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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
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
6,376 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.
avanti1960
1,471 posts
 
almost a year ago- but do you still have the El34s in the cronus mag?  I would like to try this also, thanks.  
stereo5's avatar
stereo5 OP
6,376 posts
 

I sent you a PM.


No, the Cronus Magnum is long gone now.

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