Mesa amps are fixed bias and generally they are very cold. The exceptions to this are the EL84 amps like the DC2 and DC3's and also the F30 and Loan Star Specials to name a few. You can actually get the bias up where it should be with warmer grade's of tubes.The Dual and Triple rectifiers are set very cold in the 6L6 mode even when in the solid state rectifier mode and the bold setting. So even the hottest 6L6's on the planet will still not get the bias as high as it could safely be run. When in the EL34 mode the bias is quite warm with even a moderate grade of EL34's which is why Mesa will tell you that if you're going to use EL34's that you have to run them in the tube rectifier mode. The reason for this is because when the stock 5U4 rectifiers are switched in the plate voltage drops from about 475 on average to about 415 volts so in turn the bias goes down.We quite often use the JJ GZ34's in place of the 5U4's to get a punchier and less mushy sound in the tube rec mode and the voltage drop is only about 10 volts down from the solid state diode mode. If the amp is played at high volumes with the GZ34's then depending on your line voltage you may need to jump the fuse up from 4 amps to 5 amps. This is what Bob does in his personal Dual Rec no matter whether running a combination of 6V6's and KT77's or KT66's and KT88's because the amp is always in the tube rec mode and usually in the Bold setting where the voltage is at it's highest. (He likes it best that way)The Dual and Triple Rec's are actually quite versatile with all of the combinations of tubes that can be run including the JJ 6V6's in the EL34 mode, 6L6's. EL34's, E34L's, KT77's, KT66's and KT88's. We also do Integrated quads with different combinations of power tubes like a pair of KT77's and a pair of 6V6's which is another one of Bob’s favorites. The Single Rec's that are switchable from 6L6 to EL34 mode can also use pairs of 6V6's, KT77's, KT66's and KT88's for a lot a tonal variations. The bias does not have to be modded and made adjustable if the proper grades of tubes are used. So if you order any of these tubes online from Eurotubes then make sure to leave us a note in the comments box on the checkout page to let us know what amp they are for so we can select the proper grade. Of course if you want ultimate control then a bias mod will allow you to dial any grade in. The front ends of the Rectifier amps are quite happy with the ECC83S's and especially the high gain ECC83S's used in specific locations. The ECC83MG’s very nice mixed in or in all positions and the ECC803S along with the Gold pin versions of these tubes are also a great V1 tube if a fat thick clean is the mission.We get lots of questions on the Mark series amps and mainly on the MK IV. The two inside sockets on these amps are biased VERY cold and need real hot tubes where the two outside sockets are biased much warmer to be able to use EL34's. We use a lot of hotter grades of E34L's and KT77's in the outside sockets on these amps along with very hot 6L6GC's or for the big box MK IV heads (not combo's!) where the tubes sockets are over 2 1/4" apart from center to center then you can run a pair of the JJ KT66's in the middle two sockets which is a great combination.Here is a great option for you Mesa Dual Rec players! This option is running the JJ 6V6's and GZ34's in the Dual Rec! The pics above show a quad of the JJ 6V6's running with two of the JJ GZ34's. These are running in the EL34 bias position and the Bold setting and in the tube rectifier mode utilizing the two GZ34's which gets the plate voltage to 450 volts and the current draw to 28mA per tube which is perfect! Using the solid state rectifier gets them a little too hot.Using the GZ34's will get you that tube rectifier sound without the saggy feel that the stock 5U4 rectifier tubes give you so it's like playing in solid state bold but you get the rectifier tube dynamics.The sound? Very cool! The JJ 6V6's have a very big low end that is tight and well defined. The mids and highs are thick, sweet and harmonically rich, and at higher volumes they crunch up like only a 6V6 can do. To date this is the warmest clean tone we've heard from a Dual Rec.So if you want to get a little more of that cranked up tone a bit earlier and want a great clean to go along with a very warm and punchy drive tone from you Dual or Triple Rec the JJ 6V6's and GZ34's may just be what your looking for!As a side note the preamp tubes we used were three of the graded high gain ECC83S's in V1, V2 and V3 with a standard ECC83S in V4 and a high gain balanced ECC83S in V5. This is the most popular set of Mesa front end tubes that we do and they are available in the online store under the title "Mesa high gain front end".Yet another option for Dual, Triple and Single Rec heads is the KT66! They have a big tight low end and a huge open mid and high end that crunches up at high volumes just like the original Genalex. These tubes draw a lot of natural plate current so the warmer grades get the cold fixed Mesa bias right up where it should be in the 6L6 mode. The pic below shows a quad of them running in the tube rectifier mode with a pair of JJ GZ34's and you can also run them in the solid state rectifier mode if you like or with the stock 5U4's.The bases on these tubes are the same large metal base that is used on the KT88's but it's black in color. A very cool look! Because of the large diameter base the bear trap spring tube retainers will not fit around the base of these tubes so they can either be pushed down as shown in the pic or removed. If you want to use these in Mesa combo's you would need to remove the retainers and install spring style retainers if the amp is a gig amp and constantly moved around. In January of 2008 during a tube rolling session we were trying to come up with a combination of tubes that was as unique as the 6V6/KT77 option but we wanted it to handle everything including high power situations. One of the complaints we receive about Dual and Triple Rec's is that the bottom end does not stay together at higher volumes especially for drop tuned players. In the past we had gone directly to the KT88's to solve this problem because they are thunderous sounding, and high volume performance is what they do best. This time we wanted more versatility with the ability to sound great at any volume. We also wanted to be able to run in any mode including “Bold, Spongy, Diode or Tube rectifier”. We won't bore you with all the combinations we tried but we will tell you about what worked! You can see the lineup below in the pic with a pair of a matched pair of JJ KT88's in the outside holes, two KT66's in the inside holes and a pair of the JJ GZ34's to the right and the amp is running in the 6L6 mode. After playing around with different grades to get the bias right we were able to run the amp either using the GZ34's or running the diode rectifier. We were also able to go from pealing the paint off the walls to a soft volume level that you could talk over and maintain a killer tone. We used what we call the high gain front end so it could go from shred to a huge clean jazz sound. The KT88's added the thunder and the KT66's fill in everything else with a wonderful harmonic complexity. Bob is currently using the KT88/KT66/GZ34 power section in his DR with the "Personal Fav" front end that he was using with the 6V6/KT77/GZ34 setup. It consists of a gold pin ECC803S in V1, a gold pin ECC83S in V2, a high gain ECC83S in V3, a standard ECC83S in V4 and a balanced ECC83S in V5. This is smoother and not quite as aggressive as the high gain front and and suits his style better. We also do this kit for Tremoverb's and Roadster's. Kits are in the online store.Here is a review from one of our customers.From: german r [mailto:] Sent: Monday, March 09, 2009 7:02 PM To: EurotubesSubject: RE: dual rectohello again, i would like to extend my deepest gratitude's towards Bob and the rest of his staff. honestly companies with your level of customer service, INTEREST IN THEIR CUSTOMERS, and just amazing work ethic is non-existent today. you guys stuck with me on the phone and never rushed me answering all my questions, that really made me feel like you guys wanted me to get the best product for my particular needs. it was like having a group of friends that knew everything there was about tubes, and we all know us musicians love talking about gear for hours on end. so thanks for making me feel like more than just a bag of money, and definitely more like a friend in need of the best gear guidance. OH! and the shipping is insanely fast. on top of that the tubes are top notch grade A quality. i purchased a KT88/KT66 matched quad, a high gain front end, and the GZ34 rectifier upgrade, all for my mesa boogie dual rectifier. i haven't yet seen this reviewed yet so i will review it and post it on forums i frequent. this setup really is all that Bob says it is. the GZ34 rectifiers are a godsend for tube purists who want all the mystique and warm tube goodiness in their clean/crunch channels but also need the tight bottom handling qualities of a silicone diode rectifier. the KT88/KT66 tubes in my amp together with the high gain JJ's in the preamp make my recto into a versatile monster. i get sweet warm cleans, i can get from a chimey bright tone to a funky twang almost. the crunch channel on this amp before was a bit unneccesary before because i couldnt dial in a tone i liked, now i can get awsome 80's style hot rodded jcm rythms to classic rock crunchy tones very cool channel now, with the solo boost it really does open up and scream if you want it to. but honestly if you have a mesa dual recto you know its pretty much all about the modern high gain in channel 3, this is where the JJ's really took my head clean off. total brutality. kill on contact tone. my amp is now a classified government weapon! seriously sustain for days, im talking clear as day high gain that just pounds your chest with every strum. palm mutes now feel like kicks to the chest, it literally makes you feel hollow, i can feel my organs being shaken (i dont know if thats healthy but i like it!). this set has made my amp not only a high gain clear as day fun to play inspiring amp, but it has also opened up new tones that i just couldn't get before. awsome tubes, awsome setup, awsome guys. keep up the great work. please feel free to post this in your reviews.thank you, Germanp.s. the prices are unbeatable.The smaller Mesa amps using EL84’s are also fixed bias but are not as cold so by using hotter tubes they sound real good and the JJ EL84’s are by far the best EL84 being made today. They rival most all of the classic EL84/6BQ5 tubes and sound great in the little Mesa’s. Here is a shot of Kris Siewell's custom Lone Star Special. It's clad in African Ribbon Mahogany and has a flame Maple front. We just did tubes and an Alnico Silver Bell and Blue Dog for him and he loves the tone!Here are a couple of Mesa 295's and a Strategy 400 that we also used the KT88's in. The EL34 sockets in the 295 run pretty hot so even the coolest KT88's I had were pulling 90mA which is too hot so we ran the JJ E34L's in the outboard sockets and the KT88's in the inboard sockets in place of the 6L6's where they pulled a very nice 45mA at 530 plate volts. This combination sounded fantastic! The lows were very deep and tight, the mids were thick and very harmonically rich. The highs were creamy without being at all brittle. Overall the amp was very punchy with a great dynamic.The Strategy was a bit different because the EL34 sockets were not biased so hot and the amp sounded best with the JJ 6L6GC's and a quad of the JJ KT88's in place of the EL34's where they pulled exactly the same 45mA at 530 plate volts.The Mesa 290 amps seem to be the "go to" amp for a lot of players and these are also fixed bias and setup to run in a Simuclass configuration with a quad of EL34's and a quad of 6L6's or more commonly with an octet of 6L6's. If you want the biggest low end from the amp then using an octet of the JJ 6L6GC's will do it. If tend to scoop your mids then I would definitely stay with all 6L6GC's, but if you like more of a pushed mid and don't mind loosing a little low end then there are a couple ways to go. You can use a quad of the JJ E34L's which are deeper and tighter than a standard EL34 or you can use a quad of the JJ KT77's to mix in with the 6L6GC's. The KT77's have slightly more low end than the E34L's but not as much as the JJ KT77's but both the E34L's and the KT77's have that mid range harmonic complexity that 6L6's don't have. You can also run standard EL34's but the loss of low end is quite pronounced so we much prefer the E34L's or KT77's mixed in with the JJ 6L6GC's which provides a much more balanced sound.You can find full retube kits in the online store for these amps under "Mesa Rac Mount Amps"If you decide to run and integrated set of tubes then you need to know which 290 you have. The older 290's have a blue pilot light and the fuse located directly above the pilot light. If your 290 is a newer model it will have a red pilot light and the fuse is (conveniently) located on the back of the amp. Below are a couple pic's of how the tube sets are orientated for series I and series II amps. KT88's in a Dual Rectifier??Yes! KT88's in a DR... This is one way to make a DR into "Frankin Rectifier" The bias on the DR's is set very cool for 6L6's and warmer for EL34's but the average KT88 draws about double the natural plate current of the average 6L6 so when you run KT88's in a DR with the amp in the EL34 setting and in the tube rectifier mode the bias comes up to where it should be and the bottom end gets VERY deep and strong along with a punch that is truly enormous! If you are going to run a DR in the solid state diode mode or with GZ34 rectifiers then it needs to be set in the 6L6 mode and we would use a hotter grade for the KT88's. When we setup a DR to use the KT88/KT66 power section we again use hotter grades so the bias needs to be set in the 6L6 mode and you can use it in the diode mode or with GZ34's.May the amplifier Gods have mercy on your speakers!!We would also like to dispel a couple myths that we've read in the past on the Harmony Central forum about KT88's ( yes we do read the forum every once in a while when we have the time ). First off Dual Rec's are capable of using either 6L6's or EL34's and LOTS of you DR players out there are using the JJ E34L's in your amps and have been doing so for years. A standard 6L6 draws .9 amps of heater current where the JJ E34L draws 1.5 amps. A KT88 draws 1.6 amps so as you can see a quad of KT88's will only draw .4 amps more that a quad of E34L's and is not going to cause a problem. Bob even used a quad of KT88's in his 1982 Evil Twin which means that it's having to deal with a 6.4 amp draw instead of a 3.6 amp draw from a quad of 6L6's and it's been running STRONG for years now. The 5150ex in our studio has a total draw of 6.2 amps running two KT88's and two E34L's once again this amp was made for a quad of 6L6's with a total draw of 3.6 amps and yes, it's a MONTSER and gets played very hard and often since 2001. The bottom line is that most of the amps out there that can run 6L6's can run E34L's or even KT88's if there is room and the bias is set properly.Now about that pesky bias question. Dual Rec's are a unique amp because of the different settings available with the "spongy" and "bold" settings along "6L6" and "EL34" settings and the "solid state" and "rectifier" modes. Since a KT88 will draw almost twice the natural plate current that an average 6L6 draws your bias will go up! An average E34L will draw only about half the natural plate current that an average 6L6GC draws. An amp that has anywhere from 450 to 500 plate volts can run a JJ KT88 at between 50mA and 90mA per tube. Here's a quick run down on the bias using a moderate grade of JJ KT88's in a DR.These setting are in the "6L6" mode.Rectifier mode in spongy setting with 5U4's = 16mA which is VERY cold so you can imagine how cold a 6L6 would be!Rectifier mode in bold setting using 5U4's = 18mA which is still VERY cold.Solid State mode in spongy setting = 28mA still cold.Solid State mode in bold setting = 32mA still a little cool.These setting are in the "EL34" mode.Rectifier mode in spongy setting with 5U4's = 44mA which is actually just getting out of crossover distortion and sounds very good.Rectifier mode in bold setting with 5U4's = 50mA nice an aggressive with great dynamics.Solid State mode in spongy setting = 78mA now this is getting some heat in the tubes and the dynamics are spectacular but this is running pretty warm.Solid State mode in bold setting = 88mA this is getting up to about 80% of max dissipation and will shorten the tube life about 10 to 15% and the tone will start to "brown" but it's a lovely sound!Now we do want touch on one more thing that is a big myth. Just because a JJ KT88 will actually dissipate 50 watts does NOT mean that because you use a quad in a DR you will have a 200 watt amp! It doesn't work that way. To get more power from an amp the plate voltage needs to increase and you can't do that unless you replace the transformers to supply a higher plate voltage. For instance Bob’s Twin when loaded with JJ 6L6GC's will make right at 90 watts before clipping and loaded with a quad of JJ KT88's it makes 98 watts before clipping which is not much and is basically perceived as a little more headroom. You have to double your wattage to gain 3db so if you calculate the difference the KT88's will breakup only a fraction of 1db louder than the 6L6's. Anyone reading this has our full permission to post it on HC or anywhere else.