Correct sockets for installing EL34 instead of 6L6GC in 395

[PanamaJack]

I have a Mesa Simul Stereo 395 Switch Track amp (from about 1990). It currently has eight 6L6GC tubes (four per side) and can run in Class A, or Class AB and A combined. Four of the tubes (two per side) can be exchanged for EL34 tubes to get more dirt in the Class A setting. The manual is lacking in which sockets to use. The 290 and 295 manuals talk about using the outer sockets, but I have heard different regarding the 395. Hoping someone has the facts on which sockets are the correct ones.

 

[Bobby Marshall]

I use this power amp but don't change tubes often.

Send me a PM with your email address and I can send you a pdf with info.

 

[PanamaJack]

Thanks for the socket layout. However, it does show the opposite of your previous post. That is, the Class A sockets are at the outside (left and right), as opposed to in the middle or across the rear. And I am still not convinced that the EL34 can be just popped in (at least to good effect). I am hearing different things from different techs at Mesa as well as people that have the amps in service. Would be nice to know if it is a good trick or if the 6L6 tubes are just as good sounding, based on the design.

 

[Bobby Marshall]

I did have it backwards. Guess I was mixed up.

These poweramps were designed to have 2 of the 4 power tubes run in Class A and the other in Class AB. This is what Mesa Simulclass means. The 395 allows you to run only the Class A tubes. Amps like the Mark IV also can mix 6L6 tubes with EL34 tubes by putting the EL34 tubes in the Class A portion of the power section. You can also tell which tubes are in the Class A sockets if you turn the switch in the back to run Class A all the time.

Running all 6L6 tubes has cleaner headroom overall. Mixing will give you more dirt sooner and with the 395 the ability to channel switch and run just the EL34 tubes that are in the Class A section.

I really like the ability to channel switch for a "lead" channel on the power amp. Great feature. I only know of one other power amp that can do this from London Sound. Their rackmount preamp and power amp appear to be extremely flexible.

Good luck with your 395. I love mine.

 

[PanamaJack]

Got through to another MESA tech today. He confirmed the Class A sockets can accept EL34 tubes, but that they won't be putting out their maximum due to the way the amp is wired and biased. However, the problem is most of the EL34 tubes are too tall to fit inside the amp cage! That's good to know. I measured the tubes inside and they are only 3-1/2 inches tall, with less than a half inch clearance to the cage. Mesa said even some of the current 6L6 production may be too tall, touch the cage, and stand a chance of breaking! Looks like short bottle 7581A NOS would work, assuming they are matched up and match the fixed bias of the amp. The trick is that eight of those costs almost as much as the amp's street value!

 

[Bobby Marshall]

Got through to another MESA tech today. He confirmed the Class A sockets can accept EL34 tubes, but that they won't be putting out their maximum due to the way the amp is wired and biased. However, the problem is most of the EL34 tubes are too tall to fit inside the amp cage! That's good to know. I measured the tubes inside and they are only 3-1/2 inches tall, with less than a half inch clearance to the cage. Mesa said even some of the current 6L6 production may be too tall, touch the cage, and stand a chance of breaking! Looks like short bottle 7581A NOS would work, assuming they are matched up and match the fixed bias of the amp. The trick is that eight of those costs almost as much as the amp's street value!

If you are going to load her up with cool NOS power amp tubes you should get her modified so you can adjust the bias. This is supposed to be a simple mod that almost any tech can do.

Tubefreak says "Mesa amps have a fixed bias and run their tubes pretty cold. Although the tubes live longer that way, the soun could be better. I have had my amp modified and installed a bias trimpot for the tubes and run my amp pretty hot now. And it sure makes a difference. The overall sound is bigger and more dynamic. Clean sounds are a lot sweeter and less harsh, higher gain sounds have a nice compression to it and sound bolder. I highly recommend this modification to any Mesa owner as it will improve your tone a lot."

Here is the link to his site- http://www.tubefreak.com/395.htm

 

[PanamaJack]

A fixed bias means the tubes have to be within a tighter spec. Mesa's theory is to use select tubes and eliminate tech quirks that not only cost you time and money, but can screw up the amp's engineering. If I was set up to bias my own amps, I would be more inclined to consider fudging Mesa's schematics, but I have been spoiled by another fixed bias amp (a THD Univalve) that lets me experiment with almost any decent tube, input or output. I found the Tube Freak link before I purchased this 395, and together with the Harmony Central reviews, figured it was a good investment. My reason for the 395 was that I reached a certain tone I like with my THD Uni, but it is a 15 watt amp and I want to use it as a front end, a pre-amp. Enter the 395. The line out from the Uni is my guitar pre, its speaker out is my stage monitor, and the 395 will drive 4 x 12 cabs. I am trying to reproduce the Class A output from the Uni (and its NOS valves) with the Mesa's 190 watts of bliss. Mesa's tech didn't push their current tube line as hard as I would have guessed, which implied to me that the amp's engineering may have sprung out of yesteryear's more tightly monitored tube manufacturing (NOS tubes were more plentiful twenty years ago). I understand that today's tube production is all over the board, hence the need to bias an amp to match the crappy current crop of cruddy tubes designed for crunchaholics. NOS tubes, especially those designed for military use, were manufactured to closer tolerances. I am taking a leap of faith, assuming the Mesa engineers used high quality, thoroughly tested NOS tubes to substantiate their schematic blueprints. I did not want to smoke this amp by rolling incompatible tubes through it. Eurotubes has some re-tubing recommendations specific to the 395 (that make it run hotter without changing the fixed bias), and Watford Valves has specially-selected tubes (for Mesa's fixed bias amps) in a few in production varieties that they hand-pick. These are a couple of end runs to avoid doing a bias mod. Not to say there is never an alternate, better way. But for now, it's time to roll...

 

[eet fuk]

I had a 395 a long time ago, and a Mesa tech said it would have to be modified to accept el34s. Presumably to tie pins 1 and 8 for the el34 sockets, as 6l6's pin 1 is unused.
You can also take off the top tube cover and easily fit any size bottle you want.

 

[PanamaJack]

As far as installing tall tubes, removing the cover (in a rack setup) would definitely work. On the other hand, no one else has mentioned anything about modifying the sockets to accept the EL34 tubes. There has been some confusion about which sockets were correct, but I believe that to be a question of the Class A sockets having a -52 bias instead of the -59 bias of the Class AB sockets. The simplest route would be all 6L6 tubes, but what fun is that? I have received some conflicting information before, so I appreciate your input. As for other posts regarding other Simul-class Mesa amps, none of those mentioned the pin 1-8 issue. That discussion had more to do with headroom versus early breakup. The Mesa techs have commented that any of the Simul-class amps will accept the EL34 substitutes (with no mods). However, within some of the models, the arrangement of the Class A sockets has changed with different revisions.

 

[eet fuk]

I have heard that the 395 and 290 don't accept el34s without mods, but I can't verify. You can check for yourself: see if pins one are tied to pins 8 on the sockets, if they are, it'a a go! if not, then they were designed exclusively for 6l6s, unlike the 295. Just look for a small piece of soldered wire between the pins, that's it. Look for the notch in the socket that fits the tab on the tube that guides the tube's installation. On either side of that are pins 1 & 8.

 

[PanamaJack]

Interesting. I will check it out. Thanks.

 

[PanamaJack]

Finally got around to opening up the amp, and the outside sockets' pins 4 and 8 are tied together, separated by a resistor. Socket 4 on the inside sockets is tied to socket 4 on the outside sockets, but not to its own socket 8. Both the inside and outside sockets' pins 1 and 8 are tied together.

 

[eet fuk]

Maybe you're confusing pins 4 and 5 for the first one. Pins 5 and 8 should be tied together via a high value resistor for the outside sockets for class A. In the Mark II and III that resistor is 980K (? hard to see), and in the Mark IV it's 2.2M. The pin 4s are switched between triode and pentode on the MkIV, and are set permanently pentode on the 395 sounds like.

Have you called the Mesa techs yet? You always have to leave a message and they'll call you back. They will tell you for sure what's up. I hope. Depends on which tech maybe. Again, when I called them, the guy said it would have to be modified to accept EL34s and that most didn't like it or were indifferent and some even switched it back to 6L6s. I don't know who the tech was or if it's actually true or not. If so, maybe the bias circuitry has to be changed like one of the dudes above said.

Btw, I have found the whole simulclass thing a little too scooped than regular ab when fully distorting with loads. You sure you want to even bother with el34s when you might just want a regular class ab for mids? Maybe try the strategy 500 or coliseum 300 or even a good ol 50/50?

 

[PanamaJack]

The pin thing could be confusing me, no question about that. What looks to me like it should be "pin 3" has no connection in the circuit, so if you are correct, Eet, it is possible that it is actually "pin 6." But the pins on either side of the guide slot are tied together.

I did talk to Mesa techs, and they sent me some schematics and tech stuff confirming that EL34 tubes should be fine. I may be creating unnecessary smoke and mirrors by trying to talk about pins. I would upload a photo, but that feature is not enabled on my account.