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What Did I Do Wrong?

I have a Rogue Cronus Magnum integrated with the KT120 tubes. I wanted to try the EL34 sound in my system so I purchased 4 Shuguang EL34B tubes which were graded and matched from Upscale Audio. I checked with Rogue and was told to set the bias at 30ma. I received the tubes today, installed them and let them warm up for 30 minutes. The sound was a little ragged and distorted and try as I may, I couldn't get a reading on the bias meter.

Thinking I wrecked the amp, I reinstalled the KT120 tubes (thank God I marked which socket they came out of) fired the amp up and the sound is again full with no distortion. I checked the bias on the KT120's after 30 minutes and the bias on one tube was off a little.

Why did the EL34 tubes sound distorted and why couldn't I bias them? What did I do wrong? Do you think these tubes are all toast now?
128x128stereo5
mofimadness's avatar
mofimadness
5,139 posts
 
Stereo5...please let us know what you find out. I'm kinda intrigued by the problem.
rleff
1,065 posts
 
This is what is best in the audiogon forum communities; great help and sometimes even a designer or manufacturer will pop in to help as well; Ralph from Atma-Sphere as a example.
stereo5's avatar
stereo5 OP
6,502 posts
 
Talked to Nick at Rogue today and he confirmed the EL34 tubes do work in the Cronus Magnum. He said I may have to turn the bias pots up to a dozen turns to get the tubes to bias.
bifwynne
2,206 posts
 
Well ... that turns out to be a very good answer. At the very least, you now know that the EL34s did not damage the amp .. a very comforting response.

It is curious that the web site didn't mention that the amp could switch hit between the two tube types. As I said, maybe the Cronus II is KT-120 only, but the earlier version is the switch hitter.

In any event, glad to hear the news.
stereo5's avatar
stereo5 OP
6,502 posts
 
I put the EL34 tubes back in today and set the bias. It is now rock steady at 30ma as per Rogue Audio. It seems the EL34 tubes needed a dozen turns each on the bias pots. That did the trick. I never would have thought I would have to turn the bias pots that many turns for this tube.

It is thanks to a fellow Audiogoner that I had the courage to try this again and the results are stellar. I am really liking the EL34 sound.
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