[Amps] Yaesu FL2000B amp and input tuning problems

[Vic Rosenthal 4X6GP/K2VCO]

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