"Asimov" wrote in message
...
"Arfa Daily" bravely wrote to "All" (19 Jan 06 01:49:16)
--- on the heady topic of " Marshall 4140 Amp question"

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AD lowering of the HT rail. Glowing anodes are the order of the day
here,
AD but I would rather have that, as the outputs will stand this for a
AD considerable time without damage, rather than have exploding or
AD shorting electrolytics, brought on by voltage stress.


Arfa,

High voltage electros have a surge rating about 20% higher than the
wvdc specified on the can. A brief surge less than the few seconds it
takes to get a reading will not usually be a problem. OTOH leaving the
tubes in runs the real risk of having a large current burn out a
transformer winding. Which risk would you prefer to take; ending up
with on your workbench: a smoking rectifier with a zapped electro, or
an open winding in a cherished rare vintage audio output transformer?

A*s*i*m*o*v

... Transformer designers take turns doing it.

Hi Asimov

Please don't get me wrong - I'm not challenging what you say. Merely
pointing out that there's two schools of thought on this. I agree that high
voltage electros should stand +20% on their indicated voltage rating, but
that is when they are youthful and in good condition ( unlike me these days
.... !! ). The point is that any measurements done with the tubes removed,
MUST be done quickly - which is why I said 40 seconds maximum in my post.

As far as the smoking reccy and zapped electro versus the open transformer
go, personally, having been in the vicinity of many exploding electrolytics
in the old days, I would rather risk the transformer damage, as I can't
remember ever having seen a transformer - power or output - in a high power
group amplifier, damaged from an output tube or bias problem. Remember that
old electro cans, at best had a ' pip ' in the rubber seal at the bottom,
and at worst, nothing to assist with venting, unlike the proper bungs or
rupture lines designed into modern ones. This is why they had a tendency to
go off like hand grenades, spewing boric acid soaked paper everywhere.

In contrast, if the output stage is drawing anything like enough current to
damage a transformer - and, IME, these will happily drip wax for at least 5
minutes without failure - then the HT fuse should ideally blow, or the surge
limiter / smoothing resistor smoke, glow, and go open.

It's probably just a matter of opinion and experience, which you obviously
have. I'm just making sure that those who might be reading, and have less
experience than you and I, understand the possible dangers and cosequences
d:~}

Arfa