[Amps] GS35B Amp 15 m Situation

[Mike]

Hello Tony,

Thanks for the suggestions on where to look.

I haven't monitored the drive signal.  The IC746 does have an 
overshoot on the leading edge as its internal ALC catches up.  The 
IC746, as observed on its display, does not exhibit the same hesitation.

The cut-off bias resistor (10K) is shorted by a relay contact on transmit.

The tuned pi input circuit is similar to the one you describe on your 
web site, Tony.  A separate input circuit is used for each band.

The amp does not have an ALC circuit.

Additional bypass capacitors will be added to the cathode line going 
to the bias board.  The same cable was used when the tune input 
circuit was adjusted, but I'll try a different length of cable in 
case something has changed.

You're probably right about this being some kind of feedback 
problem.  Just have to find the path and eliminate it.

Thanks again for your suggestions and your web site.

At 11:55 PM 7/2/2006, Tony King - W4ZT wrote:
>Mike,
>
>Have you monitored the drive signal at the same time? Does the drive
>signal come up immediately? Are you using a dynamic bias circuit? How do
>you switch your bias? Are you using a tuned input circuit? Are you using
>ALC?
>
>There may be a problem with RF getting back into your driver. Do you
>have everything bypassed well? Since it is frequency dependent, I would
>suspect that cable lengths may be causing an RF feedback problem either
>due to no tuned input or needing more bypassing.
>
>73, Tony W4ZT
>
>Mike wrote:
> > Hello,
> >
> > Having an issue with a homemade GS35B operationon 15 and 17 m.  When
> > keyed, the power seems to hesitate before going up to 1.5 KW
> > output.  CW is a mess on these bands due to this situation.  On RTTY
> > the power hesitation is so short that it doesn't present a
> > problem.  Amp. seems to be okay to about 1200 W, but the hesitation
> > returns above this level.  Other bands, 10, 20, 40, 80, 160 work okay.
> >
> > Using a TIP147 transistor in a bias circuit similar to Ian's Triode
> > Board bias circuit.  Cut-off bias is provided by a 10 KOhm resistor.
> >
> > The filament choke is ~25 bifilar turns of #16 on a 10"X1"X1/4"
> > oblong ferrite rod.  An additional choke of 20 turns #20 on a ferrite
> > rod, 3.5" long and 3/8" dia., is in the line from the filament
> > secondary center tap to the bias circuit.
> >
> > This a strange situation as seen on 17 and 15 meters using either an
> > antenna or a dummy load.   On the other bands, the power smoothly
> > rises to full output.
> >
> > Any ideas on how to fix this situation are appreciated.
> >
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         73,
         Mike, K4GMH