[R-390] in rush current limiters

[Bill Smith]

> 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.

A ballast tube won't help that problem, the two tubes in series will always
draw the same current.

> Second thing
> was my freq calibration was off over 2 kHz.

This is something I wish to examine in my receiver.

Bill


----- Original Message -----
From: "Helmut Usbeck" <vze2gmp4@verizon.net>
To: "Bill Smith" <billsmith@ispwest.com>
Cc: "Jim Shorney" <jshorney@inebraska.com>; <r-390@mailman.qth.net>
Sent: Sunday, December 22, 2002 5:38 PM
Subject: Re: [R-390] in rush current limiters


>
>
>
> >
> > 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.
>