[R-390] On 3TF7's...

[Drew Papanek]

Bob Camp wrote:

>The main issue with the 3TF7 is that is is designed to regulate around
>a line voltage of about 108 VAC. With 122 VAC line power you are
>getting close to the "unregulated" end of the 3TF7's range.

The 3TF7's voltage range is 8.6 to 16.6 volts; at 122 VAC line the 3TF7 
would see  about  16v.
One could insert resistance in series with the 3TF7 to reduce voltage and 
lengthen that tube's life.

With 13 ohms in series the 3TF7 would see about 12v at 122 VAC line.   If 
the line were to then drop
to 110v the 3TF7 would see 8.6v; at any lower line voltage it would drop out 
of regulation.   Most of us do not run our R-390x  at less than 110VAC; 
series resistance might be a good option.

National issued a service bulletin instructing NC-300/303  users to insert  
resistance in series with
the radio's unreliable 4H4C ballastube; R-390x owners are hardly alone in 
dealing with ballasfailures.

>It's not very clear exactly how important the inrush effect is on receiving 
>tubes. As far as I can see >tubes are pretty reliable as long as you don't 
>vibrate them. That makes it a bit tough to quantify an >improvement from 
>inrush limiting.

Good point.  How many of our tubes fail from open heaters?  I find 
relatively few.

>The whole issue of half wave rectification to run the tube filaments has as 
>you mention been >thrashed out at great length in the past. One idea that 
>has not been tossed around is to lift the
>far end of the regulated filament string and then full wave rectify the AC.

The grounded far end is the PTO tube heater.  That tube's hot side is 
filtered by an LC network.
If the far end were lifted it would be necessary to add there another LC 
network to keep RF signals
where they belong.

It would probably be easier to full wave rectify (bridge) right at the 
secondary terminals of the power transformer and use pulsating DC for the 
whole radio's 25.2 VAC needs (don't forget to add a hash suppression cap 
across each of the bridge's diodes).  A solid state ballast replacement 
module would then have a diode at the input to isolate the module's filter 
capacitance from the rest of
the 25v circuits.  The other circuits would otherwise see about 35VDC 
filtered instead of the intended pulsating nominal 25VDC; the ovens (we all 
have those turned off, right?) would fry eggs and the antenna relay would 
pull in with a heated vengeance.

Drew

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