[ARC5] Tube-tester Question and selenium rectifiers

[Kenneth G. Gordon]

On 20 Sep 2015 at 13:41, Randy Randall wrote:

> be in circuit while adjusting the line setting. The other thing I can see
> glaring on the tester is it uses a selenium rectifier.

Yes. I had not thought of that, but that is a very important matter!

> The internal resistance
> of a selenium rectifier tends to go up over time.

What happens with those is that they become less a rectifier, and more like 
two slightly different values of resistor.

> to move the shunt control knob as it was set at the factory.  You can replace
> the selenium rectifier with a silicon one, but you also need to insert a
> resistance to bring the voltage down to a correct level as the silicon diode is
> more efficient than the old selenium unit and has a lower internal resistance. 

Yes. My Hickock 3444 has selenium rectifiers in it. I did some research some 
time ago on replacing those with silicon. At the time, as I remember it, I 
needed to add a 20 ohm resistor in series with the silicon to get close to the 
values of the selenium, but before I make the mod and recalibrate, I'm going 
to recheck.

One can find the necessary info on the web somewhere.
 
> Unfortunately there are no voltage readings on the schematic so trying to figure
> out The size of the resistor will be harder.

Well, you don't really need to know that for a tube tester. You should try to 
equal the overall voltage drop of selenium vs silicon, but that is not really 
critical.

In order for me to recalibrate my own tube testers (I have the 3444 and two 
TV2s), I am going to have to replace the seleniums with silicon asap.

BTW, be very careful about selenium rectifiers: when those things burn up, 
they release a toxic gas.

I really don't like the darned things...

Also, for those who have TV-2s, the first model had problems with oscillation 
at times, especially when testing VR tubes. Later models had at least a 
million ferrite beads added to practically every wire in the things...

I have two TV-2s: one works, but it is the earlier model without the ferrite 
beads, and it does, most definitely oscillate heavily with VR tubes, and 
another one, which has some sort of intermittant short in the main power 
transformer, which I am hoping is external to the transformer.

That transformer has at least a million wires coming out of it...

Ken W7EKB