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-   -   WD 88VA, one KT88 red hot (http://www.world-designs.co.uk/forum/showthread.php?t=7830)

MarkL 8th December 2019 10:33 AM

WD 88VA, one KT88 red hot
 
Hello everyone, I have one KT88 glowing red hot in my 88VA. I have had this problem previously with my KAT88, but I can't remember what I did to fix it. (must be something to do with age...)

When the valve glows like this, has it usually taken some other components with it?

Any assistance most welcome!

Mark

Richard 8th December 2019 11:18 AM

Re: WD 88VA, one KT88 red hot
 
Hi Mark,
It may be a valve getting old and drawing too much current but more likely it's an (electrically) leaking coupling cap C5/6 0.47uF 630V. This is particularly likely if they are PIO type as they start to allow leakage of the HT which raises the bias to KT88. Change them all to plastic film type for reliability.
Or, electrically shorting cathode bypass caps C7/8 47uF 100V which are electrolytics and only last so long, if they go short they directly connect the cathode to ground. Change them all if they are more than a few years old.
Check cathode bias is 40V nominal at pin 8 KT88 :thumbsup:

MarkL 8th December 2019 03:17 PM

Re: WD 88VA, one KT88 red hot
 
Thanks Richard, I'll get right on it and report back

MarkL 9th December 2019 08:00 AM

Re: WD 88VA, one KT88 red hot
 
Yes, it was one of the electrolytics and the 470R resistor with it. Will replace all. Many thanks again. Mark

John Caswell 9th December 2019 08:59 AM

Re: WD 88VA, one KT88 red hot
 
Careful,
Usually the elco failing is due to it being overvoltage, generally due to the associated valve going faulty g2 to g1 s/c.
I would suggest a new valve or matched pair this side.

John

MarkL 9th December 2019 10:04 PM

Re: WD 88VA, one KT88 red hot
 
Thanks John, I've got a spare pair I can use. Might be a good excuse to do a little research on a new set....:) And now the bottoms off the amp, maybe a little upgrade of other bits!


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