[ARC5] 6AC7 and 6AB7

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Tuesday, February 05, 2013 9:07 AM
Subject: Re: [ARC5] 6AC7 and 6AB7


> On 5 Feb 2013 at 8:23, Ian Wilson wrote:
>
>> Where were 6AC7's actually used? They show up regularly 
>> in fairly
>> large numbers in piles of metal octals, etc. My guess is 
>> that they
>> were used as IF amplifiers in radar sets. I don't recall 
>> seeing any in
>> the Surplus Conversion Manual schematics, though, so I 
>> wonder when
>> they were introduced?
>>
>> 73, ian K3IMW
>
> You are pretty much right about how they were used.
>
> They were used in large quantities in the IF stages of 
> WWII VHF FM
> receivers like the R-19, and in radar receivers, for 
> instance. I think they were
> first introduced about the same time as the 6AG7 for use 
> in TV receivers, but
> am not certain.
>
> As Carl pointed out, they tend to load the grid input 
> circuits when used as RF
> amps. unless the connection to the grid is tapped down or 
> up to compensate.
> This makes their casual use as substitute RF amps not 
> particularly useful.
>
> Many hams used to plug them in place of the usual remote 
> cutoff tube,
> expecting lower noise and higher performance. In many 
> cases, all that would
> do is turn the RF stage into an oscillator.
>
> They were often used like this in BC-348s. I have no idea 
> if this helped with
> that particular receiver or not. The BC-348 has a 915 KHz 
> IF, so using the
> 6AC7 wouldn't hurt its image response too much, I suppose
>
> However, the 6AC7 makes an excellent low-noise mixer and a 
> good IF amp
> when the circuit is designed for them. They also make an 
> excellent oscillator.
>
> The military used millions of them and NIB versions are 
> still available in bulk.
>
> The 6AB7 is far more difficult to find. It is listed in 
> the RCA HB-3 as a
> "Television Amplifier Pentode" with the same two-condition 
> screen operation
> as the 6AC7. The 6AB7 has a transconductance of 5000 
> umhos, while that
> of the 6AC7 is 9000.
>
> Ken W7EKB
>

      The three types, 6AC7, 6AG7 and 6AB7 seem to have been 
designed as a group. The 6AC7 is a sharp cut-off pentode, 
the 6AB7 is a remote cut-off pentode for IF or RF use, and 
the 6AG7 a sharp cut-off power pentode meant as the last 
amplifier in an IF stage.  My tube handbooks are too late to 
have much detail. I think these were all early 1940 tubes 
featuring much higher Gm than previous tubes for similar 
applications.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com