[Test-Equipment] Hickok 800 Tube Tester

[Bill Abate]

I've had this tube tester for a few years and it has performed well when
I needed it.  I typically use it to separate out the bad tubes I pick up
at fests or flea markets.  Recently I bought some 6146 tubes and noticed
that the reading stated out fine but then dropped after a few seconds or
so of holding the test button in.  It eventually dropped almost 40%
after a minute or so.

This puzzled me but I thought it was a weak tube so I went to the next
one.  Same thing.  After some head scratching and testing I found out
that the filament voltage was low but sank even lower during actual
testing.  After inserting the 6146 and adjusting line voltage, I found
5.6 volts on the filaments.  About 10% low. I felt this was borderline
voltage but I was not happy.  When I pushed the test button, the voltage
dropped even lower to 4,8 volts!  Rats!!!! 

Evidently the test voltages on a 6146 load the transformer even more. 
Anyone else notice this?  I doubt if the transformer is defective. 
Bedsides compensating with more line adjustment which would be hit or
miss, I figure I can add another transformer.  I would add a 5V, 6.3V
and 12.6 volt 4 ampere transformer to the tester.  I figure to
disconnect the original transformer taps for those voltages, tie the
transformer commons together at the original tapped secondary common and
attach the other ends of the supplementing transformers to the switch. 
Hope I'm saying that clearly.

My question is am I going to mess up the original circuitry.  Tube
tester schematics always make my eyes glassy.  I figure the directly
heated cathodes require same connection to the tester, e.g. 5R4.  So I'm
trying to maintain continuity.  BTW, the 5R4 voltage measured 4.4 volts
but did not change under test, which I would expect.

With testing miniature tubes I never noticed it before because the
filament load was minimal.  Any thoughts??

Bill Abate