[ARC5] 6AC7 and 6AB7

[Kenneth G. Gordon]

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