Faulty Power Tube?

[Kyaz Music]

Hey everybody!

Hope everyone is doing fine during these weird times. I'm trying to diagnose a problem with a Marshall JTM60 amp from '95. I'm experiencing almost gate-like sound dropouts every now and then. The interesting thing is that the sound comes back immediately after I play another note. Almost as if the coming transient in the voltage opens up a noise gate. While trying to examine those moments I managed to catch a moment where the sound went of completely, including the ground hum and it was completely silent until I played another note at least 10 seconds later. This made me come to the conclusion that the problem is in the power tubes and that the voltage flow gets interrupted.

What would you guys recommend me to try?

Thanks in advance for the replies!

[Chuck H]

Sounds like an intermittent connection to me. A bad solder joint, faulty pot, dirty tube pin, dirty jack switch, etc. I would start by using a cleaner/de oxide product on all unsoldered contacts. Pot cleaner is different from contact cleaner so you should keep them exclusive.

[Mick Bailey]

I always carefully inspect the PCB under magnification on the solder side with these amps. They can develop circular cracks around component legs and varying degrees of failure. Be really thorough and re-solder anything that looks suspect.

[pdf64]

One bad power tube of a pair couldn’t cause complete silence. It must be something common to both; sure the speaker / OT secondary circuit is good?

[Kyaz Music]

Thanks for the replies people. Another useful point is that I haven't received this issue on the overdrive channel so far.

Sounds like an intermittent connection to me. A bad solder joint, faulty pot, dirty tube pin, dirty jack switch, etc. I would start by using a cleaner/de oxide product on all unsoldered contacts. Pot cleaner is different from contact cleaner so you should keep them exclusive.

Great, took the amp apart now will start with some inspection and some contact cleaning.

I always carefully inspect the PCB under magnification on the solder side with these amps. They can develop circular cracks around component legs and varying degrees of failure. Be really thorough and re-solder anything that looks suspect.

On a quick inspection, some solder joints have a small oxidation around them but nothing that looks like a crack on the solder itself. I will take a closer look once I take the pcb out.

One bad power tube of a pair couldn’t cause complete silence. It must be something common to both; sure the speaker / OT secondary circuit is good?

Good point. I'm not sure what you refer to by speaker / OT secondary circuit. I'm not very experienced with amp circuitry but would be glad if you can elaborate a bit. Thank you.

[Jon Snell]

The first thing I would check is the effects send/return sockets. Insert a jack plug in both of them about 10 times and the same with the speaker out socket.
That will clean the contacts.

[mozz]

Is that the model with all the problems of overheating?

[g1]

Another thing that could cause this symptom (if it's not an intermittent connection) would be ultra-sonic oscillation.

[Chuck H]

The first thing I would check is the effects send/return sockets. Insert a jack plug in both of them about 10 times and the same with the speaker out socket.
That will clean the contacts.

And it will clean them even better if you apply contact cleaner. Also, many effects loops in amps use isolated switches actuated by a plastic button. Plugging in and out doesn't help much for these. Better to "see" the contact and "clean" it. Though I've gotten myself and a few other guys through a gig by plugging in and out several times to establish a patch of decent contact on a jack.

[Jon Snell]

And it will clean them even better if you apply contact cleaner. Also, many effects loops in amps use isolated switches actuated by a plastic button. Plugging in and out doesn't help much for these. Better to "see" the contact and "clean" it. Though I've gotten myself and a few other guys through a gig by plugging in and out several times to establish a patch of decent contact on a jack.

I was merely suggesting a solution to fault finding.
Most issues as described are caused by lack of movement within the jack socket switch contacts, causing tarnish to build up and a poor connection.
Always replace faulty sockets for your own peace of mind.

[g1]

Another troubleshooting option is just putting a short patch cable between send and return (or pre-out to pwr.amp in).

However, post #5 he stated he hasn't had the problem occur when using the OD channel, so the fault may be specific to the clean channel.
Still, always worth verifying any switching jacks, like FX return or inserts, when dealing with any intermittent fault.

[pdf64]

... I'm not sure what you refer to by speaker / OT secondary circuit...

Anything that if open circuit, would prevent current flowing in the OT secondary circuit; eg a bad speaker, cable, connector, impedance selector.

Another thing that could cause an amp's output to go totally silent is something open circuit in the HT current path, via the tubes, OT primary.

[Pedro Vecino]

The speaker jack (8 Ohms extension output) can be problematic since the 16 Ohms output depends on its internal contacts. A quick way to check it is connect momentarily the speaker to the 8 Ohm output when the problem appears.