Transformer question

From: T N Nurse (tnnurseNOUCE99_at_hotmail.com)
Date: 09/25/04 

Date: Sat, 25 Sep 2004 14:35:33 +0100

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?

Relevant Pages

  • Re: Best valve config for bass
    ... in the 10 to 200 Hz range, and not of a fullrange amp; ... transformer will perform even better, if I'm not asking the blood out of it. ... > Iron is important, but also the number of turns. ... >> the trade" to improve OPT frequency response without using tons of ...
    (rec.audio.tubes)
  • Re: IDing a Fender Bassman to reverse mods
    ... >>>A variac is very useful for checking the impendance ratio of unknown ... A variac works just fine. ... >>>unknown power transformer, ... If I'm buildng an amp that requires 330 volts AC ...
    (rec.audio.tubes)
  • Transformer question
    ... with the transformer *_completely out of circuit_*, ... The transformer is from a push-pull standard guitar amp (Trace ... Elliot 50 watt combo, 2 EL34s) and one of the valves was glowing ... the centre tap to either of the primary outer connections, ...
    (sci.electronics.repair)
  • Re: Does the power supply limit the actual power of a tube amp?
    ... I rebias the amp (perhaps ... you'll blow your power transformer." ... ALL current from the power transformer's collective secondary windings, ... Any increase in current draw from ANY one of the secondaries, ...
    (alt.guitar.amps)
  • Re: Problems with custom amp...
    ... >Anyway, it's basically a 50 watt Fender Reverb, single channel style amp. ... >Bassman size output transformer, no choke, yada-yada. ... >power tranny center-tap not connected, ground side of screen supply filter ... >the underrated filter supply caps. ...
    (alt.guitar.amps)
Loading