I am a beginner so bare with me
Some time back I came across an ebay listing of a guy selling a step down transformer for my 539c
in the listing the guy said the voltage was higher today then when the old stuff was made
and so it makes sense to step down the voltage to the old 110 instead of the new 120 we have coming out of the wall sockets today as to not stress the old girls shiny parts
I have been using an old acme step down
as seen here in this pic



but after going threw some of my older finds in the barn I came across an old Ohmite vt10gaa as seen here in this pic



I think this may be the better of the 2 transformers but as I was testing and calibrating it I saw that the Amperes gauge
never moved as I was changing the value of the power
I do not know this device well enough to say its not working right, as far as I know the amperes gauge might not come into play unless some thing-a-ma-jiggy if hooked up to some other what'cha-Mcdoogle
so my question is...
is this the Ohmite the better transformer and is the amperes needle supposed to be working along side the volt meeter
thanks for any insight you may have

The old Acme will be fine, no need for a variac. Unless you are leaving the tester on for long periods and have 125 or more line voltage, you really don't need anything as the tester has a pot to adjust for the line voltage built in

The idea that line voltage was 110V back in the old days is persistent but not true. 120V was not unknown during the 1920s and even 125V was standard in a few places (though mostly for DC systems).

As of 1926, sales of lamps broke down as follows: 110V lamps accounted for only 12% of demand, 115V for 47%, 120V for 35%,  125V 4%, and all others  2%.

WOW!!
that is why I like this board
so would it be fair to say that most of these old tube testers where designed on pretty much the same power as what they are running on today?
and if so, is it harmful to run these old testers on a lesser power of 110 Vt ?

Yes they were made to run on a range of voltages. Generally, the lower the better (less heat dissipated in the line-adjust rheostat).

interesting,
the red line test set line on my Hickok 539c looks to be on 100 volts
so I wonder if I set the Variable transformer (out side the tester) for just over that, lets say like about 105 volts
and then used the "in tester" (line-adjust rheostat) to bring it down the rest of the way to 100 Volts
would that be OK or would the low voltages off set the the readings
maybe its silly to worry about over heating on the old girl but since I found out how much this old tester is worth I find these thoughts creeping into my head on long test periods

Hickoks were designed with either 93V (early models) or 100V transformer primaries. The rest was dropped in the line-adjust rheostat. I don't know of anyone who has ever set the rheostat to zero and run them from 100V. it should work, but the Gm readings were determined on the assumption that there would be some series resistance in the AC line. That might also make the tester more vulnerable to shorted tubes (the fuse lamp might tend to overheat faster than you could react). That's only a guess however.

The nominal residential line voltage in the US has increased over the years. Prior to 1900, 104 volts was common, then around the turn of the 20th century, they went to 110 volts. 115 volts appeared sometime around WW-1. 117 appeared in the 1930s, and after WW-2, they went to 120 volts. The adjustment to 125 volts was made about 10 years ago. Of course, not all areas had the same voltages depending on the ages of their distribution systems, and the actual voltage at any particular location could vary several percent either way from the nominal rating.

The Hickok 539C was made in the late 1960s, by which time 120 volts was well standardized. It should not have any problems on 125 volts. Unless you live in an area with high line voltage, a voltage-lowering transformer for your tube tester is probably not worth the bother.

In rural areas, a house by the transformer may have seen as much as 140v during light loads and a house at the end of the run may only see 80V during heavy loads. We tested to these limits at Tandy/RS into the '90s.