Thanks for the info Jim. This amp project has been a fun learning experience. I
appreciate all the help I have gotten along the way from more experienced guys.
73,
Roger
> On Jun 2, 2018, at 12:31 PM, Jim W7RY <jimw7ry@gmail.com> wrote:
>
> "the heatsink is cooler running at 200W output than at 125W output"
>
> This is typical on both VHF and HF amplifiers. Trying to turn down a
> commercial VHF or UHF repeater amplifier (or mobile radio pressed into
> repeater service) down to keep it cooler doesn't usually work. It just gets
> hotter at low power. Karl, AK2O, has had success reducing the DC voltage from
> nominal 13.8 volts down to 10 volts. The PA is much more efficient. A 100
> watt PA can easily be run at 90 watts without over heating when run at 10
> volts.
>
> Same for an HF solid state PA. Much less efficient at lower power that it was
> designed for.
>
>
> Thanks
> 73
> Jim W7RY
>
>
> -----Original Message----- From: Roger Graves
> Sent: Thursday, May 31, 2018 11:19 AM
> To: Manfred Mornhinweg
> Cc: amps@contesting.com
> Subject: Re: [Amps] new 2200m/630m amplifier - distorted waveform output
>
> Manfred, the distorted waveform output (observed before at amp output when
> driving the antenna either with or without a T form LPF) is fixed. Thank you
> for suggesting using a Pi form LPF. When I built and installed a Pi filter,
> the waveform at the output of the filter (antenna matching system input) was,
> as expected, a nice sine wave. The waveform at the input of the filter
> (amplifier output) was also a nice looking sine wave, not visibly different
> than the waveform at the LPF output. This was the case up to 200W into the
> antenna after which the amp output began to show some waveform distortion
> (flattening of the peaks). This was all at 137 kHz - I have not built a Pi
> form LPF for 475 kHz yet. Interestingly, the heatsink is cooler running at
> 200W output than at 125W output.
>
> The turn-on transient was unchanged - no surprise there. I will work on
> trying your other suggestions later.
>
> 73,
>
> Roger
>
>
>
>> On May 28, 2018, at 10:05 AM, Manfred Mornhinweg <manfred@ludens.cl> wrote:
>>
>> Roger,
>>
>>> I will try lower value gate resistors. That will also have the
>>> advantage that I will be able to drive to higher output on 160 I
>>> think. (My 1W drive is not sufficient on 160.)
>>
>> Ops... We have a misunderstanding there. I was talking about gate LOAD
>> resistors - the ones that connect from the gates to RF ground. Now I realize
>> that you are talking about resistors connected in series with the gates.
>>
>> With such big FETs I would use resistors of roughly 10-20 ohm from each gate
>> to ground (through a larguish bypass capacitor), making sure that the bias
>> supply has an impedance not over a few hundred ohm and that it is coupled to
>> the gates in such a way that the bias supply's internal resistance loads the
>> gates fom DC up to beyond the frequency where the reactance of the mentioned
>> bypass caps has become irrelevant. This gives you gate loading at a few
>> hundred ohm from DC to some low frequency, and of 10 to 20 ohm from there on
>> and into the VHF range.
>>
>> The gate series resistance for such a big FET is typically just 1 to 3 ohm.
>> 50 ohm gate series resistance is what I use for 6 watt FETs (like the RD06)
>> in the HF range. At your low frequency the resistance can be higher than at
>> HF, but what you have, for your FETs having huge capacitances, is far too
>> high.
>>
>>> I can substitute braid for the wire I used for the gate and drain
>>> leads for lower inductance.
>>
>> That would be an improvement. Anyway I wonder how much effect it's causing,
>> given that you are running this at relatively high drain impedance and very
>> low frequencies. Still, just to be on the safe side, I would still use RF
>> construction techniques, with wiring lengths approaching zero as much as
>> possible.
>>
>>> Yes, as far as I could tell, the 5023 is just a 5022 with 0.23 Ohm
>>> Rds(on) instead of 0.22.
>>
>> OK. I have a bag full of APT5020, which I got for free. Given their
>> extremely high capacitances, I'm using them in DC applications only!
>> Definitely not at RF. Even in switching applications that need to run at
>> 50kHz or so, I prefer buying more modern, better suited FETs, than using
>> those 5020.
>>
>>> I will try adding the negative feedback if other changes do not
>>> eliminate the transient.
>>
>> OK. It also aids linearity. As soon as you find you have any spare gain, you
>> should use it up in negative feedback, instead of an attenuator.
>>
>>> The APT502X series is obsolete and hard to find. Can you suggest a
>>> possibly better replacement that can work on 48V and provide 200W? I
>>> found the relatively low transconductance and high power dissipation
>>> ratings of the APT units attractive.
>>
>> Have a look at the IXFQ20N50P3, AOK42S60L, IXFQ30N60X, and IXFH26N50P3. They
>> are all current and available at Digikey and other distributors. Each of
>> them replaces the APT5023 with considerable advantages, either in most or in
>> all areas. Compare the output and reverse capacitances of these to the
>> APT5023's ones...
>>
>> Any of these will provide far more gain.
>>
>> All four are rated at 500-600V, which is really a lot of overkill for a 48V
>> supply. But in the optimal range, say, 150-200V, there isn't much to choose
>> from. Those lower voltage, high power FETs have extremely high current and
>> low RDSon ratings, which brings along extreme transconductance and very high
>> capacitances. So it's probably better to stay with the 500-600V FETs. These
>> should also be more resistant to hotspotting in linear operation.
>>
>> > The power output increases
>>> linearly with input up to the 1dB compression point at close to 200W.
>>
>> In fact with 48V and a 3:6 turns output trafo you should be getting closer
>> to 300W at the -1dB point.
>>
>>> I have not used any compensation capacitance across the output
>>> transformer primary. The measured Z of the transformer was better on
>>> 160 with 500pF across it, but I was concerned the 500V silver micas
>>> (two 249 pF in parallel) might not survive. Do you think C across
>>> either the primary or secondary would be beneficial for either the
>>> transient oscillation or the harmonic distortion?
>>
>> Probably not.
>>
>> A properly compensated transformer has a pretty flat response up to a
>> certain frequency, then falls off rapidly. Without compensation it will
>> start falling off at a lower frequency, but less abruptly. I would think
>> that when operating with 2MHz as the top frequency, compensation won't be
>> needed. But it really depends on the details of transformer construction. If
>> you find that your transformer works well on the VLF bands but struggles on
>> 160m, consider adding properly calculated compensation capacitors. The
>> measurable symptom of a transformer that's not able to cope with the
>> frequency is that the secondary voltage no longer is in the correct ratio to
>> the primary voltage (bad coupling factor). In that case one would add the
>> correct amount of compensation capacitance ON BOTH WINDINGS, thus absorbing
>> the unwanted leakage inductance, that causes the poor coupling, into a
>> PI-type low pass section, whose cutoff frequency hopefully ends up above the
>> highest frequency of operation
.
> That's all there is about compensating a transformer. It's not directly
> related to linearity, although the frequency characteristics of the
> transformer do affect the harmonic structure of the signal.
>>
>> Since compensation capacitors will shift existing resonances, stability can
>> change. But it could change for the better or for the worse. Typically for
>> the worse, though, given the fact that adding reactance tendsa to raise the
>> Q of those resonances.
>>
>> Manfred
>>
>> ========================
>> Visit my hobby homepage!
>> http://ludens.cl
>> ========================
>
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