[Amps] damping resistances

Carl km1h at jeremy.mv.com
Sat Jul 19 17:18:02 EDT 2014

But now we are getting far away from the typical ham amp tube where the 
damping resistance is simply part of the parasitic suppressor and that 
resistors true purpose is to stop a parasitic from even forming. The coil is 
self resonant hopefully near the known parasitic frequency and the resistor 
is the load of a very low Q circuit. Ideally it should not absorb any 
parasitic power since it wont even start.

In some amps, such as the SB-220 family, the so called VHF choke RFC-2 is 
self resonant close to the 3-500Z parasitic and invites fireworks. Replacing 
it with a true wirewound "glitch" resistor serves a double purpose....snubs 
the VHF energy on the B+ line and limits discharge current during a tube gas 

With HF and low VHF SS a ferrite bead and 1-2 turns at the input serves as 
the damper. At microwave/millimeterwave a small piece of ferrite sheet is 
placed over the over the input microstrip for the same purpose, plus as a 
brute force impedance matcher. I spent years of R&D lab time getting a 
circuit stable enough that it could be manufactured and production tech 
tuned with a minimum amount of time. There is still a bit of black magic 
involved at those frequencies; Ive been as high as 300 GHz with DoD aviation 

Taming a glass tube on HF or 6M is childs play (-:


----- Original Message ----- 
From: <donroden at hiwaay.net>
To: <amps at contesting.com>
Sent: Saturday, July 19, 2014 4:17 PM
Subject: [Amps] damping resistances

> Quoting John Lyles <jtml at losalamos.com>:
>> In cavity circuits, we can use  damping resistances instead of  parasitic 
>> suppressors
> I installed an RCA 20KW FM transmitter in 1976 that included a couple of
> ceramic insulators that supported a 50 ohm /100 watt film resistor in  the 
> cavity. There were brass dipole elements on each end of the  resistor.
> I looked on the schematic to see which wires connected to the 
> resistor.....
> I couldn't even find the resistor on the schematic.
> There was a brief mention of the resistor and it's function in the 
> instruction manual.
> Don R  W4DNR
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