[ARC5] Audio stage impedances.

[Kenneth G. Gordon]

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