[R-390] The Penultimate R-390* Ballast Replacement

[Bruce Hagen]

I find myself somewhat mystified with all of the ballast tube solutions. 
Most very clever and well thought out but were am I missing it? It seems to 
me that an inch or so of wire bent into a "U" shape for the ballast and then 
pulling two 6.3 volt tubes and replacing them with two readily available and 
cheap 12.6 volt tubes is the logical and easiest solution if you do not want 
to spend a few dollars and buy a 3TF7. Bruce


----- Original Message ----- 
From: <ToddRoberts2001 at aol.com>
To: <r-390 at mailman.qth.net>
Sent: Sunday, February 13, 2005 9:54 AM
Subject: Re: [R-390] The Penultimate R-390* Ballast Replacement


> Most of the 3TF7 substitutes I have read about involve buildng something
> small enough to plug directly into the 3TF7 socket or on a nearby bracket. 
> This
> comes with problems of heat dissipation or else installing unsightly 
> brackets
> near the I.F. subchassis. Some of the recent ideas got me thinking - Why 
> not
> build a small separate power supply module with a
> well-regulated/filtered/bypassed 12.6 VDC output that could be placed next 
> to the receiver and the only
> connection would be a small umbilical cable with a 9-pin plug that plugs 
> directly
> into the 3TF7 socket? You could run the umbilical through the side of the 
> R390A
> chassis thru one of the large holes and tuck the power supply and cord out 
> of
> the way next to the receiver. This layout is similar to the way some
> audiophile preamps use a separate power-supply module with an umbilical. 
> You could
> modify one or two pins of the 3TF7 socket with a jumper to ground to 
> provide a
> ground return for the 12.6VDC so you wouldn't have to tie down a separate 
> ground
> lead with a terminal and screw somewhere else on the chassis. When you 
> plug
> in the umbilical it breaks the 25.2 VAC circuit and connects the 12.6VDC
> circuit and ground. The ground pins would have no effect on the original 
> 3TF7 if you
> wanted to plug one back in.  This way you could build a nice little husky
> separate  regulated/filtered/bypassed 12.6 VDC power supply and not have 
> to
> miniaturize it or compromize the performance. If you want to go back to a 
> 3TF7 just
> unplug the DC supply 9 pin plug and put the 3TF7 back in. No unsightly
> permanent wires or brackets hanging off the I.F. subchassis.  The 
> regulated DC
> supply should give the ultimate in stability and pure DC on the filaments 
> of the
> BFO/PTO tubes. 73 Todd WD4NGG
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