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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
lewm
12,470 posts
 
How do you know the original tube with the low bias current was actually defective or aged really? Did you check the bias voltage on the grid of that tube? It could be a problem upstream, affecting grid voltage. In which case a new tube inserted in that socket would display an analogous difference in bias current compared to other new tubes in the other sockets. 
jchiappinelli's avatar
jchiappinelli OP
116 posts
 
Lewm8

i only surmised that the original tube was bad because it was running at about half temperature than all the others. On the Six Pacs, you can’t check or set individual tube bias. Bias is set for the block of six tubes. When I put the new set in, I was able to set bias to spec which held rock steady and the tubes all displayed the same operating temperature. SQ also returned so it had to be the tube I suspected was bad.
j.chip
jjss49's avatar
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jjss49
6,609 posts
 
@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
atmasphere's avatar
atmasphere
11,588 posts
 
You *always* set the bias to the manufacturer's recommended setting. If you run increased bias, this could put the power transformer in the amp at risk. Hopefully not of course, but its not worth the risk!
oldhvymec
5,101 posts
 
39.00 usd Handheld automotive/HD thermal gun. -50 to 600 degrees Fahrenheit. It will also tell you when it's Filter cap time.
Cary, Mac, VTL, and Marantz, the transformers ALL run extremely HOT when the filter caps start to take a dump. A pyrometer is good, it TOUCH the object that's being tested, they (when calibrated) have a much higher and lower reading. The accuracy is greater than a handheld thermal scanner also, (hardwired to the pickup sensor)

It (hand held) will pick up the difference in heat, in a wall outlet though, loose connections, or failing switches, nice tool for the ol pouch, for sure.

A new Maxi Tube tester was just finnish and shipped. My 533, and 539 are getting OLD, still great for small valves, but to get a good read on pairs or quads, of power valves.  The Maxi is suppose to be the cats meow.. We'll see.

He has one for signal valves but is behind on receiving logic boards. Out of Washington area, also has a burn in grid, 16 valves at a time. MAXI is the company..

Can buy in bulk valves out of Turkey, and the Russian region.  Not so much with Chinese valves. They (Chinese valves) seem to be bought by the Turks and Others and they sell the bulk Chinese valves, to silly westen suckers, like ME... Can't get a break.. :-) 1-5 per valve in bulk.
BUT you have to check them, 3% have issues, after that 24 hours they usually give great service life, 72 hour burn in is VERY rare, BUT the valves is just bullet proof until the grave...

Regards
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