[ARC5] Audio stage impedances.

[Richard Knoppow]

     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.  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  taking the reciprocal gets 0.2/1 dividing 7500 ohms by 
this gives 1500 ohms.  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.
    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.
     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.
     This is a long thread so I hope I am not going over 
stuff that has already been posted.

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

----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at frontier.com>
To: <Andrew.MacAllister at emerson.com>
Cc: "ARC-5 List" <arc5 at mailman.qth.net>
Sent: Wednesday, January 23, 2013 9:06 AM
Subject: Re: [ARC5] Audio stage impedances.


> On 23 Jan 2013 at 5:34, Andrew.MacAllister at emerson.com 
> wrote:
>
>> Thanks!
>>
>> I knew there were variations, but from reading and your 
>> info, there
>> are more than I expected. I should get some higher 
>> impedance
>> input-side xfmrs, like the 10K to 4 Ohms. After 40+ years 
>> of
>> collecting these things, I now have 5 units working well 
>> and covering
>> 190 kHz to 9.1 MHz, AND they look great.
>
> Great!
>
> From the tube data, the output RESISTANCE of the 12A6 
> audio stage at the
> tube's plate is around 7.5K.
>
> However from the turns ratio vs output impedance of the 
> transformers listed
> in the service manuals, it appears that the 12A6's plate 
> output-IMPEDANCE
> is around 20-24 Kohms.
>
> I took the turns ratios of the transformer data given, 
> squared that value, then
> used the resulting value to calculate the plate impedance 
> from the output
> impedance. The result in all cases was 20-24 Kohms.
>
> The output impedance from the original audio transformers 
> is as I have
> stated: it depends on the transformer: 8K, 4K, 600 ohms, 
> 500 ohms, 300
> ohms, or 250 ohms.
>
> You can find the necessary data in the original service 
> manuals, or perhaps
> Mike (Morrow) or Mike (Hanz) or even Mike (Everett) will 
> jump in here and
> give us the correct info.
>
> The above is what I have arrived at by comparing what Mike 
> (Morrow) and
> Robert have told me, coupled with reviewing the tube data, 
> and comparing
> the turns ratios that Mike (Morrow) passed on to me/us 
> here.
>
> Ken W7EKB
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