[Amps] GLA-1000B Issue

[Adrian]

Have had this amp (working ok last couple of weeks until two days ago)
for a few weeks and have serviced it, 
cleaned all components and changed out to a good set of NOS tubes.

Recently I tuned (962D Tuner on output of amp) my ocf dipole on 80m
(direct swr = 5:1) and then tuned the amp.
The swr seemed to change with amplifier use.

I noticed one tube glowing and changed that out to a good one.

Now on 40m I can tune up ok with a modest am carrier on 962D tuner, and
amp seems to work ok with good gain etc,
but as soon as I change mode to LSB, the swr (reflected power needle) on
952D meter goes up.

I can detune the amp a bit and swr will return to good with sma powerl
gain.
I tried another good quality 2k swr/power meter and does shows the same
thing, so I don't
think the tuner is the issue. I pulled down the ocf antenna and redid
all 4:1 balun internal connections and 
saw no issues with it.. Checked all amp inductors and capacitors (incl
variable ) ok.

What would cause the output impedance of a amplifier to change when
changing  from am to lsb drive
 (obv a power increase on drive with lsb here too)

The driving transceiver works all day at v-low swr, on same setup when
amp is off, ie bypassed.

Still trying to figure this out, and would appreciate input please?

73

Adrian

VK4TUX