Re: {Collins} 75A-3 tube

FYI. There is more to the choice of an RF front-end tube than noise and
gm.  Linearity and AGC characteristics are also important.  I'm certain
Collins took all of this carefully into account in making their choice.

Dennis AE6C

On Thu, Sep 29, 2016 at 9:16 AM, Michael Tortorella <w2iy@xxxxxxxxxxx>

> I also remember using a 6GM6 as the RF amp in the 75S-3B on the theory that
> it had lower noise than the 6DC6.  I have also read somewhere that the 6DK6
> is also an acceptable, lower-noise sub for the 6DC6.  Jeff, if you want to
> try a 6DK6 I can send you a new one for cost of postage.
> 73
> Mike W2IY
> -----Original Message-----
> From: Collins [] On
> Behalf Of Glen Zook via Collins
> Sent: Thursday, September 29, 2016 11:22 AM
> To: Jeff Brown; collins@xxxxxxxxxxxxx
> Subject: Re: {Collins} 75A-3 tube
> There was a production change and the later versions of the 75A-3 do use a
> 6DC6 tube in the "front end".  The 75A-4 as well as all of the 75S- series
> receivers use the 6DC6 for the r.f. amplifier.  There are slight
> differences
> and the 6DC6 has a lower transconductance than the 6CB6.
> According to my RCA tube manuals, the 6DC6 was designed to be used as r.f.
> amplifiers as well as television i.f. amplifiers whereas the 6CB6 was
> designed only for i.f. amplifiers.  I believe the 6DC6 has a lower noise
> figure than the 6CB6 which can have an effect as the frequency increases
> even on HF.  That noise difference would be noticed on the 15-meter,
> 11-meter, and 10-meter bands.
>  Glen, K9STH
> Website:
>       From: Jeff Brown <n9iz@xxxxxxxxxxx>
>  To: "collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
>  Sent: Thursday, September 29, 2016 9:03 AM
>  Subject: {Collins} 75A-3 tube
> I've been cleaning up my project 75A-3 receiver and testing all the tubes I
> have.  There are four or five I need to order that were missing or no good,
> and I don't have the in my stash.  I've found a bit of a discrepancy for
> tube V1.  My manual says it's a 6CB6.  However, the chassis on my radio is
> marked 6DC6.  Not sure which I actually need.  The marking was definitely
> put there intentionally.  It's not the exact same print color or font, by
> was made by an ink stamper and then "painted over" with something that
> looks
> live a varnish. Not hand scribbled or anything like that.  So, before I can
> order my three 6AL5's and a 6DE7 for a Heathkit, I need to figure out
> whether I need the 6DC6 or 6CB.  At least I'm making progress on the
> chassis.  It's much cleaner and the pilot lamps (that aren't burnt out or
> missing light up.  So far all the filaments glow when powered up.  Any
> suggestions on the V1 would be much appreciated.  Thanks.
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