[HBR] The long, SLOW HBR project

Sun Sep 18 20:13:42 EDT 2011

Thanks for the update, Walt.....Kinda reminds me of an old Republic Pictures
serial...."next week....Chapter ten..."The Deadly 10 Meter
Drift..........",  BTW, I am a HUGH fan of Republic Pictures......
Anyway, how about some pics of your radio?  Would love to see it!  I feel
like its an old friend, after following its' progress.
-I have a question for you-or anyone else, for that matter.  Given a typical
rf amp-6ba6-what would be the best l/c ratio for the (input ) tuned
circuit?  For example, an inductance of -say 280uh with a capacitance of
365...versus a cap 0f 150pf and a much higher L of 650.....both will
resonate at the wanted frequency...but which combination will give the best
transfer of energy to the tube?
-Thanks !
-And keep the saga coming Walt.....can't wait for the next "episode"!
Don in Nashville

On Sun, Sep 18, 2011 at 6:35 PM, Walt Hutchens <waltah at earthlink.net> wrote:

> > I will redo the [20M] coil in the next couple of days.
>
> Done.  Downward warmup drift of ~3 kcs over about an hour.   (Worse than I
> thought it was, but this was a better quality measurement.)
>
> 3 kcs is awkwardly high: further work seemed necessary.   I rewound the
> coil
> again, with 8 instead of 7 turns but with the turns spaced about 1/16".
> The concept is that (with the right spacing) the length expansion of the
> coil (which reduces the inductance) can compensate for the diametric
> expansion (which increases inductance).
>
> (In general a coil with a bit of spacing needs one or two more turns to hit
> the same inductance as one that is closewound.)
>
> The space wound coil got the warmup drift down to an honest (carefully
> measured) 2 kcs or so.   Then I noticed that I had a paper cap bypassing
> the
> cathode of the oscillator tube; replacing it with the largest mica I could
> lay hands on lowered drift still farther, to about 1.6 kcs.   You can still
> buy leaded NP0 ceramics up to 0.01 mfd with 500 WV ratings and I may get
> one
> of those for this job.
>
> (Any reactive part in an oscillator circuit through which RF flows will
> affect the frequency.   Thus such parts should be eliminated when possible
> and those that remain must be as temperature stable as possible.)
>
> There, for a day or so, I stalled.  Nothing else in the circuit jumped out
> as the cause of drift taking place over most of an hour.
>
> This AM I warmed the set up thoroughly with the 20M coils, then switched to
> the cold set of 80M coils with previously measured drift.    The drift
> pattern changed substantially:  Almost no upward drift in the first 2-3
> minutes so that's in the chassis -- the oscillator tube would be a good bet
> as it takes a few minutes for emission to stabilize.   This drift could be
> reduced if the tank circuit/tube coupling could be further reduced but
> since
> it happens so quickly, this isn't a priority.
>
> (The 6688 is a sharp cutoff pentode with a transconductance of 16,500.
>  It's
> intended for use in radar IFs so it ought to have low interelectrode
> capacitances.   It should be a good LO tube, but I haven't looked at the
> spec sheets -- only the summary in the RCA manual.  6.3V/300 mA.  Naturally
> there is no 150 mA equivalent ...)
>
> After five minutes or so the warm chassis/cold coil combination drifted
> downward at about half the rate found when warming up everything at once.
> The chassis buckling effect shouldn't be much of this since the chassis was
> already warm and I made the coil switch quickly.   However the 5651
> regulator tube has a temperature coefficient:  Rising tube temperatures
> decrease its output voltage by perhaps 0.1 volt over the range of temps to
> be expected during warmup.   I turned the hair dryer on this tube for a few
> seconds (then turned it off) and sure enough, the oscillator frequency was
> a
> few hundred cps lower, then gradually went back up over the next few
> minutes.
>
> It's mounted where I had a good space: Near the 117N7 rectifier/2nd audio
> tube.   That's obviously not ideal and in a full rebuild, I'd put it in a
> cool spot.   It should not be too near the oscillator, however, since RF
> fields also affect it.
>
> The other way to go is to run the oscillator with the plate voltage roughly
> three times the screen voltage so as to reduce the sensitivity to voltage
> variations and -- I hope -- eliminate the need for regulating the voltages.
> I will probably try that shortly as it is not hard to do and would not only
> eliminate the issue coming from the regulator temperature coefficient but
> also deal with the problem the regulator was supposed to solve, namely, the
> variation of frequency with AGC action.
>
> That ~0.1 volt changes can affect the LO frequency is not a good sign: It
> indicates that the oscillator doesn't have a bunch of gain to spare.   And
> indeed when I dropped the oscillator plate (tube screen) voltage by half as
> an experiment, the LO stopped at the low end of the tuning range on 20M.
>
> The rest of the drift is in the coil assembly.   I could add some more
> space
> to the windings -- that is, I could wind that coil for a fifth time with
> greater turn spacing.   Hey, what's another two hours and 3" of PVC pipe?
>
> Use of one or more temperature compensating caps is a possible final step,
> when drift is at a practical minimum.
>
> Perhaps I'm over-obsessing but these questions are interesting.  Anyway I'm
> going to try to get a bit better stability before making the 10M coils.
>
> Walt
> KJ4KV
>
>
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