[ARC5] Audio stage impedances.

[Geoff]

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Wednesday, January 23, 2013 6:05 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

You would have to know all the tubes parameters in the receiver running as 
designed to start and then go to work on the published curves. The rest is 
then simple formulas.

Gm, mu (amplification factor) and Rp are all interelated.


The published values are the design bogey and will vary from tube to 
tube....some say +/- 20% was acceptable in production testing at RCA while 
Sylvania held to +/- 10%

The 6K6 is very close in most respects to the 12A6.


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