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Old 8th December 2019, 11:18 AM
Richard Richard is offline
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Join Date: Dec 2005
Location: Notts
Posts: 5,203
Default 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
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