Tracking down a short

[dewman]

Any sort of qualitative flow chart-like procudeure for isolating a short in a tube amp? (removeing certain tubes, in certain order, disconnecting certain components, etc.). I'm rebuilding a home brew amp with a short and would like to isolate it to the preamp, tremelo, or power amp circuit in a systematic way...

[paulg]

The proceedure as outlined in Jack Darr's book is a time proven technique. Basically you check the plate voltage and pop test the grid with your probe. Do this test at the tube socket and work your way from there.

[dewman]

From this book: Electric Guitar Amplifier Handbook?

[WaltC]

That's the one. I know that Antique Electronics Supply has them.

www.tubesandmore.com

search for Jack Darr

[soldersucker]

Google it diy guitarist has it free online.

[John Phillips]

Originally Posted by paulg

The proceedure as outlined in Jack Darr's book is a time proven technique. Basically you check the plate voltage and pop test the grid with your probe. Do this test at the tube socket and work your way from there.

That's fine for finding an open circuit, but it won't help much with a short circuit, which is most likely causing a fuse blow or other nasty problems that will stop you getting even as far as a pop test.

It's difficult to outline a procedure since if you understand what to pull or disconnect, you'd probably know where to start looking anyway... there will be different symptoms depending on where the short is.

The best is to start by removing the power tubes (and rectifier tube if it has one), since they are some of the most likely components to be shorted.

Don't remove all the tubes in an amp with a solid-state rectifier - you need at least the first preamp tube in place to provide a load on the end of the B+ chain, as otherwise the voltage there can rise dangerously, for the filter caps. (This also means you must put in the preamp tube when you put in a tube rectifier too.)

Then meter the power supply voltages, and down the B+ chain, inserting tubes as necessary from the rectifier downwards to find what causes the short to appear. Meter the tube pin voltages, and you should be able to spot where the low one is (or the fuse will blow when you put in the bad component or the one that feeds it - eg if the fuse blows after you put in the rectifier tube, it's almost certainly either the tube itself or the filter cap it feeds).