Anyone else have bad luck with tube amps?

[formula73]

Re: Anyone else have bad luck with tube amps?

I've been to a couple techs that found nothing wrong with it. I think they're just lazy and/or unable to handle newer technology. One place that called it good was Top Gear, one of the two top amp shops in San Diego.

Edit: it never really occurred to me until now, but it's probably a lot like new cars vs old ones. I can pull a motor and rebuild a carb, but when it comes to new stuff, I'm lucky to even find the plugs. To a guy who mods plexis, 3 channels with midi switching must be a bit of a chore.

[glassman]

Re: Anyone else have bad luck with tube amps?

I've had no less than 3 amps in the shop in the last month that were cutting in and out due to HT fuses that were intermittently opening and closing. None of these fused blew; they simply opened...most likely due to vibration. All were slo-blo fuses like the one in the photo earlier in this thread. This is most likely what happened to your Bogner.

Fuse reading 101:
* Large black burn: a catastrophic failure: usually due to a failed power tube.
* Small black burn: usually caused by a chronic over current condition such as bias set too hot but also may be caused by a number of other things.
* No black burn: mechanical failure of the fuse caused by vibration.

5150s have very little current limiting on the power tube screens. This can result in the wire-wound sand block resistors over-heating and opening. The symptoms include an amp that seems to surge in power when a note or chord is hit, dropping back down as the note or chord decays. Sometimes it goes the other way with the resistor functioning until it gets a surge, casing it to heat up and open. The only way to find this is to monitor the current on each tube as a signal is applied. This phenomenon occurs almost exclusively in 4 (or more) power tube amps where each tube has an in-phase mate that is still operating properly. Best to let a tech sort this out.

In the JTM, the usual culprit is a poor ground connection on one of the power tubes. If this is the case, when it cuts out, the sound will drop in volume significantly and get a really nasty distortion character. If the tone remains the same, it could be any number of things including a screen resistor, a bad tube or a poor power tube or PI connection. If the tone cleans up when the volume drops, it's likely a tube.

Hope this helps...and don't go inside your amps unless you know what you are doing and have taken all the necessary precautions.