[ARC5] Interesting occurrence...

Henry Mei'l's meils at get2net.dk
Sat Apr 2 11:28:24 EDT 2011 

Thank you Dennis for your very informative comment.
I learn a lot from your very thorough contributions

Henry
oz3o n2nr

----- Original Message ----- 
From: "Dennis Monticelli" <dennis.monticelli at gmail.com>
To: "Leslie Smith" <vk2bcu at operamail.com>
Cc: <ARC5 at mailman.qth.net>
Sent: Saturday, April 02, 2011 4:54 PM
Subject: Re: [ARC5] Interesting occurrence...


> 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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