Another silly tube yesting question

dperry · 20th August 2010, 10:06 AM

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 · 20th August 2010, 02:59 PM

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 · 03:20 PM

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 · 20th August 2010, 03:28 PM

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.

20th August 2010, 10:06 AM	#1
dperry diyAudio Member Join Date: Aug 2010	Another silly tube yesting question 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

20th August 2010, 03:18 PM	#3
pjp diyAudio Member Join Date: Nov 2006 Location: under the rainclouds	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 ? Last edited by pjp; 20th August 2010 at 03:20 PM. Reason: q

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20th August 2010, 02:59 PM	#2
TheGimp diyAudio Member Join Date: Aug 2009 Location: Johnson City, TN	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.

20th August 2010, 03:28 PM	#4
dperry diyAudio Member Join Date: Aug 2010	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.

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