm just getting started at this so I could really need some help to get me going in the right direction. The popping is not very loud, you won't notice it much when playing, but it is very irritating none the less. I have traced it to appear before the power tubes, but I can't quite track it down. It's so brief that my scope (or my eyes?) doesn't pick it up everytime so it's time consuming to trace. I have tried to replace the tubes by the way. I could, of course, replace all caps but I'm wondering if this typically could be filter caps or other caps in the circuit? I have attatched a picture of my scope with a click that I managed to freeze 
). I immediately turned it off and checked everything. After some cooling, I turned it back on and it seems like it is okay. But could this be a pointer to a bad power transformer? Could that cause these pops? Also, I poked a bit more (after getting the trigger settings right) with my scope and it seems like the pop is also present at the input jack... could it be as easy as a bad jack? The jack is also the main ground for everything except the first power section (theres two ground points in this amp, one ground point on one screw of the power transformer and the other at the sleeve of the input jacks).