[Milsurplus] Dust-Off those TU-10s; BC-375 on 30 Meters!

[David Stinson]

I always meant to try this and it works: 
fixing BC-375 thermal drift with reduced B+.

The major reason guys don't try to operate the BC-375 on 
40 and 30 meters is thermal drift. Install a TU-10, let everything 
warm-up for 15-20 minutes, set it to 10108 KC, key-down and 
listen to the MO slide smooothly up the band as the tubes and 
tuning components heat up. On 40 meters, it can move as much
as 3 KCs over a single CW exchange.

Now, with only 550 volts on the plates, using either the 211s or 
the 1625s at 20-25W out, the thermal drift is cured as long as
you use a gas-free tube. It does chirp, but no worse than many of
the rigs during the Classic Exchange. 
Proper adjustment of the keying relay will help a great deal. 
But it no longer wanders, even on 10.1 MC.

How do you tell if your NOS tube is really gas-free?
Stick it in the MO stage *at reduced B+*.
Let everything warm-up, then key-down for a few seconds. 
If the MO wanders around,the tube is gassy. 
Even a small amount of gas that won't hurt in the PA stage 
will booger-up your MO. These tubes are 70 years old 
and many of them have at least some gas, 
so swap them around until you find a "good-un."


Once finished with the tube-substitute experiments,
gonna get that keying relay "right" and make some 
BC-375 contacts on 30 meters, just to fluster the noobs ;-).

73 DE Dave AB5S
 
P.S. got the rig to modulate the 1625s properly for AM.
The 10Y stopped emitting for some reason.
Changed it and will resolder the pins tomorrow.