5150II Not playing once warmed up?

[frozen]

Hi there. A little issue with my 5150II.

Everything is fine, until after about 15 minutes of playing, in which the amp starts to cut in and out, and then after a couple more minutes, does nothing at all. Turn it off for a minute, and then back on, and again it'll play for a couple minutes until it hits a point, and then cuts out again. I can hear the power tubes vibrating and they're glowing good... Maybe the OT on it's way out? I haven't yet tried the preamp out/effects in to/from another device, however I think my problem lies somewhere after that since my tubes are vibrating as normal... Opinions?

[Chuck H]

First thing I'll say is , don't play the amp until it's fixed. If you can hear the power tubes vibrating then they are working. That there is no sound could mean they are trying to play into an open load or some other improper condition. The OT probably isn't responsible for a failure like this. Which means you may also be running the OT into an open load. The OT and power tubes are expensive parts. Running them into a grossly improper load will kill them sooner than later. You should not do that anymore until the amp is fixed.

With the symptoms you describe I would start by checking the connections to the speaker out jacks in the amp and the speaker in jacks on the cabinet. Also check the impedance switch on the amp and if the cabinet has one check that too. I would then clean all the speaker connection jacks and replace the speaker cable. If that doesn't work I would try a different cabinet. If your using an attenuator that could also be responsible. Stop using it for now until you trouble shoot the problem.

Chuck

[Amp Kat]

Have you tried replacing your power tubes. It sure does sound like power tubes as that's what they do when they go and as Chuck said when they do go hard it will take something out with it. Another sympton is loop jacks and patching a cord from send to return will test that but it really sounds like a bad tube.

[frozen]

I'll check all that and get back here. The whole amp got new tubes, I think the Sovtek 6L6's were replaced with J&J's. The tubes do light up properly, however I still have the old tubes which still worked so maybe I'll put those in to see what happens. These tubes still glow nicely though, I'll swap em out to be safe.

[frozen]

Alright, I replaced the speaker cable, and replaced the power tubes, and still have the same symptoms. Everything works until about 15-20minutes of the amp being on, after which output ceases. And it's not like the guitar input is bad or anything, there is no hiss, no nothing from the speaker, and the tubes still vibrate. I don't get it. What other components characteristics change with temperature rise?

edit: It does this on both rythm and lead channels, if that helps. Are there any measurements I can take to help solve this? I have the schematics, and can read prints well, just no experience with this stuff.

[Enzo]

How about the small tubes behind the litle cover? Do they ALL remain lit when the sound fails?

[Chuck H]

I think the mention that he can hear the power tubes vibrating sympathetically is indicative of a speaker connection failure.

Chuck

[Enzo]

You're probably right.

[frozen]

Yeah the preamp tubes are staying lit. I think it's too consistent to be a speaker failure, considering it only acts on the time the amp has been on. If I play for 15min straight, then it starts kicking out. However, if I play it for 5 minutes, and then put it on standby for 10 minutes, and then turn it off standby and try, it still doesn't play, even though the speakers were playing 10 minutes again when I stopped playing, and they have not had anything through them for 10min. Guess I'll bust out the prints and the multimeter and see what I can trace it back to.

[jamez]

Hi,
when it hits its state of not working, before turning it off check the heater voltage is still what it should be (6.3 VAC) .. Just a thought !

[pdf64]

Is this the issue of JJ tube pins being skinnier than standard, and so not making good reliable contact with the tube socket (or was that 9 pins tubes only?). Peter.

[HTH]

Is this the issue of JJ tube pins being skinnier than standard, and so not making good reliable contact with the tube socket (or was that 9 pins tubes only?). Peter.

that issue was with the octals too - I had some of the early JJ KT77s with the skinny pins (what a pita).

[frozen]

I put the old Sovtek's in and they experience the same problem so I doubt it's the JJ tubes.

[frozen]

fixed

Just wanted to post what the problem was. I finally opened it up last night, and made my friend play while I was checking things out. Turned out to be the OT transformer connector was loose on the pins. I added some solder on the pins to bulk them up a bit, and tada! She runs like a dream again. Anyways, thanks for the help guys.