Line6 HD100 (all tube power section) screeches for 3 seconds now and then

[edfardos]

I have a new-to-me Line6 Spider MkII HD100 head which has two issues so far, I was hoping somebody might have heard of these.

The first issue seems to be resolved, but the second issue is new:

 

1) soft static-y noise during note decay on clean channel, notable on open-A

 

pulled center tubes, went away

put in new center tubes, still fine

rebiased all tubes, static is back

ran compressed air through the amp (with all tubes out)

starting with outter tubes, problem was gone

put original inner tubes in, still fine

rebiased all tubes, still fine ---- problem solved at this point

 

a day or two later,

 

2) amp screaches LOUDLY for 2-3 seconds, 3-4 minutes after powerup

 

first time it did it once

second time it did it twice within 10 seconds

screaching/static zipper noise (increases in pitch a bit)

amp works perfectly before and after the screaches

no visible arcing or strange lights in tubes during screach

replaced left 12ax7 tube to see if that changes anything

screech still happened, only 3-4 screaches over 12 second period

removed inner power tubes to see if screech goes away ----- waiting