KAT 6550 problems

[gerrygwm]

Hello. I am having trouble with a KAt 6550 power amp. I built it years ago and used it for many years with no problems.
Then it starts to overheat V8 only. That is the 6550 nearest the back of the amp.
The problem is only on the left hand channel. The amp was retired for a few years and now I am trying to resurrect it.
Tried swapping valves but always the same one getting red hot.
I can run the amp for about 3 minutes then have to turn it off because it looks like the valve is about to blow.
The amp runs fine with V8 removed. Checking voltages I can’t find anything wrong.
Anyone got any ideas?
Or better still is there anybody out there who nows more about it than me (not difficult!) that would be willing to have a go at fixing it?
Thanks Gerry

[John Caswell]

Hi Gerry,
The fact the valve is overheating points to several things
So....
Faulty valve - eliminated you say by swapping them over and the same position gets hot
Leaky coupling capacitor - would cause the valve to overheat, increase cathode volts drastically, positive voltage on g1 (Pin5) - measure again
Short circuit bias capacitor/overheated or wrong value bias resistor - valve will overheat - measure bias resistor and capacitor (out of circuit) or just replace
High Frequency oscillation - tricky, really need a 'scope to see this.
Damaged OPTX - Unlikely but possible - Once again a 'scope needed
Yes I can fix it no problem but am a fair distance from you so carriage would be necessary. Cannot really offer for you to bring it to me to try to fix on the spot as I am on of the "at risk ie 79 yo" persons so am steering well clear of any contact at present, sorry.

Try these things then get back here

John

[gerrygwm]

Many thanks John.
I will try your suggestions later and get back to you.
Will be in touch thanks.

[Richard]

Hi Gerry,


you don't say how old the amp is but it came out in 2001 so I'd be inclined to replace all the electrolytic caps, at least C15/16/17/18 and C1/2,

and check the coupling caps C11/12/13/14 are plastic film, not paper in oil type which may leak electrically turning on the valve, as John says.

Check resistors R35/36/37/38 for value, which may be higher than the published 500R, perhaps 560R or even 680R etc as some folk lowered standing current to prolong valve life.

[gerrygwm]

Hi Richard.
Many thanks. I check all this out this afternoon and get back to you.
I can`t remember exactly when I built the amp but it would have been
about 2004.
Its had a lot of use. It was used daily for about 8 years with the only maintenance being the fitment of a new set of valves at some point.
Thanks for your help, Gerry

[gerrygwm]

Hello John and Richard.

With your help I think I have found the problem.
The electrolytic bias cap c16 for valve 8 is short circuit.
Would this cause the problem?
I have some spare caps I could try but unfortunately the four ceramic resistors
R35,36,37 and 38 have fallen to bits as I was trying to remove them. (they are 560R 7W incidentally) One of the resistors on the right hand channel was open circuit so pin 8 of the 6550 was at about 89v instead of 42V
Unfortunately I do not have any spare resistors that I could fit.
Would you know of anywhere that would sell and send me some in these troubled times?

Again, many thanks for your help.
Gerry