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Re: [Amps] Alpha 78 B- Resistor

To: "Michael Tope" <>, <>,"AMPS" <>
Subject: Re: [Amps] Alpha 78 B- Resistor
From: R.Measures <>
Date: Sun, 25 Jan 2004 07:32:26 -0800
List-post: <>

>----- Original Message -----
>From: <>
>To: "Michael Tope" <>; "AMPS" <>
>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).

**  I would try putting the over-current sampler R in series with the 
glitch resistor - and then protect the sampler R with paralleled 
200A-peak 1n5400-series Si diodes.
>> >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  that use 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: ]
>> >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. 

**  possibly two. different events?

>The body of
>the resistor doesn't touch the burnt area, and there
>is no soot on the resistor body. 

**  Wirewound vitreous resistors have no organic parts.   

>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?

**  agreed

>If all the crazing came from a very short event,
>then how do explain the baked epoxy resin
>the PWB?

**  Beats me, but gold leakage can cause high anode current flow, even 
during standby.  Two events seems possible.
>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

**  As I understand it a 3cx800A7 will do c. 800mA of cathode-I before 
stripping occurs.
>> >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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