Amp sound cutting out

[Singlebladepickup]

My PRSF sounds awesome but the sound started cutting out last week and now it's more often than not, like the sound is now cutting in. When it comes on it sounds good and the reverb and vibrato work fine. Any idea what might be wrong with it?

[Stosh221]

Could be a number of things, but you might start by removing the tube covers and try gently wiggling each tube while the amp is on. Start at v1 and go from there. If you hear a loud crackle and/or the amp cuts in and out it could be dirty sockets, or maybe the pin receptacles are not tight enough to make reliable contact with the tube pins.

I recently had the same symptom with an old Fender amp of mine and it turned out to be a preamp socket that needed to be cleaned and re-tensioned.

I assume it goes without saying, but don't clean the sockets when the amp is on and if you're not certain the filter caps have been discharged. I think the plate voltage on a PR is north of 400 volts.

[Singlebladepickup]

Let's assume I know nothing about amps...how do I know if the caps have been discharged

[Stosh221]

Just to explain why this is worth trying, if the sockets are internally corroded (common in old amps) and the pins are making intermittent electrical contact, the vibration from playing can jiggle the tubes enough to cause them to cut out. Could of course be some sort of internal component failure, but this is a simple thing to try first.

You're safe doing the first part—wiggling the tubes while the amp is on to see if oxidation in the sockets is causing them to crackle and/or cut out. The only hazard would be that the tubes could be hot if the amp has been on for a while. I usually just touch the tip of the tube glass.

Here's a YouTube video that shows what to do.

https://www.youtube.com/watch?v=d_3KMXEBBJQ" onclick="window.open(this.href);return false;

If you get that sort of crackle or cutting in and out, then the sockets need to be cleaned. That's when you need to make sure the amp is off.

There are a couple of ways to clean the pins and contacts, some of which you can do without discharging the caps. First though, see if you're getting the loss of signal from wiggling your tubes, which will let you know if you need to clean the sockets.

[Stosh221]

. . . If after the wiggle test it's evident that your sockets need to be cleaned, here's a way to do it without sticking anything directly in the sockets. Watch from 1:46 to 2:30.

https://www.youtube.com/watch?v=hg2bGlfHI9w&t=100s" onclick="window.open(this.href);return false;

You don't have to remove the chassis from the amp, as in the video. Just turn it upside down and take off the back panel for easier access. Make sure the amp is off and unplugged! When you squirt the cleaner into the sockets don't shove the nozzle right into the pin receptacles. If you do it'll squirt back out into your face and eyes. (Don't ask me how I know this.) You can see in the video how he's holding the nozzle up a bit, and at an angle.

You don't have to use the cleaner being advertised in the video. I use red label Deoxit. All of these cleaners are designed to evaporate quickly. They are also very flammable, so I wait about a half hour before turning the amp back on to see if the cleaning worked.

If you do clean the sockets and you're still getting the cutting in and out, the sockets may need to be re-tensioned. It's not hard to do, but you have to know how to discharge the caps (and to confirm that they're discharged using a multimeter). I learned it from watching a lot of online how-to videos, but as those tutorials will generally emphasize, 400 volts DC can do you in if you make the wrong move.

If any of this looks too nerve wracking certainly don't attempt it.

[Stosh221]

Oh, last thing. Contrary to what they say in the second video, it's perfectly okay to handle (cooled) tubes with your fingers. The thing about finger oils cracking the glass is bunk. It's an issue with halogen light bulbs that burn at over 1000 degrees, but not audio tubes—which don't get much toastier than 200 degrees.

[Singlebladepickup]

Wow thanks. I'll probably try wiggling the tubes but I don't really trust myself not to get shocked doing any of that other stuff.

[Stosh221]

No prob.

BTW, if you try the wiggle test and it's clear a socket is dirty, and if you're hesitant to use the spray cleaner, you might have luck just removing and plugging the tubes back in four or five times. Sometimes that's enough to burnish off the oxidation or corrosion on the pins and sockets.

[Ursa Minor]

I'm currently troubleshooting my own SFPR for simialr reasons. Mine will intermittently just drop to like half volume. It doesn't happen often and it seems I can usually get it back to normal by turning it off and on again but thats not very good.

With stuff like this I always start with simple things to eliminate like cables, then on to tubes. My sockets are clean and not the culprit in this case. I rebiased my power tubes (JJs) since they were reading a bit low and then it seemed to help but then it started doing it again. Next I was going to start swapping preamp tubes one by one until a possible result presented itself. So far it seems like the 12ax7 phase inverter was the culprit. I haven't had an issue since i replaced it.

YMMV, but just a reminder to be patient and try things with a very systematic approach when troubleshooting.

Check this out if you haven't yet. http://www.geofex.com/ampdbug/lowpower.htm" onclick="window.open(this.href);return false;

[Singlebladepickup]

Mine was doing the half-volume thing at first, now it's often cutting out completely. The guy who sold me the amp said it probably needed to be rebiased, but didn't really explain why. Maybe he's right and it just burned a tube up most of the way. I haven't had a chance to mess with it at all though, so I'm hoping it's something I can just wiggle or clean.