[ARC5] The added 6F6 output stage again

Sat Oct 20 07:49:53 EDT 2012

Thanks Dave for a great article..I thought it was just me having those 
thoughts about the chainsaw articles..like the one removing the bfo coil 
from the BCB set to convert it to a car radio..goodbye BFO forever..funny 
when I ran the ''improved'' set with the 6J5 and 6F6 the audio was really 
dreadful and I just thought that there was a fault in the stage..but the 
fault was in the idea as you say..not only did I take out the 6F6 but I took 
out the 6J5 as well and went back to the detector for the proper audio and 
there it was..these little sets were made so well that in my opinion even 
Einstein would have thought twice before trying to improve them..there is 
plenty of good audio from the 12A6 thru a matching tfr to 4/8ohm 
speaker..for more into a real external af amp..73 de Pat/EI7CN

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Subject: ARC5 Digest, Vol 105, Issue 107

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Today's Topics:

   1. Re: Bare Aluminum USN sets. (Brian Clarke)
   2. Re: Filaments - How Low Can You Go? (Leslie Smith)
   3. Re: Added 6F6 audio stage. (David Stinson)

----------------------------------------------------------------------

Message: 1
Date: Sat, 20 Oct 2012 17:32:26 +1100
From: "Brian Clarke" <brianclarke01 at optusnet.com.au>
Subject: Re: [ARC5] Bare Aluminum USN sets.
To: "Mike Morrow" <kk5f at arrl.net>, <arc5 at mailman.qth.net>
Message-ID: <D3C18CFB0AA24DA98DDF3110D8A65135 at WORKSHOP>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original

Not quite, Mike.

The painted sets' chasses appear to have been spot faced where the serrated
teeth of the lock washers under the screw heads bit in. But, the serration
could allow air and moisture to get to the aluminium - so, a small
electrolytic cell would have been set up, eventually offsetting any
grounding and hastening what I like to call 'aluminium rusting'.

When I refurbish Command sets, I use some zinc-rich grease over the serrated
washer to prevent oxidation or the setting up of any electrolytic cell. You
could use such products as Almanox. Also, use of such grease makes future
replacement of the potted capacitors, removal of the top and bottom a piece
of cake - rather than the sheared screws I have often seen. As you probably
know, drilling out a brass screw from an aluminium 'nut' is not easy. The
potted capacitors were not so bad as they were generally brass - so, all you
got was general copper-zinc electrolytic erosion - much easier to refurbish.

73 de Brian, VK2GCE.

On Saturday, October 20, 2012 4:38 PM, Mike Morrow said:

>
>> If the corrosion gets between the potted capacitors and the chassis,
>> the designed RF bypassing is gone and instead you have interstage
>> coupling.
>
> A "problem" that, one suspects, would likely **not** have been ameliorated
> by paint on the externals of the equipment.  And one that the VHF AN/ARC-5
> units, lacking such paint, seem to have avoided.
>
> 73,
> Mike / KK5F

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Message: 2
Date: Sat, 20 Oct 2012 21:05:39 +1100
From: Leslie Smith <vk2bcu at operamail.com>
Subject: Re: [ARC5] Filaments - How Low Can You Go?
To: Robert Eleazer <releazer at earthlink.net>, "ARC-5 List"
<arc5 at mailman.qth.net>
Message-ID:
<1350727539.17263.140661143155161.689BD847 at webmail.messagingengine.com>

Content-Type: text/plain

Hello Wayne,
Good question.  Speaking generally, it's hard to go lower than a snake's
belly, but that idea's got nothing to do with ARC-5 sets, has it?
Speaking of the sets, I think there are useful "clues" to answer your
question in the tech manual. I refer here to the 250 odd page document
commonly available on the net, (AN 16-30ARC5-2) also called AN-ARC-5
Aircraft Radio Equipment.

Generally speaking the ARC-5 sets (I include ARA in this) are
transformer coupled.
The exception is the supply to the BFO and local oscillator circuits,
where a good sized dropping resistor is in series with the supply.  Now
check the diagram of the plate voltages.
This is figure 5-1.  Check pin 6 (anode) of the detector (12SR7).   The
plate voltage is 85.

Now check the circuit and find the plate dropping resistor - in the R-25
(and above) it's 100k.  It's higher in the R-23.

Next, turn to the mixer circuit.  The plate dropping resistor, R6, to
the LO is in the order of 200k.  According to the diagram (5-1) the
plate voltage lies between 13 and 29 volts.  Yes!  I took the
opportunity to measure the plate voltage when I ran a set from 60v B+
supply.  It was not significantly "off" the spec - but the set suffered
from distortion because the injection power was down.  I replaced the
dropping resistor with a lower value (can remember the new value) and
the distortion was gone.

There has been some discussion here about audio output, and output
power.
Others on this list have noted a need to correctly match the speaker
impedance to the set.
I found a set would deliver listen-able levels to an 8 ohms speak from a
60V B+ supply, and this was without the lower value cathode resistor
said to increase power out.

As for starving the filaments, I wouldn't under-run them by more than
5%.
There is little disadvantage in a lower B+ supply - see my notes on
ARC5_Radios at Yahoo.

Best wishes from Australia

  Les Smith
  vk2bcu at operamail.com

On Fri, Oct 19, 2012, at 23:45, Robert  Eleazer wrote:
> Here is a stupid question for y'all.
>
> Considering that the overall recommendation is to operate the command
> receivers with reduced B+, just how low can you go on the voltage for the
> filaments before you have a problem?
>
> With say, 125V B+ and 12V filament power with a command receiver that has
> 12V tubes and wired for 24V, how well will it work?
>
> In experiments with my first command set, an R-11A, I was astonished that
> I could get the B+ down to around 28V and it was still working, if rather
> faint on the audio output.
>
> Thanks!
>
> Wayne
> WB5WSV
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------------------------------

Message: 3
Date: Sat, 20 Oct 2012 06:18:49 -0500
From: "David Stinson" <arc5 at ix.netcom.com>
Subject: Re: [ARC5] Added 6F6 audio stage.
To: "ARC-5 List" <arc5 at mailman.qth.net>
Message-ID: <1B27069B5D6B4A239A13E3AFBA288C4A at CompaqSR5710F>
Content-Type: text/plain; format=flowed; charset="iso-8859-1";
reply-type=original

OK... I confess.

I think I *do* understand why people chain-sawed extra
audio stages into mil rigs, and I've already illustrated it,
but I will again because I'm a meany.

1.  A cool and lovely fall morning in Chicago, 1961,
which is about to turn ugly.... very ugly.
Doofuss #1, with no understanding of the original circuit
nor any inclination to find out about it, hooks-up an 8-ohm
speaker to a 4000-ohm output.
*Surprise!*   He gets low audio.

2.  Doofuss #1:  "Why it's just like my buddy Clem said-
them military engine-ears don't know nothin'!  I'm a
Cornditional Class by God an' I know more than Marconi!"

3.  Out comes the chainsaw-
"Waaa-wub-wub-wub-wrrrrrrrrrrrrrrrrrrrr...."

4.  In goes some half-baked, upside-down "audio amplifier"
with 87% harmonic distortion and a junk-box output
transformer which, by the way, *still* doesn't match
the old speaker (including bonus moth holes)
taken from a junked Philco.

5.  The poor, mangled radio, instead of delivering .5 watts
of nice, clean audio, now delivers
.75 watts of nasty, distorted audio.  Such genius!
In fact, Doofuss #1 is so proud of his "craftsmanship,"
he writes an article for CQ magazine,
which publishes it to a waiting world.

6.  Doofuss 2, 3, 4 etc. read the short article-
which is so much easier than reading boring old texts about
how the stage was supposed to work-  and tell all their fellow
Doofussi about the "wonderful modification improvements,"
who parrot it forevermore, Amen.

7.  For the next 5 generations
(or until the supply of doomed radios runs out),
where ever soldering irons are heated,
from sea to shining sea,
if one listens in the wee morning hours,
one can hear:
"Waaa-wub-wub-wub-wrrrrrrrrrrrrrrrrrrrr!"

OK.... that's really what I think
is behind the audio "improvements."

But I don't think I'll write about it.
Because Doofuss-1286 and Doofuss-1342
will get mad and call me nasty names.

73 DE Dave AB5S

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