[ARC5] Transmiitters: Parameters?

[Dennis Monticelli]

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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