It might be interesting to see what was said in the 1938 Radio Handbook:
"Parasitics are undesireable oscillations either of very high or very low
frequencies which occur in radio-frequency amplifiers.
...
A familiar type of unwanted oscillation often occurs in shunt-fed circuits
in which the grid and plate chokes resonate, coupled through the tube's
interelectrode capacity. It can also happen with series feed. This
oscillation is generally at a lower frequency than the desired one ... One
cure is to change the type of feed in either the grid or plate circuit or to
eliminate one choke. Another is to use much less inductance in the grid
choke than in the plate choke ...
Parallel Tubes - A very high frequency inter-tube oscillation often occurs
when tubes are operated in parallel. ... "
Note: Years ago, I had an amplifier which had a burned bandswitch. I fixed
the bandswitch and fired it up. Same problem. I then decided it was
parasitics, so applied all of Rich Measures "cures". I then fired up the
amplifier and had exactly the same fireworks.
I then decided to understand the circuit and use my own knowledge and
understanding of this particular circuit. I found it quite refreshing to
think for myself and use my own reasoning to solve the problem. I knew that
it was rf voltage causing the problem. Once I decided it was not
parasitics, I began to think. Eventually, I found that the L network was
incorrect, allowing a high voltage to be built up because the energy was not
being transferred to the load.
In order to solve my problem, I used some fairly simple instruments.
Placing a resistive load at the plate, I could use one of the antenna
impedance meters to see if the transform ratio was correct.
For me, assuming the problem I had was a parasitic problem caused a lot of
wasted time. I had repeated damage to my bandswitch, although I was able to
fix it.
I do have objections to the approach that almost all amplifier problems are
parasitic related. No doubt parasitics can cause some problems. Indeed, I
understand some of the tubes replacing the Eimac 3-500Z may have higher
gain, causing a marginal amplifier to have parasitics.
It is difficult for me to understand how an amplifier that has been working
faithfully for 40 years suddenly becomes a poor design because there is a
failure. And, it is also difficult to understand that no matter what the
symptoms, the cause and cure is always the same.
Frankly, I enjoy thinking and solving problems. If all there was to working
on amplifiers was to install a special parasitic suppressor, I would
probably lose interest in repairing them.
Colin K7FM
---
Outgoing mail is certified Virus Free.
Checked by AVG anti-virus system (http://www.grisoft.com).
To: <amps@contesting.com>
Version: 6.0.459 / Virus Database: 258 - Release Date: 2/25/03
|