[Amps] Alpha 78 B- Resistor

[Michael Tope]

----- Original Message -----
From: <amps-bounces at contesting.com>
To: "Michael Tope" <W4EF at dellroy.com>; "AMPS" <amps at contesting.com>
Sent: Saturday, January 24, 2004 6:05 AM
Subject: Re: [Amps] Alpha 78 B- Resistor


> **  GLOBAR-KANTHAL's Type SP resistors are superior to garden-variety
> wirewound resistors for glitch service.  (phone order # 716 286 7610)
>

Even if I replace the 25 ohm wirewound with a globar style
resistor, it is still shunted with R112 and R113 which
for a voltage divider which drives the plate overcurrent
relay circuit. These are small carbon composition resistors
that will likely flash over during an anode glitch. On the
other hand, if I put a 25 ohm globar in series with the
anode, during a glitch the anode supply will try to divide
between the anode glitch resistor and R111 in the B-
return. In this case, R111 would flash over and the anode
glitch R would take the full anode voltage and ultimately limit the glitch
current to a safe value, so perhaps this is the way to
go (leave R111 as a wirewound and put a globar or
other pulse rated high voltage resistor in series with
the anode supply).

> >While I was in the process of installing a new bandswitch
> >in our club's Alpha 78, I noticed that the plate overcurrent
> >shunt resistor R111 was cracked into two pieces, and
> >that the G-10 PWB underneath this resistor was burnt
> >pretty badly. It looks as if the resistor was running too
> >hot which in turned slowly cooked all the epoxy resin in
> >the G-10 laminate. When I scrapped away the soot, all
> >that remained was the criss-cross pattern of the fiberglass
> >layup weave.
> >
> >In any case, I don't think this was a "big-bang" kind of
> >failure.
>
> **  Amplifiers with tubes that utilize tubes with gold-plated grids
> usually don't go bang because a parasite that might induce a stentorian
> HV-chassis discharge in air in TL-922s, SB-220s, et cetera is typically
> replaced by a discharge inside the envelope through the gold meltballs
> that stuck to the anode-grid insulator during the parasitic oscillation.
> [ref:   http://www.somis.org/8877.gs2.JPEG ]



> >The resistor was not blackened at all. Instead
> >the resistor's ceramic form material is "crazed"
> >indicating that was overheated.
>
> **  Crazing is typical of a sudden overload.
>

Perhaps, but how do you explain the burnt G-10
underneath the body of the resistor. The body of
the resistor doesn't touch the burnt area, and there
is no soot on the resistor body. This leads me to
believe that the resistor ran hot for a long period
of time and that the radiant heat from the ceramic
form cooked the nearby G-10. If the resistor body
was running that hot (e.g. hot enough to cook the
board), then would it not be plausible that the
ceramic form was cracked and discolored due
to prolonged operation at elevated temperature?
If all the crazing came from a very short event,
then how do explain the baked epoxy resin
the PWB?

I think the resistor was sized for 1978 regulations
(1KW DC input, 2KW PEP input) whereas the
HD transformer and the combined plate dissipation
of the tubes are more in line with 3KW input power
rating.


> >Presumeably the hot
> >resistor proximity baked the resin in the adjacent laminate
> >material over time. The resistor is a 25 ohm x 25 watt
> >unit, which in my mind is too small. This resistor carries
> >the full supply current of the amplifier which is probably
> >close to 1.4 amps when running full bore!!
> >
> >Anyone run into this problem before with Alpha 78?
> >By the way, this might be related to the problem that
> >Jorge, EA2LU is having with his Alpha 76.
> >
> >73 de Mike, W4EF
> >
> >
> >
> >
> >
> >
> >
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> >
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