[AMPS] parasitics

Peter Chadwick Peter_Chadwick@mitel.com
Wed, 20 May 1998 18:16:48 +0100


We seem to be getting multiple threads again.

There are three, I think, and muddling them up causes misunderstanding.

1. Are parasitic suppressors of nichrome more effective than those of
copper?

2. Do parasitics lead to high enough currents to bend filaments?

3. Can parasitics boil gold off grids?


I'd suggest that Rich has presented more than enough evidence to suggest
that the answer to 3 is YES.

To 2, I am still totally unconvinced that you can get enough current from
the space charge and the filament in 3-500Z to do this. So I'm a flash arc
believer.

To 1, I'd say  'It all depends'.

My idea of the parasitic problem is that the lead inductance (including that
of the tune capacitor) resonates with the stray capacitance to give an anode
tank circuit at VHF, which resonance is at some frequency where the feedback
is enough to cause oscillation. Increasing the amount of stray inductance
has been known to stop the parasitic by moving the  resonant point of the
'stray' tank circuit to the right point in relationship to the self
neutralising frequency of the tube. I suspect that's why some amplifiers get
away without a parasitic suppressor - until the tube is changed.

By adding the usual shunt parasitic suppressor, the amount of inductive
reactance in the parasitic tank is increased. This, by virtue of a parallel
- series transform, puts a resistor in series with the parasitic resonator
and damps its Q, which reduces the load impedance (and thus the gain at the
parasitic frequency) seen by the anode.

Given this, the portion of the parasitic tank circuit made up of the
suppressor is dependent upon various factors, such as component size, lead
length, etc. A series resistor of the right value would do the same job, but
would dissipate power due to the fundamental current. A shunt L of suitable
size to leave the series resistance at VHF, but bypasses the resistor at LF,
reduces the dissipation in the resisitor.

The problem comes when the value of the resistor required to damp the VHF
circuit becomes so large that the volts appearing across the shunt inductor
is also large, and the losses go up.

After all this, I don't disagree that a good suppressor can be made with
nichrome tape. I don't say that it's the only way, though. The measurements
of Q, Rp etc don't tell us too much, because the tube sees the total VHF
tank circuit, not just the suppressor.

The flames are expected...................


By the way, has anybody experienced holes in the envelopes of 3-500Z caused
by parasitics? I've seen it in other tubes, where a component close to the
tube has enough capacity to allow a VHF current at the parasitic frequency
to flow from the anode to the component through a localised area of the
envelope: the dielectric losses are enough to heat the glass, and a pinhole
appears. Seen it in 6146s and 572s.


73

Peter G3RZP

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