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

[Roy Morgan]

On Mar 1, 2011, at 10:50 PM, Barry wrote:

> This is one of the early models
> I found the DC calibration pot is quite bad.

One trouble reported with the HP meters is that they used a wire wound  
(200 ohm, I think) pot for some of the cal or settings pots.  These  
develop intermittent or high resistance at the ends where the winding  
is crimped by the solder tabs.  Treatment with De-Oxit might help.   
you may have to re-crimp the crimps if you can get to them.  Or  
replacement pots would be a good idea.  if you find this is the case,  
let us know where  you found the pots.

> I didn't check the 12AU7A's for gas.  That also might be a problem.

You might also simply let them run for a week, then try calibrating/ 
zeroing again.

There are many tubes similar to the 12AU7 that might work in it's  
place.  The 5963 is a "computer rated" tube built to avoid "cathode  
interface" from long periods in the Off mode.  The 6211 is another  
similar tube meant for long periods of cutoff conditions and may be  
useful.  Sometimes the A suffix on a tube number indicates that it is  
selected for balance between units.  Often the military version of a  
dual tube has tighter specs for inter-unit balance.

Note that the 12AU7 filament is 150 mA on 12.6 volts and the 5814 is  
175 ma.  I doubt that this difference is going to matter anywhere near  
as much as the likely higher than used-to-be line voltage. I suggest  
you measure the filament voltage and your line voltage to see what is  
really happening there.

Roy

Roy Morgan
k1lky at earthlink.net
K1LKY Since 1958 - Keep 'em Glowing!