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?
From the manual:


Tube will not bias:

If the meter provides a reading but the tube will not bias, the tube is probably bad. If no meter reading can be attained, check the fuse (1/4 Amp slow blow).

If the fuse is bad, replace the fuse and then set the bias.

If the bias will not maintain itself and the fuse blows again, the tube should be replaced.

Bias Meter reading is too high:

Check that you do not have two of the bias switches
turned on.

Output sounds distorted - Check the bias on the output tubes to ensure that the tubes are operating properly
I went through all of that. It was distorted sound and no reading at all on the bias meter. The kt-120's I put back in biased fine after that.
Just a thought...
Do the EL34s use the same voltage range (on the bias meter) as the KT-120s?
The range on the bias meter is 0 to 100ma.

The KT120 tubes use 35 ma and the EL34 tubes use 30ma.
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?
So u got a reading fine with the 120s?
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).
09-05-15: Analogluvr
So u got a reading fine with the 120s?

Yup, everything is fine with the KT120 tubes back in.
From similar experience I think it's a bad tube.
Call Kevin and get your money back.
Can anyone please tell me the difference between the EL34B and the regular EL34? I'm curious.....
Joe - From thetubestore.com's page for the Tung-Sol EL34B - "When driven into distortion it starts off a bit crunchy like a 6L6 but as you push it, the sound blooms into that singing EL34 tone with great sustain and modest compression." From the manufacturer: "The Tungsol EL34-B is a higher impedance EL34 tube design with increased touch responsiveness and a unique clipping characteristic with a hint of the 6L6 sound." Hope this helps!
Further comments found on EL34 vs. EL34B comparison: "The Tung-Sol EL34B's have tremendous bass presence compared to the stock EL34. They were noticeably different in the way they sounded. The Tung-Sol EL34'B had better, fuller, brighter, and all around warmer tone. At low volume they sound similar to 6L6 tubes but once you crank them up you get that gooey EL34 tone but with more saturation, presence, and volume. I definitely recommend these tubes. A couple days ago I switched out my EL34B"S and put the stock tubes back in. The sound was subpar and flimsily compared to the EL34'B and very noticeable difference."

Also from thetubestore.com - "They are a direct replacement. They use the plate voltage supplied by the amplifier. They are a slightly higher impedance tube which means they will bias differently that a regular EL34 (but still within the range of the bias pots). They will not damage your preamp tubes.


The Tungsol EL34B is likely to become the #1 choice of currently produced EL34 tubes. The build construction is great all around and the move to welded plates will likely keep “EL34 rattle” to a minimum over time. The manufacturer claims that this tube has a hint of 6L6 in its tone thanks to manipulating the plate impedance. This is absolutely true, but the effect is most notable when playing clean and to a lesser extent when the tube is driven hard. At lower volume settings you hear a very pure balanced tone with a bit of the 6L6 sparkle in the higher frequencies while providing a midrange that is a little less prominent than most EL34 tubes. The tubes were very open and airy. The bottom end was percussive in the 2x12 open back combo. When driven into distortion it starts off a bit crunchy like a 6L6 but as you push it, the sound blooms into that singing EL34 tone with great sustain and modest compression. I love these tubes. Over the years I’ve tested many tubes but this is the first time I’ve had to tell myself to stop playing and get on to the next set!"
Sounds like these tubes are preferred in the pro audio setting, to be utilized in a guitar amp! There the ability to fall into distortion quicker would be a value. The exact opposite of what we want in the audio world.
As a guitar player, that's the kind of tube I would search out for my amp, something that can add some distortion and tone without me having to push it too hard. May explain the OP's findings in his system!
May explain the OP's findings in his system!

And you know this how??
Have you been provided with a definitive indication, from the manufacturer or elsewhere, that EL34's/EL34B's can be used in your amp? And by someone who seemed to know what he or she is talking about? The EL34's/EL34B's differ dramatically from KT120's in several respects, although I am aware that some other amps, such as some of the PrimaLuna models, are designed such that they can accept either tube.

In looking at this datasheet for the KT120, I see that the suppressor grid is connected internally to the cathode, with both being connected to pin 8. And pin 1 is connected to nothing internally. While on the EL34/EL34B the suppressor grid is just connected to pin 1. So if the design does not specifically address that difference, probably by externally connecting pins 1 and 8 to each other, the tube won't operate properly.

Also, the EL34 has a plate resistance of 15,000 ohms, and based on the info Bill provided above the EL34B is apparently a bit higher than that. While the KT120 is indicated in the datasheet I linked to as having a plate resistance of only 3000 ohms.

Also, the amp is rated to provide 100 watts/channel, which is far more than can be expected from a push-pull pair of EL34's, and I believe also from a push-pull pair of EL34B's. Which would seem to reinforce doubt that the amp has been designed such that it can accommodate those tubes.

There are several other differences that could also be significant, but you get the idea.

Good luck. Regards,
-- Al
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.
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.

-- Al

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.
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
"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.

Thank you so much! The info helped.
Forgive me, but I find this whole thread bizarre, particularly the OP's statement that:

"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." (** = emphasis added by me.)

Based on numerous past experiences with Kevin Deal at Upscale, he always asked me the amp brand I was retubing. If that happened here, I surmise it highly unlikely that Kevin, who knows tubes and amps pretty darn well, would have sold the OP EL34s for a KT-120 tube amp.

And even harder to understand is that the OP wrote that he checked in with Rogue and they even told him how to bias the EL34 tubes, implying that Rogue approved the switch. Weird.

Perhaps Al, Gary (Hifigeek) or one of the other electronics techies can help me out here. I would have thought that an EL34 amp would employ a very different circuit topology than a KT-120 amp. In short, I surmise that the two tube types are not electrically compatible.

Ok, I just embarrassed myself by delving into tech stuff I know knowing about. Go ahead tech guys and slap me down. I deserve it.

Btw, ARC performed the SE upgrade to my Ref 150 several months ago. Kal mentioned in passing that the 6550, KT-120 and KT-150 tubes are electrically similar. Not to say one should just drop a KT-150 tube into an old 6550 tube amp without first checking with the manufacturer. Just making the point that the 3 tubes just mentioned share similar electrical characteristics and properties.

Just caught the 9-6 post from Mofimadness ... well yeah. If Kevin said don't do it, why would the OP go against Kevin's recommendations.

I surmise that if the Rogue amp works with the KT-120s back in place, then everything is probably ok. But I gotta tell ya, if I shoved EL34s into the tube sockets of my Ref 150 SE, I'd have a lingering concern that I damaged the amp.

Very odd thread.
Hi Bruce (Bifwynne),

There are a considerable number of differences between the EL34 and the KT-120, some of which I described in my initial post above. However it is still possible to design an amp such that it can use either tube. As I mentioned, some of the PrimaLuna amps are designed in that manner. One consequence of that versatility being, I believe, that all of those PrimaLuna amps are rated to deliver significantly less power than the 100W rating of the OP's Rogue amp, even with KT-120s.

And as I indicated in my initial post above, there are grounds for suspicion that the OP's Rogue amp is not designed in a manner that would make it possible to use EL34s or EL34Bs in place of the KT-120's it is supplied with. It seems that the person at Rogue to whom the OP spoke may have simply been mistaken.

Best regards,
-- Al
FWIW, I just checked the Rogue Cronus Magnum website. No obvious clues that it could switch-hit KT-120s and EL34s.

Al, I don't doubt that some amps like the Primaluna can switch hit, but as stated, not apparent that the Cronus Magnum can.

Btw, the web site presented the Rogue Cronus Magnum II amp. Maybe the original series I could switch hit.

Btw, btw, am I reading the web site correctly ... the Rogue Cronus can switch between triode and ultra-linear operation?? That's kinda cool.

Btw, btw, btw, I didn't know that Rogue is all USA made in Brodheadsville, PA ... not far from the Poconos. My family and I love the Poconos .. great all-year round place.
The tubes were already on route to my house before I talked to Kevin at Upscale. My only reason for contacting him was because I had received a partial refund for the order. I had ordered the tubes be cryoed and paid the 8.00 per tube. When I got a 32.00 refund, I then called Kevin. During the course of the conversation, I mentioned the Cronus Magnum and he said he wouldn't do it. When I pointed out that Nick at Rogue said it would be ok, he then said "go ahead and try it."
Call Nick at Rogue back or send him an email and tell him what happened. He will help you out and explain what is going on. Rogue has excellent service and is always willing to help owners put, even if the aren't the original owners. They've always been prompt in answering my questions and usually answer the phone when you call.
I already sent Nick an email and stated I would follow up with a phone call tomorrow.
Stereo5...please let us know what you find out. I'm kinda intrigued by the problem.
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.
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.
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.
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.
almost a year ago- but do you still have the El34s in the cronus mag?  I would like to try this also, thanks.  

I sent you a PM.

No, the Cronus Magnum is long gone now.