The AM Forum

The AL-80A in my vintage station died today and I'm looking for troubleshooting suggestions.

The Amp was working normally and as I unkeyed it from a tune up there was a loud POP.  No sizzle like an RF flashover, sounded more like a pistol being fired.

So I shut it down, looked around inside for flashover residue, found none.  Pulled the tube and checked for shorts with a VOM, found none.  AC fuses are good.

Powered it up on the bench, HV looks normal.   The grid current meter pegs violently as soon as the amp is keyed (no drive).

Any suggestions on where to start looking?

73 Jack KZ5A

Barring any other discoveries, and no rf drive, It sounds like a grid-to-plate short in the 3-500Z or in the circuitry/wiring underneath.

Which way did the meter swing?

I have seen the grid-to-X element shorts clear when the tube is cooled.

Phil AC0OB

   Jack, Did you put the tube back in? If so, maybe repeat the test with the tube out, and the plate lead away from ground. The symptom you describe might have been a parasitic oscillation. These can occur in some amplifiers, and afterward the power tube is damaged forever. Indirectly heated tubes are more prone to damage than a tube like a 3-500.

Jim
WD5JKO

i heard timtron say this once of how he cleared a grid short out on a tube was to hook one terminal of a car battery to the grid of the tube and the other side to the other element that is shorted out. it will blow the short out. so if it's shorted to the plate, connect the battery to the grid and plate, and if it's to the filament, do the same thing just connect the one side to one of the filament pins.
shelby

That sounds like somehing he would do....create a permanent weld if it doesnt blow.

Use AC and be quick, Ive done it at about 24V on a measured short and 120V on one that measures OK. You need some healthy amps at the low voltage or use a medium size variac. I have heard of success by discharging a HV electrolytic when the unmeasurable short is fairly wide spaced; thats a quick one time discharge.

Sometimes you get lucky and have output as before, other times enough grid wires have been vaporized to reduce gain. The bad ones just stay bad.

BTW, a 3-500Z will arc when it has outgassed from storage, those tubes need to be run at color on a regular basis. Dont blame an arc on voodoo parasitics unless the suppressor resistor is blown.

Carl

that's the reason he used a car battery, capable of putting out a high amount of current at a low voltage.

That definately sonds like somethin Timmy would do! But...................

One wrong slip and you end up with a Slimeatron........... ;D  ;D

Smart Man!!!

Found the source of the loud pop.   The grid current meter shunt, R3, 1.5 ohm 3W, exploded.  The remains are just below the pot near the center of the attached jpg.

I'm still not measuring any shorts in the 3-500z, but I'm thinking there must have been one momentarily.  Maybe it won't come back ::)

I sent an order off the Mouser for a new resistor and a few other odds and ends.

Thanks.

73 Jack KZ5A

Here is a neat video I found that is on topic to this thread.  A bad 3-500z brought back to life.

http://youtu.be/69Kt0YxUm_M