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

[Barry]

> 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.
>

The pot acts rather weird.  It's a 500-ohm WW.  Checking the "unused" side, it shows about 1000 to 1200 ohms as I turn the wiper.  There's a company (KISS Electronics) that services these (and other model) VTVMs and they seem to have replacements for a lot of the parts.  I've asked Ashley (the owner?) about some replacements.  There are also some good candidates from West Florida Components on eBay right now (in case KISS doesn't have any).

> 
> 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.

On the VARIAC at 115, filament voltage is right on 6.3VAC; however, one of the tubes is operated at a reduced filament voltage and that may account for some of the problem with a marginal tube.

> 
> Roy

Thanks for the reply.

Barry - N4BUQ