[R-390] B+ Dropping Resistance (was Drill a Hole...)

[Drew Papanek]

K2CBY wrote:

>Perhaps this is heretical but I saw little point in retaining the tube 
>sockets
>on the PS chassis once the rectifiers had been replaced with solid state 
>diodes.
>I yanked the sockets and plated over the holes with 16 ga. aluminum.

<snipped>

It would seem that this list has defined "heretical" as major 
drilling/blasting or ripping out wire harnesses.

>The VFO and BFO shouldn't chirp no matter what, since they derive regulated 
>B+
>from the VR tube mounted on the audio chassis.

VFO has unregulated plate voltage and regulated screen voltage.  Neither 
plate nor screen supply of BFO is regulated. VFO and BFO are both electron 
coupled oscillators; frequency stability is relatively insensitive to plate 
voltage variations.    A given percentage frequency shift in BFO also 
represents less absolute shift than with higher frequency oscillators.

When operated from single unregulated supply (as is BFO) the electron 
coupled oscillator can be made to have very flat frequency vs voltage 
characterisic by correct selection of screen resistors (ART-13 PTO is a good 
example).  I would be surprised if Collins didn't take that approach when 
they designed BFO.

I doubt that voltage variations caused by slightly degraded power supply 
load regulation (when adding series resistance to drop silicon rectified B+) 
would have noticeable FMing effect .  Perhaps frequency variation claimed by 
that list member was caused by another fault.

>the only thing that bothers me about the solid state rectifier conversion 
>is
>that B+ comes on full blast almost instantaneously.

Some have claimed that delaying application of B+ until after heater warmup 
is of benefit only to extend life of thoriated tungsten (transmitting type) 
tubes and is not necessary for indirectly heated tubes as used in R-390 
series.  It is noteworthy, however, that Tektronix tube type oscilloscopes 
(which use indirectly heated tubes) have B+ delay feature.  By use of 
thermal relay (as in Tek scopes) or solid state devices such a feature could 
be easily added to the R-390 series.

>For essentially the same reason, I favor replacing the ballast tube with a
>12-volt tube having a controlled heater warm-up time (in my case a 12BY7 
>from an
>old Tektronix scope) rather than a fixed resistor.

From low impedance (non current regulated) heater supply the 6BA6 draws 
about 2 amps startup surge; that surge would likely be duplicated in the 
characteristic of 12BY7or other tube used as ballast replacement.   The 6BA6 
VFO and BFO tubes whose heater current is normally regulated by ballast do 
not have controlled warm up characteristic so any advantage of that 
attribute in a ballast replacement is lost.

With the traditional 42 ohm resistor used as ballast replacement the startup 
surge would be about 530 mA-considerably lower than when using 12BY7.  The 
lower (as compared to some other ballast sustitution schemes) surge may 
reduce thermally induced heater mechanical stress and possibly increase VFO 
and BFO tube life.

Drew

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