[R-390] On 3TF7's...
Bob Camp
ham at cq.nu
Thu Sep 30 19:28:55 EDT 2004
Hi
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.
Since the 3TF7 works just like a tube filament it has the same inrush
current issue as a tube. Other types of current regulation will take
care of the tube inrush but a 3TF7 will not. 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.
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. That would
at least reduce the level of pulsation on the AC line. I have never
taken a look at the connectors involved to see if there are enough
spares to make it something you could do.
Take Care!
Bob Camp
KB8TQ
On Sep 29, 2004, at 8:49 PM, Drew Papanek wrote:
>
>
> On the topic of the 3TF7 ballastube, Dallas Lankford wrote:
>
> (snipped)
>
>> ...I found that the 3TF7 does not do a very good job of stabilizing
>> the
>> BFO and PTO frequencies when the AC line is varied.
>
> There are other failure modes for the 3TF7 other than just going
> "open".
> I have one which "regulates" at 350 mA; another list member reported
> having
> one at around 240 mA (the spec is 300mA). The degree of regulation
> probably
> is degraded with that type of failure. It could be that Dallas' 3TF7
> has
> that defect.
>
> Dallas continued:
>
>> Whoever designed the original BFO and PTO filament stabilization
>> circuit
>> was on the right track. They just used the wrong method to stabilize
>> it. Current regulation is the wrong approach; voltage regulation is
>> the correct
>> approach.
>
> A good 3TF7 is specified to hold current constant within +-1% over the
> voltage range. Perhaps that performance is not adequate for the
> ultimate
> in PTO/BFO frequency stability.
>
> Either current regulation or voltage regulation would accomplish the
> same
> desired result in terms of frequency stabilization. The designers used
> current regulation because that was far simpler to implement with the
> technology of the era. Their current regulators weren't as good as
> those
> we can inexpensively build today.
>
> Current regulation has an added benefit in that it eliminates inrush
> current
> surge on startup. That high current (a bit over 2 amps for a 6BA6)
> causes
> a brief heater hot spot which eventually burns open. Tube heaters are
> generally designed to reduce that effect, but it still remains to some
> extent. (Extreme example: I have a few Bugle Boy 12AX7's the lowest
> part
> of whose heaters flashes brilliant white on startup).
>
> A few current regulators using the LM317 have been described in this
> forum.
> One is Dr. Gerald Johnson's simple AC regulator. While his circuit
> does not
> get the best performance from the LM317, it is still quite good and
> much much
> better than a recalcitrant 3TF7. It dissipates no more power than the
> 3TF7 and
> places neither asymmetric load nor current spikes on the power
> transformer.
>
> Another is Dave Wise's LM317 DC circuit (not his phase control based
> "3DW7"
> designs). Its regulation is excellent and is adjustable from 270 mA to
> 330 mA IIRC, but it generates more heat than Jerry's circuit (I
> calculated
> something around 7 watts compared to the 3.8 watts of Jerry's design).
> With its half wave rectification it places an unbalanced load with high
> current peaks on the transformer. Dave incorporated series resistance
> to
> reduce those peaks somewhat.
>
> Dallas' voltage regulator circuit generates higher heat and without the
> aforementioned refinement used by Dave draws even higher current spikes
> in its unbalanced load from the transformer. Dallas' circuit is not
> adjustable and is simpler than Dave's.
>
> May I suggest yet another DC current regulator circuit? Connect 25.2
> VAC
> from ballasocket to anode of diode (1N4002 and up suitable). Connect
> cathode to + side of 1000 uF 50V electrolytic capacitor. Ground - side
> of
> cap. Run DC thus formed from + of cap to input of LM317 (pin 3).
> Connect
> load at 3TF7 socket (the connection to the seriesed BFO and PTO tube
> heaters) to LM317 adjust terminal (pin 1). Connect a current sense
> resistor (4.166 ohms, 1 watt) from LM317 output terminal (pin 2) to
> the
> adjust terminal. A 4.0 ohm, 1% resistor will give nominally 313 mA
> - close enough to the desired 300 mA. For those who want to set the
> current exactly (given the LM317's Vref tolerance) use 5.0 ohms in
> parallel
> with 20 to 33 ohms. Add the obligatory short leaded, grounded .1 uF
> caps
> on LM317 input and output to quell LM317 oscillation/noise tendencies.
> Heatsink well.
>
> That circuit would have close DC current regulation, not be as readily
> adjustable as Dave's, be simpler than Dave's, more complex than Dallas'
> and have the same high input current spikes, unbalanced load, and
> excess
> heat as Dallas'.
>
> Both the circuit I propose and Dallas' could be made to dissipate a
> little
> less heat and be a little less "spikey" by reducing the filter cap to
> 470 uF
> thereby allowing more ripple (not Chuck) at the regulator chip's
> input. That
> would have no noticeable effect on regulation.
>
> While the current spikes and unbalanced load of half wave
> rectification with
> capacitive filtering will cause additional heating in the R-390x power
> transformer, I'm of the opinion that there would probably be no
> detriment
> given the transformer's conservative design and massive size. The
> transformer's existing load is symmetric and spike free, comprising
> resistive and full wave rectified choke input. The added half wave
> rectified load is small by comparison and hence would be "diluted".
>
> On 3 terminal regulator noise Cecil Acuff wrote:
>
> (snipped)
>
>> There are any number of complex ways to solve the 3TF7 issue but we
>> should
>> not be short sighted and forget about any noise that might be
>> generated by
>> the solution. Linear regulator circuits work great but are quite
>> noisy.
>
>> The new fangled sand box radios suffer from many problems associated
>> with
>> noise generated by devices internal to the radio...
>
> I don't know how much of a problem that would be for an R-390x using a
> 3
> terminal device for PTO/BFO tube heater regulation. The radios having
> that
> malady have operating bias voltages so regulated/adulterated. OTOH,
> heater power is not directly associated with the signal path, though
> there can
> still be some leakage. In the R-390x, encountered first after the
> tube heater
> regulator's output is the BFO tube heater. There and at the detector
> the
> signal level is high enough that 3 terminal regulator noise would be
> miniscule by comparison. Next in line is the PTO tube heater. That
> is fed
> through a brute-force LC noise filtering circuit included by the
> designers
> to keep PTO signal in and noise out; no trouble there.
>
> Any noise problem caused by the aforementioned heater usage of 3
> terminal
> regulator would most likely be due to radiation from heater wiring
> inside
> the IF module. That would be dependent on existing lead dress and
> shielding.
> An inductor and another .1 uF capacitor could be connected to the
> regulator
> output to form a pi-section brute force filter thereby addressing any
> noise
> concerns.
>
> Not all of us demand the ultimate in stability from our R-390x. On
> ballasubstitution, Jerry wrote a while back:
>
> (snipped)
>
>> The purist restorationist will want to use ballast tubes until there
>> are
>> no more. The picky will want to go solid state regulation, and the AM
>> listener probably will be super happy with a pair of 12BA6 and a
>> jumper.
>> Since the 12BA6 was the standard IF tube in 4 and 5 tube AC/DC radios
>> using miniature tubes, there should be a million of them about or
>> more.
>
> A schematic of Jerry's AC current regulator and a component connection
> description of Dave's DC current regulator can be found under the
> "Ballast
> Tube" heading in Wei-i Li's brilliantly conceived "Pearls of Wisdom".
> Go to r-390a.net . Select "References", "Pearls of Wisdom". There
> reside
> postings from this forum painstakingly distilled over the years.
> There is much
> elightenment to be gleaned and amusement to be had by perusal of the
> lively
> and animated discourse over this most controversial of R-390x topics.
>
> Vive le Caballo Muerto!
>
> Drew
>
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