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Old 24th September 2020, 03:00 PM
Dave999 Dave999 is offline
Join Date: Dec 2007
Location: Twickenham,Middlesex, GB
Posts: 35
Default KAT6550 query glowing valves

Hello all

just a quick question

my Kat 6550 has always caused the KT88s in the left channel to glow brighter at the top and bottom supports above and below the main Boxed plate section than those in the right. did it with 6550s as well.
can swap valves around and although the degree of brightness changes the ones on the left are always brighter and have been for the life of the amp

and the valves in the left have been the reason to get a new matched quad in the past. left channel started making the odd noise like i'd plugged a 1970s fridge into the same socket.

if i wasn't listening every day.. and every day since march, i'd have the cover off and get out the meter. I've spent a long time looking at the thing over the last 6 months and have a growing concern...

is it warranted?

main caps in power supply done, did that March April, need to do the one near the silicon based rectifier for the heaters...

and i have some balance resistors to go in as suggested elsewhere, but something is obviously a little out of balance.

sounds great and works perfectly

aint broke don't fix
ive been lucky?

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Old 24th September 2020, 05:26 PM
bob orbell bob orbell is offline
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Join Date: Nov 2009
Location: kettering northants.
Posts: 2,298
Default Re: KAT6550 query

Hi Dave, Hmm, all valves should have the same intensity, your problem is most likely C15/17 or C16/18, one pair is right the other is left, you should have about 42v at the pos, side of the capacitors, test between earth post and the capacitor, i would not run the amp until you locate the problem. Bob
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Old 24th September 2020, 05:58 PM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 5,111
Default Re: KAT6550 query

Hi Dave,

Yes, as Bob says, but also check the voltages on G1 Pins5 (should be 0V) in case the 0.22uF coupling caps C11, 12, 13, 14 are leaking electrically and letting through some HT which is turning on the valve.

This is more likely with paper in oil type coupling caps rather than plastic film so change them to plastic if in doubt.
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Old 24th September 2020, 07:06 PM
John Caswell John Caswell is offline
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Join Date: Dec 2005
Location: Wokingham, Berkshire
Posts: 1,668
Default Re: KAT6550 query

It could depend on how the heaters are wired and also with what wire gauge.
If, for instance, you start at the transformer and wire all the valve heaters as a "daisy chain" then the last one will have a lower voltage.
Just out of curiosity measure the heater voltages on the various valves, you may be surprised.
If you want to be pedantic then bring the transformer wires to a tag strip then wire each valve heater to that tag strip.

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Old 28th September 2020, 09:11 AM
Dave999 Dave999 is offline
Join Date: Dec 2007
Location: Twickenham,Middlesex, GB
Posts: 35
Default Re: KAT6550 query

many thanks guys

a good few things to check

the unit was built by WAD and still, apart from the 4 big power supply caps (two above and two below the board) its all still the same parts it was built with...

slight oddity is the left and right channel tag boards have been populated differently... i.e pick two from a box of completed boards and fit to amp, looks like 2 different people soldered up the boards, or a change over in layout right in the middle of my amp.

I will investigate further. no need to worry about this strangeness until i have checked the rest

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