[Patrick Turner]

On Oct 15, 3:47*pm, flipper wrote:
On Fri, 14 Oct 2011 19:42:32 -0700 (PDT), Patrick Turner

wrote:

snip

In fact the 75V
zeners had a zener voltage of around 82V when new. Their working
condition was with about 20mA in each, and 82V, and that's 1.64Watts,
and I'd cramped them between plate of aluminium and the aluminium
chassis with lots of heat paste to keep 'em cool as cucumbers.
Didn't stop them frying it seems.

You didn't mention what the part number was but, FYI, if they're the
typical plastic case 5 Watt zener then heatsinking the case doesn't
accomplish much, if anything. The rating depends on dissipation
through the leads soldered to the 'heatsink', which is typically a
'large' PC board trace area but can be something more robust.

The leads 'to the heatsink' should also be as short as possible.

The "75V" zeners are of unknown asian origin, maybe 8mm long x 5mm dia
hard plastic bodies. So when they are cramped between Al plates with
generous heat paste, they still may get warm internally. 1.64W isn't
much though. But at just after amp turn on, and before OP tubes
conduct, the current is 50mAdc, which is 4.1Watts, which one might
think was OK until the screens conducted some current within 15
seconds and Ea dropped to reduce the total Ig2 supply to a steady 42mA
with about 1/2 that in Zeners and 1/2 in KT90 screens. So probably the
repeating heating cycles on the zeners affected their zener voltage.
I've known zeners to be a bit dodgy in this aspect, but as long as one
doesn't exceed 1/3 the Pd rating for continuous use they are OK at
least for the short term. I could have used say 10 x 33V x 5W Zeners,
which may have coped better.
Sometimes I've wrapped a U shaped piece of copper around zeners with a
screw to hold them to a chassis and with silicone or heat paste, and
such allows one to hold a finger to the zener indefinately, ie, T
45C, which should be OK. I have used other smaller sized zeners also
rated for 75V and 5W in other locations with no troubles, ever, but
usuually Idc max is 10mA, for 0.75W Pd max, so hanging them across a
pair of lugs on a tag strip is OK and they run cool.

I'm ammending my 8585 schematics to show the use of bjts to shunt reg
the Eg2 supply. The reason for shunt reg instead of series reg should
be obvious; if you have a wayard tube with excessive Ig2, say 50mA,
the series R of 3k6 between B+ of +480V and the screen shunt reg
develops a voltage of 180Vdc, and of course other tubes still draw
Ig2, so maybe the drop is 252V, so that Eg2 then drops to +228V, and
the amp becomes silent, and action is taken to find out why. Where you
have a series reg, the Ig2 might rise from 20mA to 70mA and the faulty
tube could more likely cause colateral damage as it melts down. Shunt
reg "wastes" some current and Watts in the reg, but it is a slight
amount compared to the hundreds of Watts a big amp uses at idle, even
with low Ia bias currents as I am using.

Patrick Turner.