Anyone ever measure the A470's impedance?

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Postby Shannon Parks » Sat May 28, 2011 8:17 am

Just did the plastic A470 with 100V @ 60Hz with no load:

Plastic A470
16 ohm -- 100V to 6.144V -- 4239 ohms
8 ohm -- 100V to 4.10V -- 4759 ohms
4 ohm -- 100V to 3.06V -- 4272 ohms

Simple test for the screen taps! Yes - right at 33%.

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Postby Tom Bavis » Wed Nov 09, 2011 10:42 am

When you measure with a load, the winding resistance is included, which will increase the ratio. This is the REAL load presented to the output tubes. I usually measure unloaded, and add twice the primary DC resistance to the calculated impedance. (This assumes equal loss in primary and secondary, a reasonable first approximation) I measure at 60 Hz, as I can get 0.1% accuracy with a DVM.
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Postby kheper » Sat Nov 12, 2011 8:12 am

separks wrote:I'm planning on making a 1625 ST70 which I'm dubbing the JAN70. I was thinking the 4.3K impedance would be too low for 1625s, so I was going to use the 4 ohm tap (for 8 ohm loads) to get a better match. At almost 6K, this seems much better. Obviously I will test at the top level and compare there, but I'm trying to do a better job of evaluating iron.


For 1625s and 807s, what has been claimed for a long time to be the optimal input impedance is 10K p-p.

http://www.retrovox.com.au/A515.pdf

For UL use, an audio transformer with a tertiary screen tap winding ratio of 50% is said to be the best for 1625s or 807s.

http://oestex.com/tubes/ulo.html

separks wrote:FWIW, it certainly seems as though the cloth lead and plastic lead trannies are functionally identical. My cloth leaded ones are definitely early - like around 1960 - due to the style of the original kit.


The cloth ones (part number: 464006) were hand-wound in Philly; The plastic ones (part numbers: 454326 and Z-326) were machine-wound in Japan.

The st-70 had the same amp design from its birth to its death, so I expect the engineers wanted the outputs to be "functionally identical", but maybe the switch in fab processes altered some of the characteristics of the outputs (differences in input impedance).
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Postby kheper » Wed Nov 30, 2011 9:26 am

In an old Acrosound catalog (page 8), there is a schematic for a PP UL and a PP triode mode 807. The B+ is 450V and the primaries are 6k and 12k, respectively. 6sn7 for the volt amps and phase inverters.

http://www.bunkerofdoom.com/xfm/ACROSOU ... d_1955.pdf
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Re: Anyone ever measure the A470's impedance?

Postby tomlang » Tue Nov 04, 2014 1:26 pm

It came up today what is the %UL and impedance of the Dynaco A470.

So I measured:
1. A cloth leaded A470 dated 1962
2. A Magnequest A470 with brass bells circa 2004

I got EXACTLY the results of Shannon above using the Variac at 100 vac (to the third significant digit, I may add) for both the impedances and UL taps on both transformers above.

So, "common wisdom" is the A470 is a nominal 4300 ohm OPT with 33% UL taps correct?

Does anyone know "why" 33% UL taps -- did they actually run experiments to determine this?
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Re: Anyone ever measure the A470's impedance?

Postby dcriner » Wed Sep 16, 2015 5:13 pm

An audio xfmr has no relevant impedance of its own. xfmr impedance at the primary is just whatever impedance is connected to the secondary, multiplied by the square of the turns radio. So, the only thing to measure or to look up in the specs is the turns ratio - then you know all there is to know.
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Re: Anyone ever measure the A470's impedance?

Postby rongon » Sat Nov 05, 2016 1:23 pm

the only thing to measure or to look up in the specs is the turns ratio - then you know all there is to know.


I'd also want to know the primary inductance (either plate to CT). Any idea what that is for a Dyna A470? 10H or so? Or higher?
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Re: Anyone ever measure the A470's impedance?

Postby Geek » Sat Nov 05, 2016 7:56 pm

About 40H with zero DC unbalancing.
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