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Wed Apr 4 14:13:43 EDT 2018

On Wed, Apr 4, 2018, 1:12 PM <arc5-request at mailman.qth.net> wrote:

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> Today's Topics:
>
>    1. Oscope Fixed (Robert  Eleazer)
>    2. ARC R-19 Receiver (Robert  Eleazer)
>    3. ARC-5 Mixer Substitution (Phillip Carpenter)
>    4. Re: ARC-5 Mixer Substitution (Scott Robinson)
>    5. Re: ARC-5 Mixer Substitution (Kenneth G. Gordon)
>
>
> ----------------------------------------------------------------------
>
> Message: 1
> Date: Wed, 4 Apr 2018 10:17:33 -0400
> From: "Robert  Eleazer" <releazer at earthlink.net>
> To: <arc5 at mailman.qth.net>
> Subject: [ARC5] Oscope Fixed
> Message-ID: <E417E8493A3D4CB4B5603FB7F89F27BD at DH26DQ31>
> Content-Type: text/plain; charset="iso-8859-1"
>
> I did some tests yesterday with my $2.50 Heathkit RF signal generator (my
> HP-606 is too heavy to haul over there) and the scope looked good, nice CRT
> display and digital counter and all stable.
>
> The $4.60 part I am using in my attempt to fix the Tek 2225 is the
> NTE535.  The datasheet and the part is available at NTEpartsdirect.com  I
> have no idea on the specs on the original part.
>
> I installed the NTE-535 with a 3.3 megohm resistor in the output (anode
> lead) to the scope this morning.  But I need to get a 1 amp 3AG fuse before
> firing it up.  I used the original fuse in the 2236 I fixed.
>
> Wayne
> WB5WSV
>
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> ------------------------------
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> Message: 2
> Date: Wed, 4 Apr 2018 10:28:33 -0400
> From: "Robert  Eleazer" <releazer at earthlink.net>
> To: <arc5 at mailman.qth.net>
> Subject: [ARC5] ARC R-19 Receiver
> Message-ID: <9CBAFD4D90B945708E1BE4E200D0086B at DH26DQ31>
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>
> Glenn:
>
> I think you'll find that the Mike Tauson book as provided on the Internet
> is not quite complete.   I think it is missing some of the pin-out info.  I
> have a hardcopy as well as a copy of the military maintenance manual on
> those sets.  There is also conversion info in the June 1965 issue of 73
> Magazine, available at Archive.org
>
> A key thing to remember is that the center connector must have two of the
> pins shorted for it to work.
>
> Good luck!
>
> WAYNE
> WB5WSV
>
>
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> ------------------------------
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> Message: 3
> Date: Wed, 4 Apr 2018 11:36:27 -0400
> From: Phillip Carpenter <carpenterpa at tds.net>
> To: arc5 at mailman.qth.net
> Subject: [ARC5] ARC-5 Mixer Substitution
> Message-ID: <ED441E05-9C19-401C-BFFD-96808A658FF5 at tds.net>
> Content-Type: text/plain;       charset=utf-8
>
> Has anyone substituted a 12SA7 for the 12K8 in an ARC-5 receiver?
>
> I?m interested in hearing about any details/performance.
>
> Thanks in advance to all who reply!
>
> Phillip W4RTX
>
> Sent from my iPhone
>
>
> ------------------------------
>
> Message: 4
> Date: Wed, 4 Apr 2018 09:38:52 -0700
> From: Scott Robinson <spr at earthlink.net>
> To: Phillip Carpenter <carpenterpa at tds.net>, arc5 at mailman.qth.net
> Subject: Re: [ARC5] ARC-5 Mixer Substitution
> Message-ID: <c3ae6ede-b82e-d41d-0be3-8d3d5a849bd9 at earthlink.net>
> Content-Type: text/plain; charset=utf-8; format=flowed
>
> Folks,
>
> The 12K8 (triode/hexode) and 12J8 (triode/heptode) mixer tubes have a
> major advantage over regular pentagrid types like the 12A8 or 12SA7.
> It's this: the LO pulls *far* less with AVC action on the -K and -J
> types. Example:
>
> GE G-106 wood radio, 1939 vintage. Has RF amp and 6A8 mixer. Aligning it
> at the top end of the SW band at 18 MHz was nearly impossible, as the
> AVC moved the LO over 20 KHZ from the start point. Substituted 6K8,
> which is a triode/hexode so the LO is better separated from the mixer
> and the deviation decreased to 1 KHz.
>
> The noise advantage of the 12SA7 is trivial. The equivalent noise
> resistance of the 12SA7 is 240K and of the 12K8 is 290K. The RSF amp is
> used partly to overcome mixer noise. 12SK7 noise resistnce is 11K, far
> lower than the mixer. These numbers come from the Radio tron Designer's
> Handbook, 4th editoin, page 938.
>
> FWIW,
>
> Scott Robinson
>
>
>
> On 4/4/18 8:36 AM, Phillip Carpenter wrote:
> > Has anyone substituted a 12SA7 for the 12K8 in an ARC-5 receiver?
> >
> > I?m interested in hearing about any details/performance.
> >
> > Thanks in advance to all who reply!
> >
> > Phillip W4RTX
> >
> > Sent from my iPhone
> > ______________________________________________________________
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> ------------------------------
>
> Message: 5
> Date: Wed, 04 Apr 2018 10:10:50 -0700
> From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
> To: Scott Robinson <spr at earthlink.net>
> Cc: Arc5 at mailman.qth.net
> Subject: Re: [ARC5] ARC-5 Mixer Substitution
> Message-ID: <5AC5071A.32550.1019159E at kgordon2006.frontier.com>
> Content-Type: text/plain; charset=US-ASCII
>
> On 4 Apr 2018 at 9:38, Scott Robinson wrote:
>
> > Folks,
> >
> > The 12K8 (triode/hexode) and 12J8 (triode/heptode) mixer tubes have a
> > major advantage over regular pentagrid types like the 12A8 or 12SA7.
> > It's this: the LO pulls *far* less with AVC action on the -K and -J
> > types. Example:
> >
> > GE G-106 wood radio, 1939 vintage. Has RF amp and 6A8 mixer. Aligning it
> > at the top end of the SW band at 18 MHz was nearly impossible, as the
> > AVC moved the LO over 20 KHZ from the start point. Substituted 6K8,
> > which is a triode/hexode so the LO is better separated from the mixer
> > and the deviation decreased to 1 KHz.
> >
> > The noise advantage of the 12SA7 is trivial. The equivalent noise
> > resistance of the 12SA7 is 240K and of the 12K8 is 290K. The RSF amp is
> > used partly to overcome mixer noise. 12SK7 noise resistnce is 11K, far
> > lower than the mixer. These numbers come from the Radio tron Designer's
> > Handbook, 4th editoin, page 938.
> >
> > FWIW,
> >
> > Scott Robinson
>
> Your information is worth quite a lot, Scott, and I am grateful for it.
>
> I HAVE subbed a 6SB7Y for a 12SA7 in an old Hallicrafters S-41G, and the
> result was a
> HUGE reduction in noise, and thus an increase in signal levels, at
> frequencies above about
> 10 MHz.
>
> I would be curious about subbing a 6SB7Y for the 12K8, although of course
> some
> allowances would have to be made for the different filament voltages. A
> resistor in series
> with the filament of the 6SB7Y may work as long as the filaments in the
> receiver had been
> changed to 12V from the normal 24V.
>
> There is no 12V version of the octal 6SB7Y, although, as I remember it,
> there may be a 7 or
> 9 pin equivalent.
>
> I have also been very, very curious over the past several years concerning
> what a sub of a
> complete Pullen mixer using a pair of 7963 sub-mini dual triodes (that is,
> 4 triodes) for the
> 6/12K8 would accomplish.
>
> One could build a plug-in adapter using an old octal tube base.
>
> Each triode in the 7963 has a tranconductance of 13K, which should
> translate to very low
> noise output.
>
> Of course, although noise output would be greatly reduced, I have no idea
> what effect this
> sub would have on both stability and tracking.
>
> Besides, the lower noise may not be worth the effort, since the 12SK7 RF
> amp does a very
> good job of overcoming the horribly noisy 6/12K8 mixer noise anyway.
>
> Even so, it would be a very interesting experiment.
>
> In my previously (1950s) modified BC-454 which now covers 10 meters, the
> receiver's
> sensistivity is still less than 1 microvolt for MDS, even on that band,
> still using the original
> tube lineup.
>
> Ken W7EKB
>
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> End of ARC5 Digest, Vol 171, Issue 6
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