WZ-68 blew up upon start up & lots of noise from right channel

Cool,And Thank you Bob, Do you have advice on the best place to get them ? Thank you for the information

HI AGAIN , I was running my VTA ST-120 for several hours yesterday and my variac blew a fuse and so the st-120 went silent . I found the variac problem and when i started the st-120 I still had no sound but I do have bias and all tubes lit . I guess I will have to take it back to the shop again .I was not running it loud because it was late and I don't need to run it loud .ANY IDEAS ? thx Scott

Bob Latino wrote:Hi Scott,

You are off a decimal point on your bias setting. Whether it is a KT88 or a KT120, the recommended bias setting is .550 VDC per each output tube (550 millivolts). This will correspond to 55 milliamps of bias current per each output tube.

Bob

One thing I've noticed with the KT-120s is that at a given set of voltages they tend to be "hotter" (pass more current) than the KT-88. I test/match KT-88s and KT-120s at 465 volts on the plate/screen and -50 volts on the control grid. The range of cathode current I see on KT-88s is about 60 ma to about 125 ma. The KT-120 under the same conditions seem to range from about 80 ma to about 140 ma. So careful bias adjustment is important - and I suspect it is possible to have a set of KT-120s that you can't get to bias down to 55 ma (.55 VDC in your meter), so if I know the tubes are going into a ST-120 I try to select tubes that bias as close to the KT-88 range as I can.

That may not be relevant in this case, but I figured the more people knew the better off they were.

Oh, and Bob - thank you for your VERY kind words! I really appreciate it.