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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
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stereo5 OP
6,464 posts
 
Rogue initially had EL34 tubes in the amps and eventually they changed to the KT120's. Nick at Rogue Audio told me they will work fine and to set the bias at 30ma. That's all I know.
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almarg
9,670 posts
 
09-06-15: Stereo5
Rogue initially had EL34 tubes in the amps and eventually they changed to the KT120's.
I believe the EL34 was used in the Cronus, rated at 55W/channel, but as far as I know never in the Cronus Magnum or Cronus Magnum II, which were/are rated at considerably higher power levels.

If you happen to have a multimeter, after the amp has been off for a good while, say an hour or more, remove one of the power tubes and see if there is continuity (0 ohms resistance, or very close to it) between pins 1 and 8 of the socket. Those are the two pins closest to the orientation notch on the socket, one of them on one side of the notch, and the other on the other side of the notch.

Regards,
-- Al
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stereo5 OP
6,464 posts
 
Almarg

Are you talking the KT120's that are in there now or the EL34B tubes which I removed already? Do you think the EL34's are toast? I should have listened to Uncle Kevin at Upscale, he advised against it.
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almarg
9,670 posts
 
Note that I referred to measuring between pins 1 and 8 of the socket, not any of the tubes. If there is no continuity between those pins on the socket, per my earlier comments it would most likely be a good indication that the amp is not designed to accept EL34s or EL34Bs. Although if there is continuity it would not necessarily mean that they can be used in the amp.

Without a schematic or other specific information on the design of the amp I have no idea as to whether or not the EL34B's may have been damaged. If I were to hazard a guess, though, just based on instinct and on what I know about the tubes, I would guess that they're probably ok.

Good luck. Regards,
-- Al
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mofimadness
5,012 posts
 
"I should have listened to Uncle Kevin at Upscale, he advised against it."

I have known Kevin for almost 30 years and have bought thousands of dollars worth of tubes from him. I have never had a bad one and he has never steered me wrong with his recommendations.

If he advised against it, you should have listened. He knows more about tubes than anyone I've ever meet.
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