[Amps] Henry 3K Premier 12 meter arcing problem

[John Becker]

I'm having a serious problem with arcing in the tank circuit of the 
Henry 3K Premier which I bought about three years ago. This problem 
occurs only on 12 meters, and it is getting worse. It used to occur only 
with the HV switch in the SSB position. Now it happens in the CW 
position as well, if I try to get more than about 400 watts out. The amp 
has essentially become useless on 12, not a good thing with all the 
recent great band openings!

I think I read somewhere that when a rotary inductor is used at the low 
inductance end of its range it acts as an autotransformer, inducing high 
voltage across the large unterminated portion. I wonder if that is 
exactly what is happening here?

I've learned that there was a late production change on this model which 
shorted out a portion of the rotary inductor when the bandswitch is in 
the 10 meter position. You can see a picture of this mechanism here: 
http://www.k9mm.com/files/3k.jpg

I've been in touch with Henry, and they confirm that this change was 
made to improve stability and efficiency of the 3K Premier on high 
frequencies. They also told me that the parts for the modification are 
no longer available.

Since I don't operate 160 at all, I'm considering putting a permanent 
short across the part of the rotary inductor which is beyond the 80 
meter setting. Henry says they have never tried this. They activated the 
short only on 12 and 10 meters.

I'm also wondering if this is a parasitic problem which might be helped 
by one of Rich Measures' parasitic suppressor kits. I have one of these 
which the original owner of this amp had purchased but never installed, 
and so far I haven't installed it either.

I would greatly appreciate suggestions on how I should best proceed to 
correct this problem. Other than this one problem, I really like the 
amp. Sorry for the cross-posting but I'm getting desperate. Thanks!

73,

John, K9MM