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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
stereo5's avatar
stereo5 OP
2,766 posts
 
The range on the bias meter is 0 to 100ma.

The KT120 tubes use 35 ma and the EL34 tubes use 30ma.
joeinid's avatar
joeinid
476 posts
 
What speakers are you using? Power with the EL34 will be reduced from the KT120. Could you set the bias to 30ma using the KT120 then pop in the EL34? Do you have a spare set of more efficient speakers to try?
analogluvr
800 posts
 
So u got a reading fine with the 120s?
dweller's avatar
dweller
2,152 posts
 
Another thought:
Does the Rogue have a pentode/triode switch?
It may be set to the wrong one for EL34s.

Also, Have you tried to bias from a cold start?
It says you biased after letting the tubes warm up for half-an-hour.
I used to bias my Almarro 318 at turn-on then check after 30 minutes (of playing music).
stereo5's avatar
stereo5 OP
2,766 posts
 
09-05-15: Analogluvr
So u got a reading fine with the 120s?

Yup, everything is fine with the KT120 tubes back in.
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