... something else going on here. One of the KT88s on each amp gone into fireworks mode, first one amp and then a 1/2 hour later the other amp. Line voltage at the time was 124 volts. This happened first thing this morning. The tubes were biased at .7mA each at a line voltage of 121 volts at that time.
Here is the history of these amps since Feb this year:
I installed Shannon Park's Poseidon boards on my MK3s. One of the features of this board is DC balance adjustments - I used 1/4 watt 10 ohm metal film resistors on each socket for individual bias monitoring. The MK3s were originally rebuilt by me back in 2000 including hexfreds to replace the selenium rectifier and new sockets (ceramic with silver plated sockets)as well as the SDS PS cap boards. After the Poseidon installation, I initially ran the amps with a quad of low milage US Tung Sol 6550s without incident. I purchased from Jim McShane 2 matched pairs of the Genelex Gold Lion reissue KT88s and installed them at the end of Feb. Since I am home all day these amps are played typlically 8 hours/day with line voltage between 118 volts and 124 volts (ususally 120-121 volts).
Around mid March, one of the KT88s started to glow bright orange with a nasty hum through the speaker. I turned the amp off and found that one of the bias resistors was open. Which brings me to a question regarding bias resistors acting like a fuse that I will ask later. I emailed Jim on the matter and sent him the pair. Jim replaced the one bad tube free of charge with another and in our email correspondance, suggested that I increase the bias resistor rating to a min of 1/2 watt since he felt that the 1/4 resistors could be stressed when music is played. So I replaced the bias resistors on the one amp to 1/2 watt metal glaze (IRC). I got the pair of KT88s back from Jim and they played flawlessly until now.
A month later, one of the tubes in the other amp glowed bright orange with the nasty hum. This tube had excessive grid leakage and Jim graciously replaced that tube with another free of charge. The bias resistor on the socket of the bad tube measured over 20 ohms so it was about to open. I replaced the bias resistors with 1/2 watt metal glaze IRCs. I got the new tube from Jim and they played flawlessly until now when one of them (the original, not the new tube just replaced) went into fireworks mode just like the other one. Like before, the one bias resistor on the socket of the failed tube measures 22 ohms.
So to recap, 2 months have passed with the amps playing on average 8 hours/day without any issues until today, when a tube on each of the amp went into fireworks mode, a half hour from each other. I will replace the bias resistors, and perhaps I should go with the 3watt metal glaze IRC resistors I have on hand. Of course, I will bench check all voltages but when I did check the voltages before, everything measured fine.
Am I cursed or could there be something else going on? It is quite a coincidence that tubes in both amps would fail the same way within a 1/2 hour of each other, even with a line voltage of 124 volts.
On the matter of the 1/4 watt bias resistors acting as a fuse, if a resistor opens, according to what I read, that tube should "shut down". What does that mean, as in each case when the resistor opened, the tube continued to act up, first with the orange glow and hum and then with the fireworks display. Doesn't seem to me that the whole fuse idea really works. After all, this noise is being played by the speaker attached to the amp and the last thing I need is for my speaker to be damaged. So, what is the expected behavior of the tube when the bias resistor opens?
Jerry