[R-390] Xtal oscillator module - Possible Mod

[Alan Victor]

DDS Noise killer... That's no surprise.

On the xtal module and gm. In my opinion and what I found was a significant
rise in the equivalent series R of the xtal's which needed replaced in the
390A I worked on. They were so far off (measured them on a hp VNA) that it
was impossible to get the PTO end points to align properly. The increase in
xtal series R, will require a corresponding increase in gm in any
oscillator to get it to start. If you want to get a real boost in gm, go
with a bipolar device or a modern FET like GaN!!! A bip will give you 40K
micro mhos at 1 mA and GaN will give you mhos of gm!

But, check out the xtal issue first before pointing fingers at the poor
tube. It would be interesting to calculate the minimum gm necessary to get
the Collins xtal oscillators in the module to start as a function of xtal
Rs. At 4 or 5000 micro mhos with the feedback ratio used in those circuits,
I suspect the design is not going to tolerate much of a rise in the xtal Rs
or lack of xtal activity. Alan

On Wed, Jul 24, 2019 at 4:26 AM John Gedde <jgedde at optonline.net> wrote:

> Hi Perry,
>
> I can't comment on the FET idea, but I did some experimentation with the
> use of DDS sources in my R390.  Phase noise is a killer.  The S/N of the
> radio worsened dramatically when the DDS source was used.
>
> John
>
> -----Original Message-----
> From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On
> Behalf Of Perry Sandeen via R-390
> Sent: Wednesday, July 24, 2019 1:52 AM
> To: r-390 at mailman.qth.net
> Subject: [R-390] Xtal oscillator module - Possible Mod
>
> TYo Bubba Dudes!.
> First thanks to Rodger and all on the tuning details.  I never new it
> could be so complicated to tune up.  I guess I should hold onto my Western
> Electric 6AK5.
> So here is my questions for those with more information than I.
> 1. The 6AK5 has a gain of 5,100 uMhos. It appears from the reflector
> observations that it is just marginal.
>
> Could it be replaced by a cascade circuit of J309 Fets which have a gain
> of 9,000 (I don't know if that's uMho's or not) or something similar?
>
> What I see as an advantage is that gain would remain constant.
> Or if one was so inclined could the 6AK5 be replaced with a higher gain
> sharp cut-off pentode?
> 2.  I've seen these DDS synthesizer boards (which I know almost nothing)
> on ebay.  Could something like this be used.  I do now that when one gets
> way, way down to the nitty gritty the frequency has minute frequency
> variations but I believe that they are so small that in our intended use,
> it would be irrelevant.
> I noticed one small detail when reading the tube manual.  The gain went
> from 5,000 to 5,100 uMhos when the plate voltage went from 120 to 180V.
> Might a low plate voltage be causing marginal operation?
> Have the receivers with these problems had all their problem caps replaced
> as well as the Chuck Ripple note on changing the voltage rating of one
> small ceramic cap from 500 to 1Kv?
>
> This is all just speculation on my part and wish that those who have more
> expertese would enlighten your humble servant.
> Regards,
> Perrier
>
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