Ron,
It is known that many pushpull transistor amps of the type in the Motorola
app notes designed by Helge Grenberg will oscillate with low pass filters on
both the input and output. The input low pass filter should be redundent if
the output filter is adequate. In any event, if both are used, then one
should be of capacitive input and the other inductive input.
When you replaced the output transistors, did you replace them with a
matched set of approximately the same gain as the originals? If the new
pills have much more gain, then this could contribute. In any event , the
input and output should not have filters of the same input type as this will
almost always cause problems.
This is about all that I can contribute.
Good luck,
Sam, W5LU
>From: Ron <w8ron@stratos.net>
>To: amps@contesting.com
>Subject: [AMPS] New to list.
To: <amps@contesting.com>
>Date: Tue, 15 Jan 2002 15:26:55 -0500
>
>
>Hello to all .
>
>I joined to pose a questionand also to copy mail to see what is new.
>
>I have a solid state push pull Broadbanded RF amp that I have replaced
>the transistors but the unit is taking off at higher power levels. I
>placed a LPF on the input drive to the amp as I would see the 3 harmonic
>at 6 dB below the carrier! THe filter cured that. Looking at the
>signal from an HP analyzer , I see multiple harmonics spaced about every
>2 Mhz below the carrier frequency into a 50 Ohm load. On lower power
>levels, the closest is 20 dB down , but at higher only 10dB On an
>antenna , It takes off and pulls horrendus currents and drags the supply
>voltage down to 9 or 10 volts if I plase it on an antenna with a tuner.
>
>?...Is it normally required to neutralize this amp when replacing the
>finals?
>
>?...What techniques are used to do this?
>---
>Ron
>
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