[ARC5] 6AC7 and 6AB7

[Geoff]

> ----- 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




The correct family description is:

1851  July 1938 grid cap TV pentode, became 1852 1852   late 1938 single 
ended 1851
1853   late 1938

6AB7 3/18/39
6AC7 3/18/39
Later versions were marked 1852/6AC7; 1853/6AB7 and the earlier pair 
discontinued as a seperate number.

6AG7 5/24/39 Video Pentode, scaled up 6AC7

All the above were developed as TV tubes; the 6AG7 became known as the best 
tube for a crystal oscillator in ham rigs, commercial and HB. The last 3 
were available as W versions as well as micanol bases for lower RF loss.

I have all NIB including the very scarce 1851.

Carl