[R-390] OT: HP-410B Tubes

[Barry]

As I mentioned before, the first amplifier's (V2) filament runs at a reduced voltage.  According to the schematic I have, there are two resistors (R51, a 10-ohm, and R51A, an 18-ohm) in parallel that are in series with that filament.

Apparently R51A was an change to get just a little more heater voltage.  Mine is an early model and does not have R51A so V2 is running with quite low filament voltage - something on the order of 3.7 to 3.9 (don't remember the exact value).

I jumpered in a 15-ohm (what I had on hand) and, with the line voltage set to 115, V2 was seeing right at 4.5 which is what is shown on the schematic; however, with that in place, the 12AU7A's still won't work in the circuit.

I'm still going to run some more tests (gas, etc.) on the 12AU7A's.  Maybe I'll find something.

Barry - N4BUQ



> I brought a couple of HP-410B's back into service a few years ago. When
> doing so, an old HP friend who "engineered" in the HP Palo Alto factory in
> the old days recounted the 12AU7 burn-in and balance test rack in the "410"
> end of the factory.
> 
> HP apparently had problems  with 12AU7 section balance and drift too.
> 
> Scott, N6NXI