[Amps] How to tune an FL-2100B

[R L Measures]

On Jul 4, 2006, at 5:08 AM, Tom W8JI wrote:

>> Anyhoo....the second time I used it, the bandswitch arced
>> and
>> burnt to a crisp! Maybe I'm not tuning it up properly?
>
> Maybe. Maybe not.
>
> Since the PEP for normal 100% sine-wave modulated AM is 4
> times the carrier power, and the FT101 can distort the audio
> and make it eight times or more the carrier, it easily could
> be you tuned wrong.
>
> You actually should load that FL2100 at full Yeasu drive,
> but that would be too much for the little FL2100 for very
> much time. The FL2100 is a terrible AM linear for the FT101.
>
> PEP from the FT101EX could be as much as 150 watts. That
> means the FL2100 needs to be fully loaded at 150 watts of CW
> carrier drive, and that is just too much.  You'd need an
> attenuator pad between the amp and rig or really watch your
> control settings.

Another approach is to add an RF-NFB (neg, feedback) R in series with  
the cathode coupling cap.
>
>> My desire is to use it on 40 meter AM.....yep! AM!!! I
>> want
>> to run about 150-175 watts of carrier, driving the amp
>> with an
>> FT-101EX through a Dentron MT-3000A to a W9INN dipole
>> (which
>> is resonant on 7290).
>
> Let's see if your numbers make sense.
>
> 175 watts of carrier. To be linear, the FL2100 would
> typically have less than 25% carrier efficiency. That means
> tube dissipation under carrier conditions would be 525
> watts. You'd have:
>
> 700 watts dc input
> 175 watts carrier output
> 525 watts heat in anodes capable of dissipating 320 watts
> 1400 watts PEP with 100% sine wave modulation
> 700 watts PEP output with 100% sine wave modulation

OTOH, Tom, an FL-2100B will do 600w PEP with voice modulation, and  
that might be enough to do 150w of carrier.
>
> That amplifier is far too small for the power level you
> expect on **clean** AM operation. It would have to be tuned
> at 1400 watts of plate input power or sligthtly more. That
> would be 600 watts or so dissipation out of 320 watts
> available.
>
> Now you see why people never ran many linears with AM...at
> least until CB came along.
>
>> How do I do this without burning up the bandswitch again,
>> assuming it was my fault in the first place???
>
> Load the amp heavy enough to handle peaks without saturating
> or over-dissipating the tubes. To do that safely, you will
> need to add two more tubes.
>
> 73 Tom

R L MEASURES, AG6K. 805-386-3734
r at somis.org