Re: Transformer question
From: philo (philo_at_privacy.net)
Date: 09/25/04
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Date: Sat, 25 Sep 2004 12:35:41 -0500
T N Nurse wrote:
> I have a 50 watt valve amp which seems to have blown its
> output transformer. A new one is on order, but I was puzzled
> by some of the reading I got off it and wondered if someone
> could give an explanation. All the measurements were made
> with the transformer *_completely out of circuit_*, both primary
> and secondary.
>
> The transformer is from a push-pull standard guitar amp (Trace
> Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing
> red hot before the fuse popped. Checking the bias voltages, I
> had around -46v on the grid of each of the El34s, so that was
> a reasonable figure. I then removed the output transformer
> and did some resistance measurements on it. When measure from
> the centre tap to either of the primary outer connections, the
> reading was 69 ohms for each. But when I measured across the
> primary outer connections, instead of the expected 138 ohms,
> I got an open circuit. I rechecked it numerous times but with
> the same result. I even removed the cables from their connector
> and measure across the bare wires, but still the same result,
> 69 ohms from the centre tap to the outers, but open circuit
> between the outers.
>
> On the basis of these bizarre reading, I assumed the transformer
> is faulty and ordered a new one, but can anyone offer an
> explanation as to why I got such resistance readings? I have a
> vague recollection of similar results on a small 15 watt amp I
> repaired many years ago and replacing the transformer fixed it
> and it went on to give good service, but I would like to know
> what is actually going on. Anyone?
there are usually two wires in the center tap...
if they are not touching when you take your reading ...there will be
infinite resistance...
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