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[Amps] Grid Current Metering FL-2100B

To: Amps@contesting.com
Subject: [Amps] Grid Current Metering FL-2100B
From: VE6TI <ve6ti@telus.net>
Date: Wed, 31 Jan 2007 17:13:24 -0700
List-post: <mailto:amps@contesting.com>
carl / kz5ca and I have exchanged some info regarding 572B's and the  
tendency of the amp to blow the 33 ohm 1 watt resistors in the grid  
bias circuit of the FL- 2100  and the lack of grid metering on that  
particular amp.  I recalled that the Heath SB200 had a similar  
circuit so I downloaded the manual from BAMA and looked at it  
closely.  The two circuits are almost identical.  The grid circuit  
resistors and caps are identical @ 33 ohms and 200p as are the rfc's  
and its shunt resistor @ 3.3K ohms.

Heath meters the grid across the bottom of the RFC and a shunt  
resistor of 1.5 ohms in series with the rest of the bias circuit.  On  
the 2100 the RFC is connected to the bias set resistor (R4 of 30  
ohms  and 10 watts) at its set position which is common to one end of  
this resistor and the antenna relay coil. The opposite end of the  
bias set resistor is switched to ground along with the other side of  
the relay coil when the operate-standby switch is put in the operate  
position.   I inserted grid metering  between the RFC and the bias  
resistor set contact.

I was expecting to see a grid current of 0 ma with the operate- 
standby switch  in the "operate position", exciter set to "on", which  
grounds the FL2100B antenna relay,  but provides no drive from the  
exciter.  Instead the meter indicted 160 ma of current.  I have  
checked this current with two different meters.  One  a 1 ma meter of  
internal resistance  86 ohms shunted with .44 ohms and the other with  
a digital multi-meter.  The latter showed 158.5 ma.  I wonder where  
this current is coming from?

Suspecting something else was wrong I recorded the following voltages:
Plate voltage:  2300 volts.
Grid RFC to ground (operate-standby switch in operate) with exciter  
on standby:   -18.4 volts.
Grid RFC to ground (operate-standby switch in operate) with exciter  
on send, but not keyed:  -2.7 volts.
Cathode to ground, measured at the centre tap on the transformer,   
Zero volts with the antenna relay not grounded and 0.007 volts when  
grounded.  I mention this because I have installed a relay to lift  
the CT ground on the heater transformer, as per W8JI, Tom's advice on  
his website regarding the FL-2100B's poor bias design, and was  
concerned that maybe this ended up floating the cathode above ground.

Again, my question, where is this current coming from?

VE6TI
gordon Hungerford
ve6ti@telus.net



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