[Heathkit] Blowing fuse in SB401 Help

[OTAKEBI@aol.com]

I have an SB 401 that was working fine last night.
After talking on the net I went to bed.
When I turned rig on this am all OK.
When I went to tune on Dummy load as soon as I turned mode  switch to tune, radio went dead and all lights went out.
Checked fuse and it was bad. I replaced fuse turned level control all the way down and when I turned mode switch to tune all went dead again.
This time I noticed a blue glow very bright coming from the finale cage 6146 that is furthest to the front of rig.
Took radio apart and checked under board for shorts around finale cage. Found none.
Placed both tubes in a good tube tester and the one from the front glowed purple and made a noise like a hum.
The other tube did not do this.
Put a 3 amp fuse in rig with final tubes out Put in a 3 amp fast blow fuse.
Turned on rig all OK.
Turned to tune and all OK.
I have an extra brand new pair of 6146's but I am afraid to put them in for fear that a short has caused the original problem.
Prior to the radio going out today I was only getting about 70 watts out on 10 meters.
I think the tubes were old and getting weak.
Can someone give me an idea as to get this radio up again without putting the new finals at risk?
Dan N4VET