Using subminiature tubes in a radio

[wa2ise]

Used a 5899 semi-remote cutoff pentode in place of a 12BA6 in an AA5 type radio. I passed the submini tube's wire leads thru the wafer tube socket pin holes and soldered the ends of those leads to the tube socket terminals. The submini pinout nearly matches that of the 12BA6, except for the plate and a heater are swapped. That's the crossover with insulation you can see. Normally a shield fits over it. It works well. Looks cool too

[bob91343]

As long as the characteristics are similar and you aren't overstressing the tube, it's fine.

Subminiature tubes never really caught on; by the time the hearing aids started using them, transistors began to appear. They were a lot of trouble, not being intended to plug in.

[Reece]

That's neat. Another use for subminis is to make modern versions of unobtainium early tubes, like WD-11's. Various methods have been used. One puts the submini in a silvered test tube with a special made base to match the odd pinout on the old tube. They look really close to the original and work the same or better.

[marty59]

What made you decide to use the 5899?

[wa2ise]

Originally Posted by marty59

What made you decide to use the 5899?

I wanted at least a semi-remote cutoff tube. It's the IF stage. And heater running at 150ma. And of course I had some.
They are supposedly long life, so being soldered into the circuit shouldn't be an issue.

[Reece]

Wonder what that tube was intended for, having a 12V heater and made to be soldered in. Most of the subminis I've run across have low voltage filaments.

[wa2ise]

Originally Posted by Reece

having a 12V heater and made to be soldered in. Most of the subminis I've run across have low voltage filaments.

It's 6.3V @ 150ma, though I suspect that current rating is an upper limit spec. And that the tube would draw a little less off a true 6.3V source. I decided that this tube heater looked a little too bright, so I paralleled a 1K resistor with it to drain off about 6.3ma of the 150ma series heater string current. So the tube heater looks closer to the normal orange color.

I haven't seen any 12V heater subminis, though I have several with 26V heaters. Not sure, but maybe aircraft had 26V power. 12V would make sense for mobile equipment in cars, but wiring a pair of 6.3V heater tubes of the same current demand would get you there. These are the indirectly heated cathodes

The filament submini tubes run at a volt or two. I haven't messed much with those, they tend to be microphonic.

[Findm-Keepm]

Originally Posted by wa2ise

I haven't seen any 12V heater subminis, though I have several with 26V heaters. Not sure, but maybe aircraft had 26V power.

Yep - 28VDC was the DC bus in Military aircraft, and I'd imagine some civilian aircraft as well. The AC bus(es) are 115VAC 400 cycle and 26V 400 cycle.

And yes, the military used TONS of subminis in Avionics - I worked on the ARN-52 Tacan receiver that used submini tubes in the RF/IF modules. They tended to go weak, never "bad" - and were all socketed.

Here's the innards of the ARN-21, it's predecessor:



Cheers,

[marty59]

I know 400Hz was common with the source voltage as it would filter down easier and cleaner. Military land vehicles (tanks) that would generate their own power were on 400Hz too. I want to say the voltage was 48V?