help : How to test output transformer?
Anthony
hi,
I have check the tubes. It turn out that the tubes were O.K. The problem
lies on the output transformer. It seems that the left channel output
transformer is causing the fuse to blow. I took out all the left channel
tubes and turn on the system with the right channel only, everything was
good. I then took out the left channel tubes and swap in the right
channel tubes , everything is o.k also. (proves that both set of tubes are
o.k). Whatever set of tubes I use for right channel (right channel only)
the fuse will blow. However what I can't understand is that it blows only
after the tubes are warmed up. Anyway to test the output transformer?
Anthony
generic
proper bias. The fuse doesn't blow until the tubes warm up, because
they can't draw plate current until they do.
The fact that one channnel works suggests that the amplifier is cathode-
biased, rather than using a separate bias supply that could serve both
channels.
A typical bias arrangement for a push-pull pair might be for
the cathodes to be connected together and then grounded through a
low-value power resistor, something on the order of 50 ohms at 10
watts. The grids of the output tubes go to ground though separate high
valued resistors, say 1 meg, whichs holds them at ground potential.
The voltage drop across the cathode resistor caused by the tube's
idle current causes the cathodes to sit at some value above ground.
Since the grids are at ground potential this makes them negative with
respect to the cathodes, and presto! There's your grid bias. Often the
cathode resistor will have something like a 100uf 50v electroltic
capacitor across it to increase the gain of the stage by providing a
low impedence path around the cathode resistor for the signal current.
If This capacitor is shorted or leaky, there will be bias problems,
also true if the value of the cathode resistor has decreased or
increased. The output tubes might have separate cathode resistors, or
there might be an arrangement with a potentiometer with one cathode
at each leg and the wiper grounded, allowing the DC current balance
of the output stage to be adjusted. There are all sorts of ways of
doing it, but you get the idea.
Another thing to look for is the capacitor which couples the grids to
the preceding stage. Often this stage is a tube configured as phase
inverter(rather than a center-tapped transformer). One output tube
grid will go to the plate and one to the cathode to get drive signals
which are 180 degrees out of phase for the push-pull outputs. If the
coupling caps which isoloate the DC bias voltage from the +DC
potentials at these points are leaky, the outputs will be bias-starved
and draw excess current. A sign of this in an amplifier which is
working well enough to not be blowing fuses is when one tube is
running hotter than the others, maybe with the plate glowing
excessively.
Output transformers don't blow that often, which is good, because they
can be expensive and hard to get. Chances are pretty good that you've
got a bias problem.
Good luck!
Frank