[ARC5] Interesting occurrence...

Sat Apr 2 10:54:35 EDT 2011

Regarding leaving on filament without applied B+, there are some lessons
from early vacuum tube computer circuits.  In binary logic operations a tube
could spend a great deal of time sitting at either an ON or OFF state.  It
was found that tubes were failing bofore their expected MTBF and that
problelm was traced to tubes that had been in the OFF state for an extended
period.  This premature failure was nick-named "sleeping sickness" but is
more correctly termed "cathode interface resistance."  The mechanism is the
formation of an interface layer between the nickel cylinder (in an
indirectly heated cathode) and the active oxide coating.  The series
resistance of this layer reduces the gm.  The work-around is to run a little
plate current during the "idle" state or better yet use a tube purpose built
for the job.  There are tubes in the premium industrial series of minatures
that are specifically rated for computer use.  These types are virtually
immume to the formation of the problematic interface layer.

In view of this known problem it wouldn't seem to be a good idea to run
indirectly heated filaments over extended time periods without some trickle
plate current.

Dennis AE6C
On Sat, Apr 2, 2011 at 12:14 AM, Leslie Smith <vk2bcu at operamail.com> wrote:

> Hello Ken,
> When I subscribed to the magazine "Ham Radio" I read an article by the
> editor (apology for forgetting his name) about set life and "tube" life.
>  His practice was to leave the filament supply "on" permanently, but
> always turn B+ off when the set was not in use.  He made the statement
> that one  set had been used in this manner since 1945 - his comments
> having been made some time in the '70s.  Some-one argued (it may have
> the the editor himself) that this practice resulted in less differential
> expansion between the glass and wire in the valve base. This resulted in
> long valve life.  I know nothing about this subject - I'm merely
> reporting hearsay.
>
> Returning to the loss of AF volume.  I suggest you try to establish
> whether the loss of signal lies before (or after) the detector.  I note
> that the local oscillator and BFO plate dropping resistors are large in
> value (in some sets they may be in the order of 300k ohms (from memory).
>  Certainly the tech manual lists the operating plate voltage of both
> oscillators as being under 20 volts.  I would check ALL the voltages in
> your set, using the diagram from the manual TM 16-30 ARC5-2.
>
> However, while the manual gives an accurate record of what the tube
> voltages should be under normal running conditions I can say from
> experience and measurement that the sets will operate at voltages well
> below those listed.  (eg all my sets run from a B+ supply of 60
> regulated volts.)
>
> Some faults can be obscure.  For example, in one set I repaired I found
> the SM capacitor across the output transformed had a DC resistance of
> approximately 350 ohms.
>
> A transistorised square wave generator is a cheap and useful device to
> suggest where problems may lie in the "command" sets.  There is no need
> to actually couple the signal directly to the valve circuits - merely
> lying an insulated cable adjacent to any valve grid (12A6 may be an
> exception to this rule) and the harmonics from the oscillator will be
> audible - increasing in strength as the injection is taken further
> forward in the signal path.  This is a "rough and ready" test - but
> effective with experience.
>
> Look forward to reading your next posting.
>
>  Leslie Smith
>  vk2bcu at operamail.com
>
>      -o-o-
>
>
> On Fri, 01 Apr 2011 15:40 -0700, "Don Davis" <dxguy at earthlink.net>
> wrote:
>  > It might have been rewired for 12 volt filaments.  Do you tubes test
> ok?
> > Bad habit to leave powered up when sleepinng.
> >
> > 73 de don ad6pb
> >
> >
> > ----- Original Message -----
> > From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
> > To: "ARC-5 List" <arc5 at mailman.qth.net>
> > Sent: Friday, April 01, 2011 3:28 PM
> > Subject: [ARC5] Interesting occurrence...
> >
> >
> > > Well, that R-25/ARC-5 I got working the other day has recently
> > > exhibited an
> > > interesting (to me) phenomenon:
> > >
> > > First, an explanation: this unit had been rather badly hacked (among
> > > other
> > > things, extra holes in the case) and all the remaining "flower-pots"
> > > were bad.
> > > Some were missing.
> > >
> > > Overall, it didn't look all that bad, so I replaced the bad caps with
> > > 0.05 mfd
> > > 1KV disk ceramics, cleaned everything cleanable, and plugged tubes
> > > into it.
> > >
> > > The power supply that I am using was built back in the 1970s by
> > > someone
> > > who did a pretty fair job. As it came, it output about 230 VDC and 24
> > > VAC,
> > > and had separate switch to turn the filaments and the B+ on and off.
> > >
> > > However, the only DC filter was a triple 40 MFD at 450 VDC upright can
> > > cap
> > > with all its sections in parallel. No choke or RC pi-filter.
> > >
> > > When connected to the receiver via an adapter made from a dynamotor
> > > base, the receiver exhibited quite a lot of hum, especially at low
> > > audio levels.
> > >
> > > So, I added a 3K ohm 5 watt resistor between the input filter (2
> > > sections of
> > > the can-cap in parallel) and the output cap. This brought the voltage
> > > down to
> > > about 145 VDC, so I moved the screen tap to the high end of the
> > > screen-
> > > dropping resistors.
> > >
> > > However, the hum was gone. I suppose I should reduce the value of the
> > > filter
> > > resistor by 1/2 and restore the screen connection to the center-tap
> > > point.
> > >
> > > I used the receiver for a couple of weeks, and was quite pleased with
> > > its
> > > stability, excellent calibration, and sensitivity. I then replaced the
> > > 12SK7s in
> > > the RF and 1st IF with 12SG7s. This dropped the noise down by about
> > > 1/2
> > > and increased the sensitivity noticeably (unmeasured).
> > >
> > > When I would go to bed, I would turn off the B+ but leave the
> > > filaments on.
> > >
> > > I started to notice that the overall audio level was slowly dropping,
> > > and last
> > > night, it was almost down to zero.
> > >
> > > In fooling with it, I found that when the tuning reached about 2.7
> > > Mhz, the HF
> > > oscillator quit completely.
> > >
> > > So, I replaced the original 12K8 with one I pulled from another
> > > receiver.
> > > Although this revived the receiver, it couldn't be accurately
> > > calibrated any
> > > longer, the low-end calibration cap being maxed out before reaching
> > > accurate calibration.
> > >
> > > Also, the antenna peaking cap was now way over against the left stop
> > > for the
> > > peak over most of the tuning range.
> > >
> > > I am now wondering if leaving the filaments on, but no B+, for
> > > extended
> > > periods has a deliterious effect on the tubes, or if some other
> > > component is
> > > failing?
> > >
> > > Anyone have an idea?
> > >
> > > Eventually, I want to use this receiver to monitor 160 meters and WWV.
> > >
> > > Ken W7EKB
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