[ARC5] 70.7 V transformers...info...

[Kenneth G. Gordon]

OK. Here's what I found: there was an article in Electric Radio Magazine by 
Mike Murphy WU2D in November 2007 beginning on page 30 entitled, 
"Those Pesky 70-volt Audio Transformers".

According to him (and I have no reason to doubt him) the "...common Radio 
Shack 70.7V line transformer, part number 32-1031..." shows the following 
impedances at the various "Watts" taps:

Watts - Impedance
0.625 - 8K ohms
1.25  - 4K ohms
2.5 - 2K ohms
5 - 1000 ohms
10 - 500 ohms.

The Radio Shack transformer listed above can handle up to 10 watts at 4, 8, 
or 16 ohms output.

So, the input impedance of the standard 10 watts-rated 70.7 V line 
transformer, is, as Bob stated, 500 ohms.

But, those other transformers which have various "watts" ratings input taps 
can be used to replace the transformers in our receivers.

As I remember it, the output impedance of the 12A6 is something like 7.5K, 
so the 8K tap would be a close match.

I have one receiver here which was hacked up long before I got it. In that 
one, the original audio output transformer had been replaced with a Radio 
Shack 32-1031.

But also, as Bob told us, audio output transformers from AA5 radios, the very 
common table-top tube-type BC sets, also match the impedances very well, 
and those are a lot easier to find than the multi-tap 70.7 V line transformers. 
They are also a bit smaller.

Ken W7EKB