JAN-GE 5751 in MarkV Head

[bandit2013]

I bought my 5751 and the tung sol's through tubedepot as well. I was a bit disappointed with Doug's tubes due to the dud I got for the 5U4G. I have yet to see a replacement. Money lost, lesson learned. I sent it back under an RMA, will see if Doug sends a replacment. I chose the Tung Sol's based on their construction and build quality. I may consider building an inventory of the 5751 as well as other tubes, I have boxes filled with mesa 12ax7's, need to test them and weed out the bad ones. Waiting for the Orange VT1000 to become available. I was considering the Combo V but already had the MKIV combo along with Rectro slant 412 (looked like something was missing so opted for the head).

what were the plate current(ma) readings for the tunsol 7581 you got from tubedepot...they are a great store and im sticking with them for awhile...
I would like to get the VT1000 as well and it not a bad price for what it does.but I was looking at a maxipreamp tester http://maximatcher.com/
but it cost quite a lot more so im not sure if I can justify such a purchase,,but then again im getting a lot of tubes that I do not know if they are really that good or not,,,i like to see what the gain readings are for most tubes...
I most likely will get a set in a couple of weeks...of the tungsols..

I purchased the 7581 from Doug's tubes.
plate current and transconductance of each tube:
23.6mA, 2360
23.5mA, 2540
23.3mA, 2520
23.4mA, 2570

I paried up the based on transconductance. 2570 went to V8, 2560 in V9, the other two in V10 (2520) V11 (2540).
I put the highest gain into V8 since that tube is active in all modes, V9 is used as a dummy load in 10W mode in class A operation.

 

[mesa metal]

thanks bandit,,,,,,doug matched those pretty well indeed
I was wondering how close the ratings were as opposed to the sed-c-6L6s...
I have a quad of those I bought from doug about 5 years ago(probably longer) that I put in my 50/50 head......going to try them out in the V
and in a few weeks ill get a set of the tungsols 7581............my 6L6s come in at
25.0mA,2350
25.0mA,,2290
24.9mA,2190
24.9mA,2390
some really good info here on tubes thanks for sharing this stuff and in what positions you have them in...
going to do the preamp tubes maybe tomorrow.....

 

[bandit2013]

Bandit,if your chassis was getting hot as you say,it is likely the power tubes were biased too hot,very uncommon in a stock Mesa with Mesa tubes.They set up their amps to be biased extremely cold.Never seen a Mesa with stock tubes biased any hotter than 25ma's,and usually colder.I have found most are best at about 35ma's and some a little hotter.While the 7581 is a 30 watt max tube,it was replaced with the 7581A which is a 35 watt tube when the 6L6GC came to be.What brand 7581 are you using?

The amp was overheating with the stock Mesa Tubes! I posted in the MARK V formum on this issue. In summary: I Was uncertain of the cause, but may have been related to bad 6L6 tubes. I red plated the center two STR440 6L6GC. I replaced all of the power tubes with an old set STR420 that I borrowed from my MKIV supply. Red plated the center two tubes again. I got around to replacing all of the V30's in the cabinet, they were 13 years old, not sure if they were the cause but after the speaker swap I was able to use the original tubes that were in the 90W (outer two) in the inner two without any issue. I had to put the STR420 in the 90W positions (I did not want to leave the sockets empty). While the amp was operating (before red plating the 45W inner tubes) the chasis would get considerably hot. Hot enought that I could not touch the rear chasis, speaker plug, and the front of the chasis was also hot). Heat soak of the filiment transformer for sure. Even after an hour (or longer) the chasis was still very hot after shutting down the amp. I would not doubt that the cathode heaters in the tubes I had issue with were in bad shape. Odd that the fuse never blew. New speakers in the 412, and all tubes changed I no longer have a thermal issue with the chassis. It gets warm but I can place my hand anywhere on the chassis without getting burned (except the tubes of course). I am not sure what the impedance on the cathod heater should be, the bad tubes I pulled measure less than 5 ohms. I never changed the bias switch ( always set to 6L6). I did check the ballast resistors (aka Bias resistors) and they match the marked resistance (not an out of spec tolerance issue). The only defect I could find was around all of the hand soldered wires and resistors on the small PCB that connected the Power tube sockets. The defect being the rosin flux not being cleaned after soldering. It should be inert and okay, but it can be reactivated by heat and pose a risk of dendrite growth which would lead to short circuit issues. Considering how hot the amp was running, I was concerned about that if I could not bring the temps down somehow. Besides that, electrolytic caps will suffer a shorter life span at elevated temperatures. I bought this amp new last November. All of the other Mesa's I have owned or still own never got hot like this one. Like I mention, it is now running much cooler and seems to be operating at normal temps now. The tubes that were stock had "10 AC RED" on the Mesa test label. A closer look at the Mesa Tubes I noticed that the "getter" or sputtered metal glaze on the top of the tube was flacking off of the glass. I can see metal fragments inside the tube where is should not be. Not sure if the tubes were damaged during shipping and the 20 degree weather I experienced when I transported the amp from PA to NC. I bought it in PA while visiting family during Thanksgiving Holiday weekend.

I do not believe there is a bias issue at this time, but was considering that. The closest certified Mesa Repair location is 3 hours away. If I could not resolve the issues, that would have been my next approach.

 

[bandit2013]

well my jan Phillips 5751 is on the fedex truck and will be here Monday..and as for the winged c tubes doug had stated he is not going to be carrying the 6L6 any more because they changed the materials being used and were not testing well and too much money,,,,,thats a shame cause I have a set in my 50/50 amp and really like the bottom end they produced...so im going to put them in the V and give them a listen.....for the V6 currently im using a high gain JJ 115/115..and for v3 there is a tungsol 110/110 in there,,,but its all going to change when I get the 5751..i love my tone now but there is something that's just not there...the treble sometimes can be a bit over the top but I was adjusting that with the eq......and I want to use channel 3 extreme mode more often......I like the brutal tone im getting from that........it seems as im getting older the heavier the music im listening too
so how are the 7581 in the V are you getting some good bottom end from them.......im starting to be a fan of tung sols but have not tried the power tubes yet..was thinking to get the Tung-Sol 6L6GC-STR when the stock mesas go blammo...and the price is better than that of the SED-C-6L6
id like to hear that koa carvin is it a neckthru version? im playing Jackson neckthru mahogonys and the sustain is fantastic but it weighs a ton..
thanks for your feedback on your tube arrangement there is a few of us on here with similar playing styles and the music we like and not to mention the tone we want from this amazing amp......for me its quite a change from my previous triaxis/2:90 rig.....the V has the best clean channel ive ever heard!!!
ill let you know how the tubes sound later in the week,,,i have to decide wich tube to put in V3.. :? .

I would not waist your time on the 7581's. The quad that I had in my V went completely dark after a month of daily use. Then one had the Zeuss effect (began to intermittently arc). I was operating in 45W mode and V10 was shooting bolts? WTF... It could have been the 5U4G but it happened again in 90W mode. Besides the fact they were no longer generating any appealing sounds, they were toast. They never red plated, nor discolored like the 6L6 tubes I fried before I installed the 7581's. I will burn through one more set before I get Mesa on the line, this time I am sticking with 6L6 (in the winged C variety).

 

[stokes]

What brand 7581's were you using?Either they were biased too hot or you have a problem in your bias supply.

 

[mesa metal]

[I would not waist your time on the 7581's. The quad that I had in my V went completely dark after a month of daily use. Then one had the Zeuss effect (began to intermittently arc). I was operating in 45W mode and V10 was shooting bolts? WTF... It could have been the 5U4G but it happened again in 90W mode. Besides the fact they were no longer generating any appealing sounds, they were toast. They never red plated, nor discolored like the 6L6 tubes I fried before I installed the 7581's. I will burn through one more set before I get Mesa on the line, this time I am sticking with 6L6 (in the winged C variety).

bandit thanks for that info on the7581....something must have been wrong with the tubes or those tubes are just not right for the V.....thats a bummer you had to find out the hard way,,,i really appreciate the input and will probably not get those..i want to try something other than the stock mesa chinese tubes!!
i do like sed-c- tubes and have been using them for years but according to doug they have been testing poorly and he is discontinuing them...so i wonder how the new Tung-Sol 6L6 STR will work....i might try those.....wondering also if you got a bad batch from doug????
havent tried out the 5751 yet,,,been buisy with work this week...should have time this weekend...
so keep us posted...let me know which 6L6 you decide to use.........your 7581s were not biased to hot at all..

 

[stokes]

Mesa metal,how do you know the 7581's were biased okay?He fried two sets of tubes in a very short time.I aint sure,but I get a sense these tubes were not checked for current draw when they were installed.A 5U4 is not going to cause a tube to cook and arc as described.A true 7581 is a ruggedized version of a 6L6GC and if biased properly will last a long time.A 7581A is a 35 watt version of the 7581.I have a set,biased on the hot side of "safe",in my IIC+ for over 3 years and they are still going strong.While its possible he just happened to have a bad set of tubes,the fact that 2 seperate sets cooked is a strong indication that something isnt right in that amp.

 

[bandit2013]

They were 7581tung sol tubes.

As for the bias on the Mark V. Never checked it since I do not have any means to do so at this time. The Mark V is new, purchased in November of 2012. Not sure if the tubes were selected for the Mark V, I bought them through Doug's Tubes and indicated they will be used in a Mark V with fixed bias. The original tubes did not last very long and red plated for unknown reasons operating in the 45W mode while in channel 3. I had rebuilt the 412 cab with EV's. Since I did not do all speakers at once, I decided to get a feel for the EVM12L compared to the V30's. Nice thing about the series parallel connections is you can split half of the cab up for stereo use. Playing through the EV's, no issues, however when I connected the amp to the V30 side, red plated another set of tubes (they were old Mesa 6L6STR420's that I borrowed from the Mark IV). It took a while to red plate, at least one hour of use. Before I installed the 7581's, I had the 412 cab completed with all EV's. Impedance of the cab was checked at all input jacks (low frequency DVM) and all was well in a DC perspective. I also checked the V30's after I removed them. The last two V30's measured on the low side (between 5.4 and 6.5 ohms, when connected in parallel the, the resulting impedance drops to 2.95 ohms). They are factory originals that were installed by Mesa when they built the cab (built in 2000). They are supposed to be 8 ohm. I will get a full analysis on the speakers at work before I sell them if I do.

With the 7581, they performed well for about a month. Tone and character was very similar to 6L6GC. Considering I play through the amp 3 to 4 hours per day, every day that would put the usage around 120 hours. The tone change may have been gradual since the amp sounded better every day I played through it. The darkness was a sudden change. Tubes sounded great the day before, the next time I turned on the amp, character changed to only bass response with moderate mids and no highs. As for the arching, I believe it was in V10. Had the amp chasis on the cabinet inverted.

Considering this is a new Mark V, perhaps some of the components are out of spec. I first had problems with it within 10 hours of use, sound would suddenly dropp off with hum. Of course at that point my reaction was to hit the standby switch. More than likely, I had one bad tube that pulled its mate down with it. The other tubes that had similar issues were used but were known to have been good (old set from MKIV). Also when the tubes went, I still had the V30's. I have not red plated any tubes since I changed the speakers. Arching may have been caused by having the amp on top of the cabinet, inverted with the tubes facing up. Any loose material in the metal glaze on the tube may have become loose due to vibration. After I removed the tubes, found metal flakes on the sides of the bottle. When looking through the tube with light in the background, spots on the glaze will pass light through which is about similar size to the flakes I found on the side of the glass.

 

[bandit2013]

Mesa metal,how do you know the 7581's were biased okay?He fried two sets of tubes in a very short time.I aint sure,but I get a sense these tubes were not checked for current draw when they were installed.A 5U4 is not going to cause a tube to cook and arc as described.A true 7581 is a ruggedized version of a 6L6GC and if biased properly will last a long time.A 7581A is a 35 watt version of the 7581.I have a set,biased on the hot side of "safe",in my IIC+ for over 3 years and they are still going strong.While its possible he just happened to have a bad set of tubes,the fact that 2 seperate sets cooked is a strong indication that something isnt right in that amp.

When I get a means to check bias, I will do so. The tubes were new production design from Tung Sol. I do not wish to cook any more tubes since that gets expensive. I never had this much issue with either the Mark III or Mark IV. Both have served me well. I have burned through many tubes with the Mark III but that was from heavy use, and took over 6 months to get there. Used to be in a garage band. 4 to 10 hour sessions on weekends during practice.

I do have concerns on the bias of the Mark V. I have always either used Mesa Tubes or requested "must be suitable for fixed bias Mesa Boogie" and always stated the amp model number with any order I placed.

 

[stokes]

The position of the tubes makes no difference.They can operate in any position.An 8ohm speaker will rarely read 8ohms on a meter.The meter measures resistance,a speaker is actually impedance,it is frequency dependent.A reading of 4 to 8ohms is perfectly normal for an 8ohm spkr.The speakers had nothing to do with redplating.The mismatch would have to be so big to redplate the tube that your OT would have cooked.There may be nothing wrong with the amp itself,its more likely the tubes fell into an "unsafe" current draw.If you use a Mesa amp as intended the tubes must be selected to match the bias the amp is set at.It is best to have an adjustable bias pot installed if you want to use NOS tubes or tubes that have not been screened to match the proper bias range.Mesa's tubes are selected to be "safe",which is usually very cold,that way there is little to no chane of them being too hot.I've seen most Mesa amps biased at 18 to 25ma's.A 6L6GC can be biased up to about 50-60ma's in a Boogie amp.

 

[mesa metal]

Mesa metal,how do you know the 7581's were biased okay?He fried two sets of tubes in a very short time.I aint sure,but I get a sense these tubes were not checked for current draw when they were installed.A 5U4 is not going to cause a tube to cook and arc as described.A true 7581 is a ruggedized version of a 6L6GC and if biased properly will last a long time.A 7581A is a 35 watt version of the 7581.I have a set,biased on the hot side of "safe",in my IIC+ for over 3 years and they are still going strong.While its possible he just happened to have a bad set of tubes,the fact that 2 seperate sets cooked is a strong indication that something isnt right in that amp.

bandit had listed the plate currents of the tubes and they fell within the mesa fixed bias point.....but you're right they may have been a defective set in the first place....you know what happens when we assume :wink:

 

[stokes]

The numbers I saw were,I thought, the plate current that was marked on the box from the supplier.Unless you know for sure that those currents were measured in a MKV you really cant be sure they arent numbers from some bench power supply the seller uses.I have one myself,I can check for current draw match without using an amp.I believe he also listed some gm numbers,that tells me it is likely the tubes were matched/measured on a tube tester.My tube tester has a current feature which measures the current draw in the tester at about 250vdc on the plates,those numbers mean nothing to a MKV.

 

[mesa metal]

very good point,,,we are i guess at the mercy of the vendors to get the right tubes we need for a specific amp..and hope they last...i have a set of JJs in my 2:90 i got from eurotubes in 2008 and they still sound like the day i put them in...go figure....
would love to get a good tester for myself.....in all my years of playing and using mesa gear i was never so "obsessed"(for lack of a better word) with tubes
be it preamp or power tubes as i am with this MKV...well except for my triaxis!!

 

[bandit2013]

Tubes will not last forever, except a set of Mesa 6L6GC STR420's. Had original tubes in the Mark IV for 13 years. I finnished them off in the MKV. As for the 7581's they lasted the longest in the MKV so far. The only issue I had with them was high frequncy drop was a bit sudden after 120 hours. Bass and midrange still have plenty of power. I removed them after seeing arcing in V10 while operating the amp in 45W mode. I have had EL34 last just as long in my MKIII under similar usage. I stopped using EL34 when I could not find the Mesa brand ( mid 90's). At that time, all I could find were 6L6's. Never had issues with them in the past. Never had to check bias in either Mark IV or Mark III... I admit, the 7581 are not Mesa tubes, nor were they screened by Mesa for use in their fixed bias circuits. I ordered them from Doug's Tubes and I clearly stated they will be used in a Mesa Boogie Mark V. For now, a mixed set of 6L6 is in use. Matched pair of Mesa 6L6GC STR440 (reds) and Matched pair of Mesa 6L6GC STR420 (GRY - assuming that means gray?). I do not even remember when I bought them, they were reserves I had for the MKIV. I did order a quad of the GT6L6-R2 from Doug. He claims they are Groove Tubes relabeled SED winged =C= tubes. I will keep the Mesa's in the amp for the time being until I can get my hands on a quad bias meter. What good will that do unless I hack into the bias circuit to add bias adjustment. Any suggestions on which meeter to get? Probably would be cheaper than burning off the cathode coatings and going through tubes once a month. Odd that the amp was running much cooler with the 7581 in the sockets than the 6L6's on board.

Plate currents listed were emperical values of the test equipment used by Doug for matching the tubes. It does not imply the amp currents as that will be different for any amp even if it is identical. I have no idea what the cathode current in the MKV is, no clue on the grid voltage, etc.... I had poor luck with the original Mesa tubes, no luck with the old used Mesa STR420 's that I removed from the MKIV (was not expecting a long life from them since they already served for 13 years was even supprised they even worked for a few hours). Sooner or later I will be able to enjoy this amp rather than spending time trying to figure out the issues.

What about that 5751 preamp tube?......

 

[bandit2013]

Mesa metal,how do you know the 7581's were biased okay?He fried two sets of tubes in a very short time.I aint sure,but I get a sense these tubes were not checked for current draw when they were installed.A 5U4 is not going to cause a tube to cook and arc as described.A true 7581 is a ruggedized version of a 6L6GC and if biased properly will last a long time.A 7581A is a 35 watt version of the 7581.I have a set,biased on the hot side of "safe",in my IIC+ for over 3 years and they are still going strong.While its possible he just happened to have a bad set of tubes,the fact that 2 seperate sets cooked is a strong indication that something isnt right in that amp.

bandit had listed the plate currents of the tubes and they fell within the mesa fixed bias point.....but you're right they may have been a defective set in the first place....you know what happens when we assume :wink:

There very well could be issues with the amp. Have not rulled that out yet. At least I did not redplate the 7581's. The blue flash in the one tube only spawned twice but that was enough to remove them. I did not state the 5U4 caused any issue, I thought at first I saw the lightning inside the 5U4 but it was a reflection on the glass since it was V10 that flashing. I was looking in that general direction at the time but not focused on the tubes when it first occured. Just before the flash over, the output went dead, then back to normal.

 

[bandit2013]

I will be checking out the terminals on the cabinet. I need to verify if there is an intermmitent connection problem which may have caused the arching. I did have a long and lenghty reply but got logged out when I submitted.

Highlights of what I had before poof. Short life of the 7581, I would agree if the cathode current is on the high side that will cause the tubes to go flat in a short time. However, it could also be related to degree of process control during manufacture (they are new production 7581, not the A type either). As for the arcing I observed, could be related to intermittent loss of load if there is a contact issue on one of the input jacks of the cabinet. The cab is 13 years old, spent most of its life in storage. Similar to the MarkIV, discovered corrsion in the speaker wire (wires inside of jack were frayed due to corrosion causing a short circuit to occur if the wire was moved). Wires in the cab were very clean, but I did not check the fingers on the jack receptacles. Vibrations could cause intermment connection if corrosion is present. Open load on the output transformer is not a good thing which can result in inductive kickback of the transfomer, or breakdown of the isolation between primary and secondary. There could also be an issue with power supply capacitors, decoupling caps, loose terminals on the sockets, or bad solder joint. Internal arcing within the tube will casue permanent damage to the tube, possible damage to other compnents as well.

 

[bandit2013]

I would assume that Mesa desgin in flyback diodes on the output transformer circuit to reduce issues caused by no-load conditions. So why the arc in one of the tubes? It may have been a plasma discharge since it followed the inside wall of the tube. Discharge path was from pins to getter. I would rather get to the cause of the issue than voice my opinions.

Some background on myself: 15 years Electrical Design Engieer. My focus has been digial/analog circuit development, process controls, integrated system development, etc.... While I was in college, amplifier design was a primary focus, but not much theory on vacuum tubes. I will not pretend I know everything about tubes, I understand how they work and operate, I am well versed on inducance, capacitance, transfer functions, load characteristics, etc... I definately know the differece between a dc resistance measurement of a speaker coil will not be the actual load with an AC source applied with varing frequency. Inductance increases with frequency XL (reactance) = 2(PI)FL where PI is the value of 3.142......, F is the frequency, L is the inducance in Henry's. DC measurement of a speaker is the series resistance of the coil. Since the speaker voice coil is not ideal, there will be a paracitic capacitance associated with the load. Enough said already.

One main reason why I joined this forum was to learn more about characteristics of the Mesa Amps. It is great to find out what others have discovered, what they dislike or like about their amps, what works and what does not. I do understand that biasing is important to prolong tube life. With a fixed bias amp, and no desire to cut or modify my amps, I will expect that some tubes will last long and some short. Over the past 20 or so year, I have found some tubes to last very long, and some were duds.

 

[bandit2013]

Decided to see if I could easily measure cathode current, not an easy task. Appears that the Mark V uses a common bus for cathode supply and is shared with the 12ax7 tubes as well. Not having a schematic of the amp and no desire to pull circuit boards to map out all circuit paths, the easy approach would be to have a quad bias meter. My guess (without a schematic) the bias resistor for the heater or cathodes is one or both of the aluminum power resistors mounted to the chasis. If my assumptions are correct, at 6.3v and 19 heater elements (all preamp tubes, rectifier and 4 power tubes) the size of the resistor would have to be 20W to 25W. The MARKIV on the other hand, has a multitude of resistors hand soldered to the tube sockets. The grid resistors are easy to locate. They connect to pin 4 of the power tubes. Two 470 ohm resistors on the outer tubes and two 1k resistors on the inner tubes. I am assuming that the 10k resistor is used for the 10W mode. Its all a guess without a schematic. Yet again, still not related to the 5751 thread.

Now that I have 6L6 winged = C= tubes, I may reinstall the 5751. So far I like the preamp tube I have in the amp. I will leave it out for reserve, may even try it in the PI position.

 

[bandit2013]

Decided to see if I could easily measure cathode current, not an easy task. Appears that the Mark V uses a common bus for cathode supply and is shared with the 12ax7 tubes as well. Not having a schematic of the amp and no desire to pull circuit boards to map out all circuit paths, the easy approach would be to have a quad bias meter. My guess (without a schematic) the bias resistor for the heater or cathodes is one or both of the aluminum power resistors mounted to the chasis. If my assumptions are correct, at 6.3v and 19 heater elements (all preamp tubes, rectifier and 4 power tubes) the size of the resistor would have to be 20W to 25W. The MARKIV on the other hand, has a multitude of resistors hand soldered to the tube sockets. The grid resistors are easy to locate. They connect to pin 4 of the power tubes. Two 470 ohm resistors on the outer tubes and two 1k resistors on the inner tubes. I am assuming that the 10k resistor is used for the 10W mode. Its all a guess without a schematic. Yet again, still not related to the 5751 thread.

Now that I have 6L6 winged = C= tubes, I may reinstall the 5751. So far I like the preamp tube I have in the amp. I will leave it out for reserve, may even try it in the PI position.

I was way wrong on my assumptions.... I will say no more on where the bias resistor is. From what I have found in this forum and others, the Mark V series run hotter than others before it. If anybody has a picture of the bias resistor please post it for the Mark V. I have seen images of the other amps which are not the same.

 

[mesa metal]

bandit im not sure where i saw this ...i think it was a tube dealer site,,but had mentioned the MKV runs hotter than previous versions...i hope i can find that info again.....let me know how the sed-c- sound.....i should have some time tomorrow to get the 5751 in there and give it a go......i see you have changed your preamp roll and included a couple of electro harmonics in there....
you have posted some really good info on how things work and i enjoy reading
iwas looking for that info and came across this link of a mkV review.......very interesting,,,,he mentions that one of the power tubes died within a week!!!
http://powerdragon-powerdragon.blogspot.com/2010/11/mesaboogie-mark-v-medium-head-review.html