[Amps] Looking for picture of SB220 interlock switch

[Jim Thomson]

Date: Wed, 6 Nov 2013 11:00:30 -0500
From: "Carl" <km1h at jeremy.mv.com>
To: <amps at contesting.com>, "Geert Jan de Groot" <pe1hzg at xs4all.nl>
Subject: Re: [Amps] Looking for picture of SB220 interlock switch

###  If you activate the HV shorting device on the drake L4B amp in slo-mo,
you will see that the B-  gets shorted to the chassis FIRST, then the B+ gets
shorted to the chassis SECOND.  Drake used a spring loaded delrin rod assy,
with top of rod hitting the top lid of the amp.  The B+ portion of the interlock
is above the chassis, and the B-  is below the chassis.  

##  Done this way, IF the top lid is removed with B+ on, the  B- gets chassis grnded
a split second b4 the B+ gets shorted to the chassis.  Then the B+ is routed directly to the
B-.... via the chassis....and the .82 ohm, 1/2 watt resistor in series with the B+.... inside the
external HV supply  simply blows clean in half.  This results in NO damage to the HV supply,and
no damage to the grid and plate meter.   Now if 6A10s are installed rvs connected, between B- and 
chassis,  you don’t require the fancy staggered  drake rod.  Any simple B+  to chassis safety assy
can be used. 

## I once fixed a ten tec centurion 2 x 3-500Z amp for a local old geezer a few yrs ago.  The HV
safety interlock was a simple offset pendulum piece of aluminum device. Curved 2 inch piece of AL,
with machine screw mounted just to one side of center, so it could freely flop about. 
With lid off, the pendulum swung over and shorted the B+ to  chassis.  When I took the amp apart, 
I could see that carbon marks  on the AL safety device!   The old fella had removed the top lid with the amp   
powered on !!   He was gonna poke about inside to find the problem.  If the safety device was not there, 
he woulda be fried.   He had removed the top lid late at night, while tired.  

##  The problem was a fil to grid short on one of the 3-500Z tubes.  I valued his friendship, and would have been real
upset if he had been electrocuted with 3100 vdc...cuz of no safety interlock. 

Jim   VE7RF  




That "safety" device can easily destroy your PS and I always disable/remove 
them and add a warning label.

If you are that worried about safety then add a cover switch to disable the 
AC line as many other amps do.  Very simple and inexpensive.

Carl
KM1H