I had a sim problem with a 4x4 p-p o/p setup fitted with kT90. These EI tubes have a reputation of s/c internally without warning. I found all the resistors connected with the tube destroyed or had signifigantly changed value. The problem with true pentode operation is to rescue the tubes from excess screen current.. I was forced to fit a 100R res in each screen leg to find out where and which was causing the problem but for some that may be objectionable.
Often my route with suspect amps is to fit the cheapest FEast 6550/88 clones in lieu for test and shan't be sorry if troubles persist. Billington A grade ain't cheap.
Double check all your voltages esp bias and screen.
richj
Often my route with suspect amps is to fit the cheapest FEast 6550/88 clones in lieu for test and shan't be sorry if troubles persist. Billington A grade ain't cheap.
Double check all your voltages esp bias and screen.
richj
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