Another silly tube yesting question

[dperry]

Hello,

I am trying to repair a piece of equipment that generates its output from the anode of a Brimar 12bh7a tube.

There is no output and some evidence of burning around the base of this tube, yet when the power is on, the tube still lights up.

I (think I) have correctly measured the heater resistance (using this pinout ... http://www.drtube.com/datasheets/12bh7a-rca1955.pdf )

And get measurements between 3 ohms and 6 ohms measuring between pins 4,5 and 9.

So would a tube light up if it was faulty?

Thanks

 

[TheGimp]

There are a lot of ways for a tube to go bad, a faulty heater is only one way.

That the heater is still good is, well, good.

More likely is a faulty component in the associated circuit.

Discoloration around hte base of the tube may simply be because tubes get hot.

A circuit schematic would be most helpful.

 

[pjp]

Yes, tubes can go bad in other ways than a broken filament.

The cathode can reach the end of its life - electron emission rate will drop. (How old is your tube?)
The tube can get gassy.
The grid wires can bend/break and short circuit to the cathode.


p.s. Why two separate threads for the same question ?

 

[dperry]

The schematic is in the other thread...sorry about the dual threads, it didnt appear instantly and i didnt realise the thread needs moderating 1st 🙁

Sorry.