Over the years there is one tube that I have had more trouble with than any other. Unfortunately, it is also one of my favorites: the EL34. I have tried every EL34/6CA7 out there, and I'm also constantly looking at what I take out of our customers' amps.
My all time favorite is still the Svetlanna (now winged C), specifically from the late 90's into about 2001. After that, there was a production change that made the tube less reliable. I stopped stocking the Svetlanna because they became very expensive and at one point were not available at all. The Svetlanna sounded great and when it failed it did so in a very graceful manner. Which is good for an EL34 because many times they fail in a take no prisoners manner.
As our regular customers know, we have been using the JJ Electronic E34L for quite some time. My main reason was it's reliability. But sadly, in 2009 I had a lot of trouble with the E34L. Switching to the EL34 had some success. What changed? I personally blame international politics. JJ Electronics is in the Slovak Republic, a small Eastern European country that would like to enter the EU. What exactly does that mean for you and your EL34 tubes? Well, for one thing they are now lead and mercury free. Unfortunately, their EL34s are now less reliable.
But in my experience, reliability has always been a problem for the EL34 in guitar amps. I almost hate to say it, but... could it be the amps? Let's think about this for a minute. Two things quickly come to mind: First, Most EL34 amps want to be a Marshall, even if they won't admit it. Second, the first Marshall amps were basically Fender Bassman copies with EL34s. So that means all of our favorite EL34 amps are based on a modified 6L6GC design.
If you enjoy the thrilling excitement of searching through manufacturer's data sheets as must as I do, you might make note of two things about the EL34/6CA7: It is not recommended to be used in a horizontal position, and the maximum screen voltage is 450 Volts, approximately 50% of the maximum plate voltage. These two attributes are not shared with the EL34's 6L6GC brethren. So any amplifier design based around the 6L6GC is going to have a screen supply that will run an EL34 to its absolute maximum.
If there is anything I have learned from movies and television of the 1980's, it's this: If you buy an exotic pet from an old man in a shop full of far east curiosities, do not feed it after midnight and do not get it wet. Ever since the first Marshall we have been breaking the rules, yet somehow we are always surprised when we find gremlins.
The wholly impractical option would be to redesign most EL34 based amplifiers made in the last fifty years, especially the high plate voltage monsters of the late seventies and early eighties. If only there was a tube with big fat EL34 mid-range that didn't mind a high potential screen supply. Well, there is, and it used to be a lot more expensive than an EL34. I wouldn't say the KT77 has come down in price as much, as the EL34 is now more expensive. For that reason I completely overlooked the KT77 until a customer inquired about installing them in his Carvin X100. I had not realized that it was a drop-in replacement for the EL34/6CA7 that will take up to 800 volts on the screen supply.
We are now stocking two options for EL34 based amps. The JJ Electronic KT77 and the Groove Tubes GT-EL34R. Lately they have been the best two tubes to replace EL34s that I have found.