The driving radio is an RCI2950. The radio output goes directly into the
input transformer. There is a VSWR of 1.2:1 between the radio and the amp. I
also tried adding various caps on the power lines to no avail. As mentioned
earlier, I have another amp with MRF455's with no problems. The only big
difference I see in the circuits is thatr the good amp has an RC network
between the input transformer and the bases.

Chris
"Fred McKenzie" wrote in message
...
| They operate normally into a dummy
| load. The problems start when connected to an antenna. At low output
levels,
| there are multiple frequencies present. It's not just harmonics.
|
| Chris-
|
| I agree with Allison that you should look at the power bus. You often
find
| designs with two or three parallel filter capacitors with values spread
over
| the range of 100 pf to 10 uf, all with shortest possible leads including
PCB
| tracks. As I recall, this was part of the cure for oscillations in the
| Heathkit 10W Walkie-Talkie amplifier.
|
| Then there is the effect of load impedance (or input impedance on the
previous
| stage) and its relationship to a VSWR protection circuit. With a
resistive
| load, even out-of-band signals are matched, so there is no drive
reduction.
| Therefore you have full power which seems to prevent the other-frequency
| oscillation from occurring. However, a real-world antenna may be matched
at
| the intended frequency, but not at out-of-band frequencies. The VSWR
| protection circuit reduces drive, which allows the spurious oscillation to
| grow, causing more out-of-band signal, reducing drive even further. The
result
| is a complex waveform consisting of intermodulation products of the
intended
| signal and the spurious signal(s). This effect would be more likely to
occur
| where drive reduction is accomplished before the stage that is
oscillating.
|
| Although not a cure, you might try defeating any VSWR protection circuit
in
| either the driver or the amplifier, and see if the problem is reduced.
|
| I found an almost identical problem in the old Regency HR-6 and BTL-301
| tranceivers. The only thing that actually seemed to cure the problem was
to
| short the emitter lead of the final multiplier stage to ground. I noticed
that
| later versions of the BTL-301 moved the VSWR protection drive reduction
from an
| early stage to a later stage. This made significant improvement, but
wasn't a
| perfect cure.
|
| 73, Fred, K4DII
|