The symptoms below are consistent with the tube working into too low a
load impedance (loaded too heavily) and not developing enough Voltage
swing on the tube's plate.
Loading comes from reducing the load capacitance...is there a parallel
capacitance on C2 that is supposed to get switched out on your operating
band and maybe it's still in the circuit?
Did the band select knob slip and not point to the correct band?
Too low a plate impedance could come from a non-resonant plate circuit.
The lack of grid current peak when swinging C1 also indicates a
non-resonant plate circuit.
Connect the MFJ259 to the antenna port and hunt around for the frequency
of greatest impedance. You don't need the 2200 ohm resistor because the
test just looks for resonance. Make sure it's in the band of interest
with nominal settings of Tune and Load.
Check the plate choke for some sort of damage that makes it not have a
high impedance in the band.
The SWR bridge has some sensitivity to harmonics, lack of resonance
might increase the harmonic content. Harmonic content would vary with
drive and might produce the effects you see.
jeff, wa1hco
Mike wrote:
> Hello,
>
> Need help in trying to isolate the problem with a homemade GS-35B amp.
>
> The amp. worked okay 6/7 January in the ARRL RTTY Round-Up. Today,
> the symptoms are:
> * SWR varies as the load capacitor (C2) is decreased when
> operating into an antenna (<1.4 : 1 SWR). Into a dummy load, the
> output increases until a value of C2 is reach and then the output is
> steady as C2 is, further, decreased. SWR does not change. (SWR
> meter is a LP-100 Digital Vector RF Wattmeter)
> * The tune capacitor (C1) does not produce a noticeable increase
> in the grid current as its capacitance is increased or decreased
> after C2 reaches the point of minimum capacitance for maximum output.
> * The output is ~900 W for 3500 V at 750 mA. Previously, this
> plate voltage & current would produce ~1400 W output.
> * Half as much drive needed for twice the plate current. The
> input is ~35 W rather than the 80 W previously needed to reach the
> 750 mA plate current.
> * Grid current is only 150 mA for the 750 mA plate
> current. Previously, >250 mA of grid current would flow for 750 mA
> plate current.
> Things tried so far and results:
> * Inspection found no burned or arcing marks on any parts in the amp.
> * Parts and connections are tight.
> * The tank circuit (PiL) was tuned to the previous tune/load
> values when the amp was working okay (~1 : 1 SWR, 50 Ohms) using
> MFJ259 on the amp. output, 2200 Ohm resistor from the tube plate to
> ground, and amp. transfer relay shorted.
> Things to try:
> * Check filament voltage.
> * Take the bandswitch out of the amp. and thoroughly inspect.
> - Look for any corrosion that may be keeping the switch
> contacts from fully making contact (some corrosion is seen on the
> silver plated part of the rotary)
> - Look for any between-the-terminals arc tracks
>
> The other homemade GS-35B amp of the same design but using a
> different type of bandswitch and output transfer relay in the output
> tank circuit is working okay.
>
> Any ideas on where to check will be appreciated.
>
> 73,
> Mike, K4GMH
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>
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