[Heathkit] SB-401 Tune-up problems

[Shawn Duley]

After reviewing all the Tempe and QTH archives for this rig, I need to
bounce some ideas off you all.  I haven't worked on a tube transmitter
for a long time.

I am in the process of final tune-up before testing this rig on the air
(recent purchase).  I have been faithfully following the manual
procedures.  Disconnected screen and plate connections before starting.
Everything was looking good until I was attempting the final coil tweaks
with the finals hooked up.  All coil adjustments were made on the
shallow slope of  each coils voltage curve.  After final tweaking of the
Driver, Grid and Het coils I have the following readings:

On 80, 40, and 20, full power out with normal (about 220-250ma) plate
current and grid indications
On 15, about 70 watts with normal plate and grid indications.
On all bands 28 and above, I get 50 watts, normal plate and NO grid
indications.

On all bands, I get above power output with the level control around 12
o'clock postion.  On the higher bands, extra drive has no effect, but
the level control will reduce power to zero on all bands.

So far, my thinking is weak tubes or improper neutralization.  My rf
probe is homemade using the circuit in the ARRL handbook and it may be
loading down the circuit giving me a false neutral point.  I know it
loaded the coils because there was quite a bit of difference between max
voltage indication with the meter and max relative power when adjusting
the coils at the end of the procedure.

Any and all thoughts would be appreciated before I attempt more
troubleshooting.
TNX, Shawn, N7PRP