[ARC5] 6AC7 and 6AB7

[Dennis Monticelli]

I have always considered the 6AC7 as a baby 6AG7.  You can use it in
the same circuits with the 6AC7 having both less capacitance and less
punch. Both have high gm and good screening of the plate from the
grid.  In osc service this helps with insuring that plate tuning
variations do not affect the grid circuit, hence very low pulling.
The gm of the grid-screen configuration is about 1/4 of the grid-plate
gm (proportional to the current split).  In a typical class C circuit
the ratio of the gm's is actually less than that because the plate
dips below the screen in actual operation allowing the screen to take
a short but healthy cathode current pulse.  Of course we're talking
large signal gm here not the small signal gm quoted in the tube
manuals.

Speaking of manuals, the 1940 RCA tube manual has good sections for
all three: the B, C, and G.

Someone noted that the GRC-109 uses it.  I have the CIA version (RT-3)
and it is similarly configured.  The 6AC7/2E26 combo work well
together, just like the 6AG7/807 or 6146.

You can find NOS 6AC7's pretty easily, even the primo ones in the micanol base.

Dennis AE6C


On 2/5/13, Kenneth G. Gordon <kgordon2006 at frontier.com> wrote:
> 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
> ______________________________________________________________
> ARC5 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/arc5
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:ARC5 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>