[R-390] in rush current limiters

[Helmut Usbeck]

>
> The idea is to build a 9-pin miniature tube plug-in replacement with no
> modification to the R-390A.  Only two wires are fed to the ballast socket.
> Ground is not available unless obtained by a tertiary wire or the socket
> shield.  Heat dissipation might be the biggest challenge in a plug-in
> solid-state replacement.

Put a diode (1N4007 will do)  between pins 2 and 7.  No muss, no fuss, no
heat.  YOu now have 12 vac for the two 6 volt tubes to fight over.


> A very simple modification is to simply jumper the ballast pins 2 and 7 with
> a simple plug-in wire jumper and replace the 6-volt tubes with their 12-volt
> equivalents.  The receiver remains very stable even when the filaments are
> not current regulated by the ballast tube.

This was the first thing I tried when my first 3TF7 went bye-bye.  One
thing I noticed was one tube was brighter than the other.  Second thing
was my freq calibration was off over 2 khz.  Measuring the voltage drop on
the tubes one was sitting at 7 volts and the other at the other at 18.  I
went though a pile of 12ba6's before I got two of them to drop 12 volts
a piece.  Similar thing happens with using a resistor and 6ba6's.   If you
want good performance out of your receiver the correct filament voltages
are manditory.  5.7-6.9 vac for 6.3 volt tubes.

>
>
> As mentioned, another way is to solder-jumper 2&4 and 5&7 and plug in a
> 12BH7.  The 12-volt filament of the 12BH7 drops the voltage in a somewhat
> similar amount as the ballast tube.


And as I mentioned, it's a useless cludge, extra work and some people
actually think that a 12BH7 has to be used.  Any 12 volt tube will do.
While one rewiring the socket, put in a 12au7 and set it up as an internal
product detector (one of those things I,m going to try when I get a round
tuit).  At least it does some thing besides producing heat and a rewired
socket.  It's attribute as having a controlled warm up filament is lost
since the other two tubes are not.

My conclusion is that the best thing to put in the 3TF7 socket is a 3TF7.
I think some people should also check the voltage drop across there tubes.
Yeah, I know they all work.  But at the expense of other problems.
Unfortunately my 390a seems to go though one every 2months or so and it's
a bit expensive.  One can buy alot of good German lager for $45.00.

-Helm.