ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.	New: view Recent Searches. New: visit Schematic Hell! The sunn still shines online!

Listen to great tunes streaming live right now!

ampage archive

Vintage threads from the first ten years

previous: Bob Hohmann No it's not gay just a little weird... -- 9/5/2003 8:20 AM	View Thread

Re: 69 twin reverb sometimes blows

9/5/2003 1:38 PM
Wild Bill	Re: 69 twin reverb sometimes blows Isn't troubleshooting fun? These things usually turn out to be something hidden well but simple but maybe you're looking in the wrong stage. You've pretty well rebuilt the output. I'd start poking aroung the preamp stages with a 'scope looking for oscillations. Maybe wiggling something will set it off. A strong ultrasonic oscillation can be a silent killer. To the output tubes it can look like a constant input of "11" or more. Are you sure that the tubes are showing plate glow? After all, you say that the fuse is blown before you can look at them. Is the fuse a slo-blo? Is 2.5amp the stock value? And what about a short in the filament wiring? Are you using a CT winding or maybe one of those 100R resistors every one uses is lightly touching something it shouldn't. Don't forget to look around the pilot light if it's fed from the same winding. It's rare but maybe you've got an OT that develops an internal short when running hot. The problem is making the fault last long enough to track it down. Putting the amp on the bench and gatoring on some metering may not help 'cuz there's now open air flow to keep things cooler. You could try putting a cardboard box over it, with the meters outside. Glad it's you and not me! ---Wild Bill

9/5/2003 1:38 PM

Wild Bill

Re: 69 twin reverb sometimes blows

Isn't troubleshooting fun?

These things usually turn out to be something hidden well but simple but maybe you're looking in the wrong stage.

You've pretty well rebuilt the output. I'd start poking aroung the preamp stages with a 'scope looking for oscillations. Maybe wiggling something will set it off.

A strong ultrasonic oscillation can be a silent killer. To the output tubes it can look like a constant input of "11" or more.

Are you sure that the tubes are showing plate glow? After all, you say that the fuse is blown before you can look at them. Is the fuse a slo-blo? Is 2.5amp the stock value?

And what about a short in the filament wiring? Are you using a CT winding or maybe one of those 100R resistors every one uses is lightly touching something it shouldn't. Don't forget to look around the pilot light if it's fed from the same winding.

It's rare but maybe you've got an OT that develops an internal short when running hot. The problem is making the fault last long enough to track it down. Putting the amp on the bench and gatoring on some metering may not help 'cuz there's now open air flow to keep things cooler. You could try putting a cardboard box over it, with the meters outside.

Glad it's you and not me!

---Wild Bill