wrote:

Why should someone replace ALL the capacitors on old Tube equipment?

It seems that some people advocate that.

One of the factors that is often overlooked is the tolerance of the
circuit to the various types of wear-out mechanism.

If a cathode by-pass capacitor on the sound output valve goes leaky, it
would have to leak very badly indeed (and measure only a few hundred
ohms) before it upset the operating conditions of the valve. On the
other hand, if an inter-stage coupling capacitor begin to leak and puts
even a small proportion of the anode voltage of the first valve across
the grid leak of the second, it will upset the second valve very badly
and may even destroy it.

Electrolytic smoothing capacitors in the HT line will leak even when
brand new, but the leakage is usually fairly small once they have
settled down. If they later begin to leak badly, this will cause
internal heating and damage which may not be obvious - the set will
appear to carry on working as normal. Eventually, when the leakage
increases even more, something in the power supply will fail due to
overloading or the capacitor itself bursts; but until that point, there
may be no hint that things are going wrong because the circuit is
reasonably tolerant of that sort of leakage.

I have repaired QuadII amplifiers which almost met specification even
though the internal voltages were all over the place, most of the
capacitors were leaking and some of the resistors had changed value too.
The initial design was intended to be tolerant of a wide range of
component values (close-tolerance components were very expensive) so it
wasn't badly upset by drift due to ageing.


--
~ Adrian Tuddenham ~
(Remove the ".invalid"s and add ".co.uk" to reply)
www.poppyrecords.co.uk