> From: Skip <mays@indigo.ucdavis.edu>
> To: amps@contesting.com
> Subject: [AMPS] a few questions...
To: <amps@contesting.com>
> Date: Mon, 7 Jul 97 15:43:41 +0000
> There was a method to measure the Z of a transformer by shorting the
> secondary leads and measuring the current in the primary while bringing
> up the primary voltage.
> I would like to know if this is a "normal/conventional" method or
> something that someone has just experimented with. I'm just looking for
> conformation from one soures or a possible text reference that I could
> look to for more details.
It is probably a valid method of estimating transformer ESR, but you
have to be VERY careful since leakage reactance will cause voltage
and current to be out-of-phase. That may cause some error depending
on how you actually measure the current and voltage.
It also does not allow for the change in ESR caused by harmonics or
dc loading in the secondary winding, normal problems with
many rectified systems. You can be sure the actual ESR will be a
little worse than what you measure.
> About ten years ago I built a clone of an SB-220 using the same basic
> design less the -130 volt supply. There was a considerable problem in the
> amplifier trying to cutoff the tubes in the standby mode.
> The first design just opened the "cathode" (center tap of the fill xmfr)
> across a 50K 10wt resistor in standby. The "diode hash" type noise in the
> receiver was horible and made the amp un-usable.
I've NEVER had that problem with 3-500Z's, or seen that problem in
PA's even when they had FULL quiescent current on standby. Could you
have had a small bad connection arcing or producing corona? Was the
TR wired correctly, to fully disconnect the tube from the input and
output lines on standby?
73, Tom W8JI
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