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.

[John Caswell]

Hi Fred,
First and foremost do not mix valve types. Each side should have either 2 x KT88 or 2 X 6550C.
Secondly it does look very much as though one of your 6550C s has had it, and the best solution is to buy a new matched pair, however if funds are low then you could get away with one, but it will probably be mis-matched to the other so you may get slight hum in that channel. Of course sods law says you will be lucky.
It is also important to to check that the cathode bias resistor has not overheated/fried up and that the cathode bias capacitor has not blown up also. If in any doubt change them.
Apart from that you should check the 0.47µF coupling capacitor to see if it is leaky/short circuit although judging by your description I doubt it.
Any more problems please report herer and one of us will pick it up

Regards

John aka Dr John

[Fred Shaw]

Dr John

Thanks for that. I'll check the amp tomorrow & lete you know what I find.

Fred.

[Fred Shaw]

Hi Dr John, all.

I checked the caps and resistors as you suggested & all are OK, so I obviously caught the failing valve before it had time to take anything else with it. I have also exchanged a pr of the 6550s for the KT88s and the amp sounds fine - the hum on the KT 88 channel down to 0.2mv. Strangely enough, at switch on, the heater in one of the 2 remaining 6550s started to switch itself off after it had heated up. Replacing it with the 'spare' 6550 seems to have solved the problem. Perhaps after 2 1/2 years use the 6550s have reached the end of useful life! It looks like I'm in the market for a quad of either 6550s or KT88s. Anyone have any recommendations as to which types/makes are the best(!) sounding and who to buy from?

Regards,

Fred.

[Global]

Good supplier and nice sound

http://cgi.ebay.co.uk/NEW-TUNG-SOL-6...QQcmdZViewItem

Mark