Blue glow and power drop??? Maybe bias?

[Twangbanger]

Last night I finaly put my 52/53 tweed pro together and played it for a while(weird thing is that I put it on a test setup about a week ago and nothing happened). Well after about 10-15 minutes of playing, the ouput dropped way too much. I checked the tubes and the left one was glowing bright with the blue hue around it. The plates weren't glowing, so I think the tubes are still good. I did switch the tubes and the blue hue followed the swap, but then the tube on the left(wasn't glowing before) began to have the blue tint. The only thing that I didn't wire to spec was the cathode bias resistor. Schem calls for a 250ohm 10watt and all I could find was a 220ohm 10watt. Is this the culprit.

Also, when it comes to cathode bias on 6L6's with about 405volts on the plates, is 58-65ma per tube too much. And do I need to raise the resistance to the spec to drop it back down to the 40-50ma range.

[Twangbanger]

ok, I'm checking the bias on the tubes. Well one socket is running a little hotter then the other. I swapped a set of known matched tubes, the current difference on the 2 sockets is about 5-6ma which is way off. Also I measured a few of the carbon comp resistors and ones that are 270k are actually 330k.

[Twangbanger]

This amp has the ole' 6sc7's in it and I have some on order. Of the originals, 1, is good. I swapped the good preamp tube with the PI tube and the bias is closer now. Another thing, my known matched GT6L6GE's are not matched. I have 3 sets of 6L6's so I've been swapping and testing each set and each socket. I have a set of russian 6l6's that are running in the 63-64ma range at idle, some 6L6's that say made in the USA that look similair to the russians@ 67ma, and the GT's that are out of spec and running 59ma and 64ma on the 2 tubes. Do I up the cathode resitor to drop the current or do I lower it? Or is that current ok for cathode biased tweeds? What was happening was the amp was fine, then it dropped power significantly.

[Stan Martin]

I think you have a few problems here.

I'm assuming the 6l6 tubes you have are the 30 watt ones, if so, 51ma would be about 70% dissipation point.
If these were JJ6l6's or Svets I think they could take the higher current.
If your resistors are supposed to be 270k and are reading 330k I would suggest replacing them pronto.

As far as the tube being off by 5ma..well I wouldn't be too concerned with that.
Your power dropping sounds like a thermal problem.
Cold solder joint? Bad resistors(yup) also how are you filter caps doing? Are they more than 10years old? The dropping resistors next to the filter caps should be checked too.

[Twangbanger]

I replaced the caps and power supply resistors. Also I set the bias to 55-57ma with a 280ohm resistor combination. The tubes that caused the problems are the GT6L6ge's that are rated 7, which means they draw more current. I noticed before the amp tried to die, that the low end response was getting bad, low "e" didn't resonate like it should. I stuck the Russian made 6L6's in there and it sounds great now. Lots of low end!