Strange pilot light experience

[Raoul Duke]

So I’m playing my 2nd gen ODS clone today and I hit an E flat at the 11th fret of the high E string and the note rings, then fizzes. At the same time - I notice the pilot light (regular socket type, not an LED) dimming. It’s like that note on that string is vibrating something and causing this behavior.

I hit the same note on the B string and it rings normally, no dimming. After going back and forth and doing it a few more times with the high E, the overall volume drops considerably. I turn down the guitar volume, put it down and get behind the amp looking and smelling for anything obvious. Nothing jumps out at me.

So I grab the guitar, turn up, and start playing again and the light brightens slightly and everything feels/plays/sounds fine. I play another 5 minutes, shut it down, let it cool, take it out of the cab and throw it on the work stand. Power it up, check voltages at all HV nodes, tube plates and cathodes, heater wires, power switches; everything checks out.

Only thing I didn’t check was the bulb - which after googling this - seems it could be the weak link here. Before tearing anything apart randomly or doing damage playing more - I figured I’d check here and see if there’s anything else I ought to look into.

Any ideas or suggestions are greatly appreciated and very much welcome!
Thanks!

[drewspriggs]

So I’m playing my 2nd gen ODS clone today and I hit an E flat at the 11th fret of the high E string and the note rings, then fizzes. At the same time - I notice the pilot light (regular socket type, not an LED) dimming. It’s like that note on that string is vibrating something and causing this behavior.

I hit the same note on the B string and it rings normally, no dimming. After going back and forth and doing it a few more times with the high E, the overall volume drops considerably. I turn down the guitar volume, put it down and get behind the amp looking and smelling for anything obvious. Nothing jumps out at me.

So I grab the guitar, turn up, and start playing again and the light brightens slightly and everything feels/plays/sounds fine. I play another 5 minutes, shut it down, let it cool, take it out of the cab and throw it on the work stand. Power it up, check voltages at all HV nodes, tube plates and cathodes, heater wires, power switches; everything checks out.

Only thing I didn’t check was the bulb - which after googling this - seems it could be the weak link here. Before tearing anything apart randomly or doing damage playing more - I figured I’d check here and see if there’s anything else I ought to look into.

Any ideas or suggestions are greatly appreciated and very much welcome!
Thanks!

Pulled the bulb out and seated it again or physically held your hand again the socket while playing (to add mass and stop it sympathetically vibrating if that's the issue)? I guess the wibbly voltage could cause some sort of interference that explains the note fizz

[maxkracht]

The amp could have an oscillation that is causing the power transformer to sag when you hit certain notes pulling down the filament, could also just be some oxidation on the bulb or a bad solder joint.

[Raoul Duke]

Understood Drew and Max, thanks for the replies!

After posting this, I unscrewed the jewel and immediately noticed that the bulb was loose in the socket. Took it out and then saw that its contact surface was sort of lop-sided - which I thought would make it prone to poor seating. So I cleaned the contact with scotch-brite and CRC electronic cleaner and then dabbed a bit of solder on the contact to re-shape it. Buffed it with scotch-brite again and put it back.

No more wiggle - nice and snug.

So I test played it again for a good 15 minutes trying to get that vibration to happen again and I couldn’t. Could it really be that simple? Guess I’ll try to repeat the anomaly again tomorrow and see.

Learn something new everyday…

[pullshocks]

Nice troubleshooting

[Raoul Duke]

Thanks pullshocks, but it turns out that wasn’t the end of it.

The next day I went to play it and got absolutely no sound (but the pilot light was consistently bright). Out of the cab again, bring it up, check voltages, inspect important connections, everything normal. Plug in a guitar and get faint sound cutting in and out. Really perplexing.

Thought about “check the simplest things first” and re-checked the 3-way impedance switch I installed a few days ago. Sure enough, the shorting jack at “speaker out” wasn’t opening all the way consistently and got worse as the amp heated up.

Gave the main contact a little “tweak”, then checked the jack operation a few times with a homemade feeler gauge (slice of a business card): plug out - card doesn’t go through contacts, plug in - card slides through. Played on the stand - guitar signal back to normal. After letting it warm up for about a half hour, re-tested. Strong signal, all voltages good. Reassembled, test - all good.

I’ll randomly re-try today, but I feel like this had to be the answer. I hope I’m right

[Raoul Duke]

Random update: tested the amp a few different times today and none of the strange behaviors have returned. Pilot light is bright and consistent; no cutting out with the sound, voltages are all good.

Great lesson for starting with the simple things first.

[drewspriggs]

Thanks pullshocks, but it turns out that wasn’t the end of it.

The next day I went to play it and got absolutely no sound (but the pilot light was consistently bright). Out of the cab again, bring it up, check voltages, inspect important connections, everything normal. Plug in a guitar and get faint sound cutting in and out. Really perplexing.

Thought about “check the simplest things first” and re-checked the 3-way impedance switch I installed a few days ago. Sure enough, the shorting jack at “speaker out” wasn’t opening all the way consistently and got worse as the amp heated up.

Gave the main contact a little “tweak”, then checked the jack operation a few times with a homemade feeler gauge (slice of a business card): plug out - card doesn’t go through contacts, plug in - card slides through. Played on the stand - guitar signal back to normal. After letting it warm up for about a half hour, re-tested. Strong signal, all voltages good. Reassembled, test - all good.

I’ll randomly re-try today, but I feel like this had to be the answer. I hope I’m right

Seems like you need a poltergeist exorcism more than tech support

[cdemike]

Do you know if the pilot light was as bright as it usually shines during the last incident? I'm tempted to say your mains AC may have dropped if you were playing during the day. If the weather there is warming up (it certainly is here in AL, but then again I've never visited Massachusetts), you may have had some dips in mains voltage. It may also have been difficult to see exactly how bright the pilot was shining if it was bright out, obscuring low voltage.

Also, is the chassis mounted in the same orientation in the cab as when it's on the bench checking out? Depending on the kind of wire you used, it seems like gravity could be moving some wires' positioning potentially causing an oscillation or opening a bad joint (both seem unlikely, though, if this amp has been working for some time prior to these incidents).