I don't see why it shouldn't work, as long as you know the correct value
of input impedance. It depends on the class of operation. The tubes must
be in place when you do it, because the grid-cathode capacity becomes
part of the network. You can't do it WITHOUT the resistor, because the
resistor simulates the load presented by the tubes when tuned and loaded
correctly.
In class AB1, when there is zero driving power, you can set up the input
cold without adding a resistor, since all the drive power is absorbed by
the swamping resistor in operation. Then all you have to worry about is
tuning out the effects of the grid-cathode capacity and other strays.
73,
Vic, 4X6GP/K2VCO
Rehovot, Israel
http://www.qsl.net/k2vco/
On 6 Apr 2015 04:59, Alek Petkovic wrote:
G'day Morris.
The method you described is probably more appropriate for setting the
output pi circuit settings.
I don't think you can use that method for the input.
I would have thought that you would be better off leaving the tubes in
situ and leave the resistor out of it and then do the procedure you
described. That should allow you to tune it with all the effects of the
tubes' capacitance/inductance in circuit. Dunno if it would work but it
must surely get you closer than with the resistor.
Good luck and 73,
Alek. VK6APK
On 6/04/2015 9:34 AM, Morris Wideman via Amps wrote:
Hello
I have a old Yaesu FL2000B amp that has been giving me fits and
thought I would see if group has some thoughts that might help out
here. I have had this amp many years and used it with very little
problems even though it has always showed reflected power on the input
side. Back when I used a Drake T4X to drive it no problems with the
reflected. I now have a FT1000mp and its built in ant tuner takes care
of the reflected. About 3 months ago one of the tubes shorted grid to
plate and burned up a grid resistor and trashed the electronic bias
circuit I had put in many years ago. I thought this was opportunity to
work on the reflected problem. I fixed the aforementioned problems and
I have a pair of tubes in that put out resonable power now so I
beleive the tubes are good. That leaves only the amps input ckt
reflected power to deal with.In my searching I found a article
detailing a method using a resistor to simulate the tubes input
impedance and use something like a MFJ 259B fo
r tuning to match with the amp off. I tried this method and used a
external supply to activate the changeover relay. I was able to change
capacitance values to obtain a very good match on those bands that were
bad not all needed this but those that did turned out very good this
way. However when firing up the amp my new match is now worse than
before. The manual for the amp recommends disconnecting the HV leads
from the transformer and tuning the input I also tried this with the
same poor results. Most of my efforts have been spent on 10M as this is
one of the worse bands with at least a 2.1 swr and I noticed that the
plate and load adjustment greatly affects the SWR reading max out is not
the lowest SWR point. I have tested the changeover relay and the band
switch to make sure they are properly making contact and swiching as
they should. I have correct idle current and decent output power.I just
need to get the reflected power down but nothing I have tried seems to
work I w
as hoping someone might have a new suggestion.Thanks for your time73
Morris wa4mit
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