Running a tube without heaters

[studeb]

i was looking at a push pull OTL amp that uses 4 36kd6 tubes.
two of those tubes have no heater connections.

Can someone please explain why this could be?

[wakibaki]

Looking at the tube datasheet these have 36V heaters, so if you're looking at a schematic, then it's drawn in error. A heater is pretty much intrinsic to a valve.

w

[leadbelly]

I think he means the other tubes in the amp.

Hot cathode - Wikipedia, the free encyclopedia

"Hot cathodes may be either directly heated, where the filament itself is the source of electrons, or indirectly heated, where the filament is electrically insulated from the cathode; this configuration minimizes the introduction of hum when the filament is energized with alternating current. The filament is most often made of tungsten. With indirectly heated cathodes, the filament is usually called the heater instead. The cathode for indirect heating is usually realized as a nickel tube which surrounds the heater."

[Wavebourn]

Theoretically, it is possible to make a cathode as a pipe that goes through top and bottom of the tube, and heat it externally, for example by a candle flame.

Still, anode power source is needed.
I remember such devices that were placed on top of kerosene lamps that generated enough of DC voltage to run battery powered vacuum tube receivers.

So, it is possible to make such vacuum tubes that require heat only to perform.

[studeb]

i am looking at the amp itself. Or i would be if i was home.
When i first powered up the amps they were not working and i thought that was why.
Two tubes had heaters worked and two did not, ergo bad heater in two tubes. Swapped the tubes out and around. No heater glow with a tube in sockets 1 and 4.
Looked at the board and i cannot see the heaters connected to anything for those two sockets.

[Wavebourn]

Hmmm... Looks like they forgot to solder some wires?

[leadbelly]

Did you get it with power cord snipped off? Somebody probably selected the 36V heaters so 4 in series could be run from 120VAC line voltage unisolated.

[studeb]

its a commercial amp

i'll try to get pics of the board when i get home tonite

[ChrisA]

Originally Posted by studeb

.. i cannot see the heaters connected to anything for those two sockets.

OK so you have an amp where 2 of the heaters are not wired and the amp does not work. What else would you except?

When I read the first post I thought you have a working amp and were wondering how it could still function with two dead tubes. I had several ideas (1) maybe the tubes are parallel and now one is taking the load, or (2) maybe it was push-pull with only the push side working and it might work but sound bad.

So now I have to ask: Did this amp ever work? If not then it is simply built wrong

[astouffer]

I used to have this old 125 watt solid state PA amp that was transformer coupled to the speaker. There was a 12AX7 tube wired but without heater connections. The socket pins were shiny and untouched by solder and the amp worked just fine. Still not sure why the tube was there