Re: {Collins} Skytec tube replacements


I should clarify my testing.  Here goes:

I measured the PD distortion with a Tektronix AA-501A analyzer, using the 75S-3C 100 kHz reference oscillator and observing changes in THD at the PD output.  When demodulated, an xtal calibrator typically has very low THD.  The testing coincided with my hum and PD distortion reduction mods - and are discussed at length in the Collins reflector archives.  

Before I modified the PD to include a cathode resistor and emulate what was done in the KWM-2, distortion was very high -- into the low double digits.  I made the PD mod and distortion out of the PD dropped to 0.5% at 1 kHz.  I then installed the SkyTec PD replacement and distortion climbed to a value mid-way between the old and new values.  Finally, I restored the PD by removing the cathode resistor and made it stock again, installed the PD Tubester and the result was that THD approximately tracked with the THD value of the PD before the cathode resistor change.  

So, in my case, I can state that the Tubester replacement did not add to PD distortion, but nor was it capable of the very low distortion attained by using the original VT and placing a resistor in series with the cathode to raise PD plate voltage.

Secondly, audio hum did climb with the Tubesters.  To test audio SNR, I again activated the xtal calibrator, set the AF control to a nominal listening level of about 9 o'clock.  The Tektronix analyzer has the ability to take any absolute audio level value and create a reference to 0.00 dB.  I then placed the 75S-3C in STBY.  What's left is residual audio/hum in the PD and audio stages.  The best I could do with the Tubesters was roughly -35 dB, no weighting.  By contrast, I could attain -40 dB with the original VTs.  Finally, by using a mu-metal tube shield on V11, and re-orienting components under the PD and audio tubes, I can easily attain a SNR of -53 dB.   Keep in mind that the SNR is based on my AF control setting but should reasonably track well at most settings near a comfortable audio level.

Here, I can state that in my 75S-3C, the tubesters were inferior to the original tubes.

Paul, W9AC
  ----- Original Message ----- 
  From: Jerry Stern 
  To: collins@xxxxxxxxxxxxx ; Paul Christensen 
  Sent: Wednesday, January 04, 2012 1:02 PM
  Subject: Re: {Collins} Skytec tube replacements

        Hi Paul,

        I don't know if you can make this generalized statement.  I have them in a 75S-3B and 32S-3 and apart from running cooler I have not seen any performance degradation.  I think many still use them and they command an attractive price even though the real-deal tubes are still plentiful and less expensive.  I would love to hear other's experience with the SkyTec replacements.

        73 Jerry

        --- On Wed, 1/4/12, Paul Christensen <w9ac@xxxxxxxx> wrote:

          From: Paul Christensen <w9ac@xxxxxxxx>
          Subject: Re: {Collins} Skytec tube replacements
          To: collins@xxxxxxxxxxxxx
          Date: Wednesday, January 4, 2012, 12:06 PM

          > What are Skytec solid state tube replacements (e.g. ebay 220920415205)? Tried google and got no applicable results. Anybody have a website?
          > Rick WA1RKT

          Rick, you can learn about them here:

          I agree with KW4DH.  You've gotta' really want to try them and odds are you won't keep them very long after the trial.

          Paul, W9AC


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