[Milsurplus] VT-25/10 subs?
Bruce Gentry
ka2ivy at verizon.net
Fri Feb 23 23:20:09 EST 2018
There are several possibilities. First of all, the 10 has a 7.5 volt
filament at 1.25 amps. That is the biggest issue. The filament is fed by
two resistors from the 24 volt source. Not only does this reduce the
filament voltage, it raises it above ground to help bias the tube. A
triode connected 6V6, 6L6, or 6CA7 should work fine as long as a
ballast resistor is added across the heater to bring the voltage down to
6 volts. A 6CA7 with it's 1.5 amp draw might be just about right with
the original filament dropping resistors and no ballast resistor. In any
of these cases, the cathode has to be dealt with. It may work fine to
connect it to one side of the heater, try each and see which works best.
The alternative would be to use two 20 ohm resistors in series across
the heater and connect the cathode to the junction between the two
resistors. It should be possible to build an adapter that will plug
into the original socket and work with the unmodified circuitry of the
transmitter.
Bruce Gentry, KA2IVY
On 2/23/18 2:48 PM, Kenneth G. Gordon wrote:
> I have recently been offered the chance to get a pristine, but tube-less, BC-375, with TUs.
>
> I have plenty of good VT-4Cs here but no VT-25/10.
>
> Is there a substitute for this tube which could be used by simply building a base-adapter?
>
> Ken W7EKB
>
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