A question of bias for new tube set


Something seemed a bit off with my system so i checked the temperatures of the EL34 power tubes on my Cary Audio Six pacs with a pyrometer and found all the tubes reading a temperature around 360 deg F +/-  but one. The errant tube was reading around 175 deg F. Checked the bias on each mono block and found that the set with the errant tube was way off the recommended bias I usually set which is 220 ma. It read 180 ma. The other mono block was operating in spec. I also noticed that with a music source, the bias readings on both mono blocks seemed to swing 10 or so ma with the music. Didn't know if this was normal or not but figured since i had a bad tube and had no idea of the age of the tube set (bought the Six Pacs used), I figured it was time for a full set replacement.

The existing tubes were Svetlana Winged C EL 34s but after some research, I decided to buy a matched set of Electro-Harmonix EL34s from Kevin Deal of Upscale Audio. According to Kevin the EH tubes were less extended in the bottom end but added some sparkle to the upper range. Since they were about half the price of other competitive tubes (I needed 12), came matched and burned in for 72 hours, I figured I'd give them a try.

Swapped them out and set the bias to the Cary recommended 210-230 ma (I chose 220 ma). Noticed a few things. First, the bias with music was now rock steady at the 220 ma setting on each monoblock. And Kevin was spot on in his description of tube sound. However, each of the tubes came marked with specs including bias which were all within a tenth of 41.6 ma. That would equate to a bias setting of 246+ ma on each Six Pac. So my question is, should I stick with the Cary recommended bias of 210-230 ma or crank it to 245 ma?  

Comments appreciated,
J.Chip
128x128jchiappinelli

Showing 2 responses by jjss49

@jchiappinelli 

learned something from you in this thread

getting a pyrometer is a good idea to monitor power tube health, at a minimum it let's you read tube running temps in a multi-tube amp 

most amps do an average bias across at least 2 sometimes 4 tubes, this is a good way to check if one of them is way off the other before more serious issues occur with the tubes

glad you found the bad tube was the culprit, and your SQ is back to normal - makes all the sense in the world

in these days with chinese and russian tubes as the only new ones available, we really need to verify proper operation, not just trust the factory/vendor
i would not exceed target power tube biasing stated by the amp manufacturer

but that is just me

if my car maker says use 91 octane and 5w40 that is what i do - i figure it is their product their engineering their testing they would know best