Announcing

Solid State Replacement for the 3TF7 Ballast Tube

When it comes to spare parts, one high profile radio collector likes to remind me, ``there are always fewer tomorrow than there are today."  For the fragile 3TF7 ballast tube used in the R390A, it appears that day is upon us.  The supply of this key part is now exhausted at what is perhaps the largest supplier of R390A and other vintage parts.

While always an expensive and failure prone part, the 3TF7 serves to regulate the filament voltage to the series-wired PTO and VFO tubes.  This design was to provide an extra level of frequency stability to the R390A.

While several well thought-out alternatives have been offered, none have been widely accepted.  One shortcut has been to remove the ballast tube and socket and substitute a resistor in its place.  While that may allow the radio to operate, the R390A does not deserve short term kludges causing permanent scars. 

Another approach was been to drill mounting holes in the IF chassis and install a regulator there.  This has several drawbacks including the introduction of noise into the signal processing chain, adding additional heat build up to the IF deck itself, and perhaps the biggest drawback, having to perform an invasive, non-reversable modification to the receiver.

There is a better answer.....     

I am offering an assembled, easy to install, high quality solid state replacement module for the 3TF7 ballast tube that is RF quiet and completely reversable.  It regulates far better than the tube it replaces and perhaps most importantly, requires no drilling or other permanent modifications to the receiver.  A typical user can install the module in 15-30 minutes.

My replacement maintains the original design theme of the receiver by locating the module and all cabline in the power supply section of the mother chassis.  The module easily installs into the existing wiring harness of the receiver with only 2 wires.  It can be disconnected and removed without leaving any permanent holes or other changes  When complete, the radio has no outward appearance of the module being in place unless the bottom areas of the mother chassis are examined.  Normally, this area is not in view. Click here to view the module installed in an R390A receiver.

Besides being more rugged, the regulation circuit actually performs better than the original 3TF7 tube it replaces.   With the BFO tube, V-505 mounted in an extender, I measured the filament voltage from pin #4 to ground with an A/C voltmeter. The result was 15.2 VAC which, when divided between the 2 tubes which are wired in series in that circuit, yields a whopping 7.51VAC across each!!  That value is far from the 12.6/6.3 VAC spec.  With the basic module in place, the new value was a comfortable and stable 13.2 or 6.6 VDC on each tube.  

The module regulates the filament voltages of the PTO and BFO tubes by dynamically tracking the amount of current drawn.  The current varies as the 2 tubes warm during use and will also change with age.  An additional benefit of the premium module is that it allows the user to adjust the filament voltage ``spot on."  The module circuit is RF quiet and thus, will not introduce noise into the signal chain.  Noise immunity is achieved by mounting all components inside a premium quality, cast aluminum sealed box.  As a final measure of RF noise elimination, brute-force filtering of the output is also provided.  Another primary design consideration was to stabilize the regulator both thermally and electrically.  This was accomplished by choosing components which are severely over-sized and capable of handling 5 times the current required by the circuit in the R390A.  For safety, the input of the module is individually fused.

Each module is individually hand built, calibrated and tested.  Please contract me for availability.  The module can be made available for shipment to both domestic and overseas users.  This replacement features with fine voltage adjustment and certain upgraded components is available at $109.  The module is also offered as installed options on all my restorations.

The modules is available on a built to order basis .  If you are interested in purchasing one, please contact me and reserve your choice.  My E-Mail link is below:

Interested? Click to  E-Mail

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