KiT88 - Red Hot Valve

[Fred Shaw]

Hi All

Last night my KiT 88 amp (with Svetlana 6550C valves) started to crackle loudly and intermittently from the right speaker and valve No 1 became very microphonic. As the loud crackle produced a very large base diaphragm excursion I switched off the amp while I thought about the problem Today on switch-on, I got the crackling (less severe) but not the microphony. To see whether it was valve No 1 that was causing the problem I switched it with valve No 2. Sure enough the crackle moved to the left speaker. After a while, when everything had warmed-up and the crackle had virtually disappeared, I put on an LP. After about a quarter of an hour I heard a pronounced hum from the right speaker (or was it both but I was closer to the right speaker?) and saw that the plate of valve No 2 (the original valve No 1) was glowing dull red. I immediately switched off before anything worse happened It seems that the original valve No 1 has developed a fault, but why the right speaker should hum I don’t know. I obviously(?) need to replace valve No 1. As I have 3 spare, part-worn, Tesla JJ KT88s I was wondering whether I could use two 6550s, in positions 1 & 2 say, and 2 KT88s in positions 3 & 4, i.e. split the two types of valve so that there is one of each per channel? I realise the valves will not be matched but will that be the worst of the problems?

I am assuming that it is a valve that is on the way out but could it be anything else that is causing the problem? Any help will be greatly appreciated.

Fred.