>DC Filaments
>
>If you run grounded cathode, passive grid with tetrodes, I don't see a
>problem with a cheap switcher for the fils - provided it goes into
>constant current, not foldback, as Rich says.
As I recall, I did not make the point about current-foldback, Peter.
...snip...
>
>Bias adjustment
>
>I'm still having trouble with Rich's idea on biasing. What's wrong with
>setting the bias and screen for the correct levels ...
For Class AB1, the correct level of grid bias V is that which produces
virtually zero grid current with max. drive applied. It's OK to increase
the bias V to decrease the ZSAC, however, it is NOT OK to decrease the
bias V - because doing so causes grid current to flow.
>...and NOT driving into
>grid current? (By suitable adjustment of the exciter) If you can't wind
>back the exciter output, then granted you have a problem, but don't most
>exciters have a power control function?
Why wind back the exciter when there is a workaround? Instead, increase
the bias V to the level needed to stop the flow of grid current.
However, an increase in bias V decreases ZSAC - - which can be restored
to the specified amount by increasing the screen V. By doing so, one
secures the max. power out, which is what running an amplifier is all
about ... ... ... in my opinion.
>If the thing is so badly
>designed that it then then gives power spikes, I guess there's a
>problem, but I don't like Rich's approach.
However, with Rich's approach, no grid current can flow, and, for Class
AB1, that is not an option - - like it or not.
>
>Parasitics.
>
>I'm getting confused. Is Rich saying that if you use a conventional coil
>with a shunt resistor, RF currents can heat the resistor so it goes high
>in value and then the parasitic can start, with whatever results are
>seen, while a nichrome wire parasitic suppressor won't do this?
>
No. The VHF suppressor resistor (Rs) is damaged after VHF regeneration
begins. The outward appearance of a VHF-damaged Rs is typically normal,
but the ohms one measures are not. I have seen resistance increases of
400%, yet with a pristine outward appearance.
- Resistance-wire (nichrome) VHF parasitic suppressors seem to be
helpful, but they are not a sure-cure. I have a TL-922 which uses them -
that makes up its mind to oscillate roughly every five years. However,
since this amplifier has extensive glitch protection, the 3-500Z
filaments don't get bent, and the meters don't blow. A few bucks worth
of parts, and in ten minutes or so, the 922 is ready to go.
- The advantage of resistance-wire suppressors is that, at 100MHz, they
exhibit about 40% less Rp than do otherwise similar, conventional
suppressors made from copper-wire.
[N7WS' measurements, http://www.vcnet.com/measures/Rp_comp.html]
The net result is roughly a 40% reduction in 100MHz voltage
amplification. The technique is nothing new. One of amateur radio's
pioneers, Mr. F. E. Handy knew about resistance-wire VHF-suppressors, and
he wrote about them on page 72 of the 1926 Handbook. ... During the
grate vhf parasitics debate, the naysayers found it quite handy NOT to
discuss Mr. Handy.
Rich---
R. L. Measures, 805-386-3734, AG6K
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