[ARC5] Transmiitters: Parameters?

Thu Mar 24 14:13:26 EDT 2016

May I enter?

Regarding the notion of « reliability » of vacuum tubes I would add “longevity”. 

Reliability is the fact that we can depend on good steady service; longevity defines the length of time we can depend on said good service. 

My experience since the mid 50’s (as a radar man) with tubes such as the “6SN7”series later replaced by  the  equivalent  “14AU7” type is that longevity AND reliability depend a lot on circuit design.

In the 1945’s, B+ was commonly 200 to 250Vdc. In some operational radar equipments it was standard procedure to replace the tubes once a month, good or bad. (I would not comment on the time spent to check and re-adjust the circuitries after the operation!).

Then in the 1965’s, in new radars still equipped with the old 12AU7 type vacuum tubes, the B+ dropped drastically to 100 to 150Vdc. In spite of the fact that more tubes were used, the overall reliability was good as the longevity increased to a point where we changed tubes ONLY when needed. 

Note:  I realize that, in the 60’s, transistors could then be used... But this is another story! 

G  De Sorne

  ----- Original Message ----- 
  From: Dennis Monticelli 
  To: Mike Morrow 
  Cc: ARC-5 Maillist 
  Sent: Thursday, March 24, 2016 9:33 AM
  Subject: Re: [ARC5] Transmiitters: Parameters?

  I generally agree with you, Mike.  I would just add that the definition of reliability used in my industry has always been "working to specification in the intended application" not simply quit outright.  If a single tube had been used it would have worked hard to obtain the AM output specs (particularly over the extremes of mil service) and thus become "unreliable" sooner than two tubes sharing the load equally.  Should the two tubes become unequal (one degraded before the other), the stronger tube would then take on more of the job and help prop up the output power.  I had that happen in my own T-22 and the CW output had indeed held up well.  Were it not for the growing emergence of a chirp I would not have investigated and discovered that one of the 1625's (tested good 2 yrs prior) had gone very soft.  

  Yes, an open filament on one tube would completely take out the other, but that is an uncommon failure.  More common in my experience with the 1625/807 is heater-to-cathode leakage (a don't care in this circuit) and emission decline.

   Aircraft Radio Corp did the right thing by employing two tubes in the final.

  Dennis AE6C

  On Thu, Mar 24, 2016 at 6:16 AM, Mike Morrow <kk5f at earthlink.net> wrote:

    Dennis wrote:

    > The configuration as manufactured  produced the required output power
    > and two tubes would provide a measure of increased reliability (and
    > peace of mind as well...).

    Two PA tubes in parallel provide little to nothing in advantages for reliability.  A mechanical element failure within either tube will almost certainly result in complete stage failure, short of simple connection loss at the base.  Likewise, the most common tube failure is an open filament.  The series-connected PA tube filaments then produce total PA stage failure.  Envelope failure in one tube with total vacuum loss will result in filament failure as well.  Thus, any argument of redundancy fails any postulated probable individual tube failure,

    An argument for the two-tube design could be made based on two tubes performing as one larger tube.  At the desired design PA stage power, that would reduce long-duration stress by reducing plate and envelope temperatures and prolonging cathode emission lifetime, compared to a one-tube design.

    DD
    ***********
    On Mar 23, 2016, at 9:08 PM, Leslie Smith <vk2bcu at operamail.com> wrote:
     I don't understand your last sentence - and I think "whine" is a typo.  Maybe you meant "while it" or "when" or "will".  Also,  I don't follow the idea that "... the oscillator provides JUST enough drive for a SINGLE 1625 ...".  Clearly whoever designed this set - whether Dr. Drake or another technical (military) type  - "knew his knitting".   Why only JUST enough for a SINGLE 1625?   What's going on here?
         73 de Les Smith

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