From Dr.Distortio–(at)–bs.mhv.net Mon Mar 13 23:13:44 CST 1995
Article: 458 of alt.guitar.amps
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From: Dr.Distortio–(at)–bs.mhv.net (Dr Distortion)
Newsgroups: alt.guitar.amps
Subject: Re: reverb transformer
Date: 13 Mar 1995 23:27:19 GMT
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Mikko Oksa (oks–(at)–its.mdata.fi) wrote:
: Anybody have data on the 125A20B impedance matching transformer used in
: Fender reverb circuits ? What kind of a transformer would I need If I
: were to build a silmilar circuit ?

I don’t have the data handy, but I can tell you how to figure it out.

1) All Fender reverb tanks have an 8-ohm nominal input impedance. So, the
secondary winding must be 8 ohm.

2) Are you talking about the transformer that was used with a 12AT7 driver?
In that case, the primary impedance of the transformer would probably
be chosen to more-or-less match the plate impedance of the tube, which
gives you maximum power transfer with a triode. Since both halves of the
12AT7 are being used, the plate resistance is 1/2 that of the stated
spec in the tube manuals. So, look up the plate resistance for a 12AT7
at the kind of plate voltage which it sees in the Fender circuit and
divide it in half; that will give you the approximate primary Z of the
transformer, assuming that they designed the xfmr to match the plate
resistance. An even better way is to obtain the actual transformer
and take measurements; feed a 1-volt, 1kHz sine wave into the secondary
and read the AC voltage appearing across the primary. This will give
you the turns (step-up/step-down) ratio. The impedance ratio is the
square of the turns ratio.
__|__ Dr.Distortion __|__
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