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Re: 500W Tube Coil Takes Off!
---------- Forwarded message ----------
Date: Fri, 17 Oct 1997 14:49:49 -0400 (EDT)
From: "Mark S. Rzeszotarski, Ph.D." <msr7-at-po.cwru.edu>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: 500W Tube Coil Takes Off!
Hello Jeff,
Great to hear you are up and running. You said in part:
>Question: The grid resistor, although seemingly overrated, got red hot during
>an extended run. Since I had already adjusted it until I got the correct plate
>current (700ma), I'm guessing that it was seeing too much voltage from the
>grid coil. I started to remove some windings from the grid coil but stopped
>after removing two, I thought I should ask the group about this first.
>
>Observation: Using a 10H filter choke to smooth the dc really killed spark
>output. From 8" to say, 1". Without the choke, the coil has a 60Hz (120Hz)
>hum that I really wanted to filter away. I placed the choke between the
>center tap of the filament transformer and ground. Maybe I should move it
>to between the negative side of the full wave bridge rectifier and ground?
The fact that the choke kills the spark coupled with an overheated
grid resistor suggests to me that you need a larger bypass capacitor on your
grid circuit. The resistor should pass enough D.C. to establish nice class
C operation, and the bypass cap should shunt the A.C. across. Try upping
the value on the bypass cap in the grid circuit and see what happens. You
may have to adjust the resistance, too. I measure grid current, the plate
current and the grid voltage and try to get all three to follow the tube
ratings charts. You may also have to reduce grid drive as you suggested.
Have fun,
Mark S. Rzeszotarski, Ph.D.