[ARC5] Audio stage impedances.

[Richard Knoppow]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Cc: "Richard Knoppow" <1oldlens1 at ix.netcom.com>
Sent: Wednesday, January 23, 2013 3:05 PM
Subject: Re: [ARC5] Audio stage impedances.


> On 23 Jan 2013 at 13:10, Richard Knoppow wrote:
>
>>      I found the 12A6 in the Radio Amateur's Handbook for
>> 1948. The plate resistance is given as 70 Kohms and the 
>> load
>> resistance 7500 ohms.
>
> Yes. That is what I have here also
>
>>  The turns ratio of the transformer
>> from the data in the handbook for the receiver is about
>> 2.22/1 so the impedance ratio should be its square or 
>> about
>> 5/1
>
> That is what I got also.
>
>>  taking the reciprocal gets 0.2/1 dividing 7500 ohms by
>> this gives 1500 ohms.
>
> OK. Here is where my audio amp theory (what little there 
> is of it) fails me:
> why do you use the reciprocal? In any case, if you DIVIDE 
> 7500 ohms by
> 0.2, you get 37,500, not 1500. If you MULTIPLY 7500 by 
> 0.2, you get 1500.
>
>>  The instructions indicate the tap on
>> the secondary is for 600 ohms and the higher impedance of
>> the entire winding is for driving several sets of high
>> impedance (8k ohm) headphones.
>
> Yup.
>
>>     Note that the specs for the transformer indicate the 
>> two
>> windings are made with different size wire so the DC
>> resistance ratio will not reflect the impedance ratio.
>
> Yes. That is why I used the turns ratios.
>
>>      The 12A6 does not seem to be similar to any other 
>> tube.
>> It is a low power beam pentode with an output of around 4
>> watts.
>
> Yes. Although a 6V6, 6K6, or 6F6 can be directly 
> substituted for it if the
> filaments are all wired in parallel.
>
>>      This is a long thread so I hope I am not going over
>> stuff that has already been posted.
>
> Nope. You haven't....however, I am still very confused 
> about what, exactly, is
> the output impedance of a 12A6 when used in the ARC-5 
> receiver?
>
> Ken W7EKB

    I don't know why I decided to multiply by the reciprocal 
instead of dividing by the value. Anyway, the secondary 
impedance is correct.
    I have not looked at the characteristics of the three 
tubes above but think they are somewhat different although 
they would work.
    The impedance presented to the tube by the transformer 
or direct load is whatever gives the straightest load line, 
i.e., least distortion.  For Class-A amplifiers its not very 
critical. The values from the tube handbook and transformer 
ratios give the values above, that is 1500 ohms and 600 
ohms, that IS the output impedance. You can check it using a 
variable resistor and a mid frequency audio tone (say 400 
hz). Like any other circuit the match is indicated when the 
loaded voltage is half the open circuit voltage. Since this 
is a very simple amplifier, with not feedback, this 
relationship should hold.
    I have encountered some curious exceptions, for 
instance, when I measured my R-388, which is supposed to 
have 4 ohms output it actually had greatest power transfer 
into about 8 ohms but when I measured distortion it was 
lower for the 4 ohm load. Collins speakers are actually not 
rated at 4 ohms but closer to 6, very curious. The 
R-388/51J-3 also has a simple beam pentode output.
     In the case of the ARC-5 receivers I suspect that they 
will work into an open circuit or into a fairly low 
impedance without harm. For instance, I think you could put 
as many high impedance headphones as desired on it until the 
levels became too low.


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