On Sun, 12 Jun 2011 09:40:53 +0100, Jim Lesurf
wrote:

In article , Don Pearce
wrote:
On Sat, 11 Jun 2011 18:20:33 +0100, "Dave Plowman (News)"
wrote:

I've been fixing a Cambridge A60 - turned out to be nothing more than
both speaker fuses blown. But thought I might as well check the
quiescent current while I'm at it. And the method given in the manual
is weird - replace the PS DC fuses with 15 ohm resistors and measure
the voltage across them.

Not having any high watt 15 ohm resistors to hand is there a different
way?

Take the fuse out and just put in an ammeter of suitable range. I think
that if it is a modern digital type, its internal resistance will be low
enough. Ohms law will translate volts across 15 ohms into current.

If you do this I recommend also having a shunt capacitor across the place
(fuseholder) where you connect the ammeter. (i.e. shunting the ammeter at
HF.)

Otherwise you may get amplifier instability due to the increased rail lead
inductance, etc. No idea if this *will* happen with the A60. But it can
occur for some designs/layouts. Use 0.1 microF or more. FWIW I always used
1 microF.

Slainte,

Jim

Shouldn't be a problem from what I remember of the Cambridge design,
but it never hurts to be safe. Somewhere in a junk box I still have a
modified fuse holder for the panel mounted bayonet fixture. It has a
couple of fat leads with banana plugs on the end for a meter for
current measurement.

One thing that could be a gotcha, though, is an autoscaling meter.
This could just induce oscillation in combination with the amp. A
fixed scale meter would be best.

d