For experienced old Mark IV owners: Mark IV tube in V4

[Russ]

First I would like to say I am a new Mesa/Boogie owner and am trying to convert from solely playing Mxxxxxx and that sort of amplifiers... (please no flames)

I just got my Mark IV. I believe it to be an early original as the serial is in the 560's. It has a 12at7 in the V4 position. On the chassis it says 12at7 in what appears to be the builder's sharpie. The new ones are all 12ax7 from what I understand. Where there any changes made that allowed the switch to a 12ax7 in the V4 position?

Would it hurt to change this out to a 12ax7 for a little more gain on the lead channel without any modding? According to the current manual it calls the V4 as the "second lead high gain stage" on the a side and "reverb send" on the b side of the triode. I asked when on the phone today with Petaluma while ordering the reverb tank and footswitch whether it would be detrimental to anything and the rep could not give mean answer because it predated his experience with their products. I am pretty sure from my experience with other amps that it shouldn't but I just don't want to risk killing this amp. I would hate to have to ship it out for repairs and not even get to play it for very long. As it is right now I am not getting the gain I would expect out of the amp. Where the early ones not as gainy? I know this is not a Recto and don't expect that gain/sound. I am just looking for a great metal (not this nu-crap) sound. I am just not getting what I think would be the full Boogie experience, yet.

I am looking for that Mesa metal sound. Those of you who know will know what I am looking for.

If changing the 12at7 to a 12ax7 means making an unstable gain stage I would not do it. If the change would cause anything to go wrong I would not change it either. It just seems like such a no-brainer to just swap it out. In the manual it says that it was designed to work like it is, but I don't hear it working like I expect it to. I have played with the knobs and switches and it still seems lacking. Is it that the rep cannot tell me to use an "improperly placed tube"? I mean, hell, the thing was out of warranty a very long time ago right?

Any assistance would be helpful...

 

[jonmm66]

russ

i have just done exactly that. i recently bought a '93/94 mark iv which had tubes in of unknown age and make so i decided a full retube was the way to go. i emailed mb about the 12at7 and was told there was no problem using a 12ax7. i got my new tubes last week (JJs) and it sounds great.

Just one question. are you replacing the reverb tank because it doesn't work.

enjoy your new boogie

 

[Russ]

I don't have a reverb tank. When I got the amp it didn't have a tank or a footswitch. Actually the head is in a Mark III head box too. I will have to manufacture a mount for the footswitch in order to hang it. Shouldn't be too hard... Cut a couple pieces of metal, make a couple bends, and drill a couple holes. Attach the little angles to the head and there you go. I might have to shave the inside of the head box though like the head boxes are for short heads.

Great, so you contacted Mike B.? How is your lead gain? Is the reverb affected? I was just scared of burning something with the additional gain going from 12at7 to 12ax7.

On a side note, I thought I would run the tubes through my tube tester while I was waiting to talk to Mike. I found my 12at7 was useless (I replaced it with one that is reading at about 80%). I checked the other tubes and V1 was barely hanging on. The front side of V2 was wasted as was the front side of V3. I couldn't believe it. I was really amazed because I thought the amp sounded great. <---- Testimonial to great tone...

I guess playing nice Gibsons through it didn't hurt either...

I am planning on now getting a full retube compliment. My other amps could use a tube check. I think this might get expensive. I might need a couple dozen good 12ax7's and 10 el-34's and the 4 6l6's, not to mention backups. I have some nice 1960 RCA 12ax7's with D-getters that I could use but I might hold on to them a little longer before putting them in the old Marshalls. They are pretty nice. I have some 1963 GE 12ax7's too but I have almost used those up. They are still testing ok but I think I will just put them in the Boogie until I get the new tubes. Man, that was the worst tube compliment I have ever tested. It looked nice though (JAN Phillips, Older EH, Older Sylvania). I think someone just played the hell out of those old tubes. The backside of V2 and V3 still test new and higher than current production. I thought that was weird. The V5 (PI) tube is ok but it isn't matched very well...anymore.

Thanks for the reply. That was perfect now I don't have to call Mike...

 

[Boogiebabies]

The MK IV A, like yours in the 0560 range to late 1993 would have used the entire V4 for the Reverb Send and Return. A late 93-94 would use a 12AX7 in the V4 and spilt it for 1/2 lead gain stage and 1/2 reverb input.
If it has a satellite send, it is a MK IV B.

 

[Russ]

Thanks Boogiebabies.

I guess the change in design made a change in the V4 tube. I hadn't changed to a 12ax7 yet from the 12at7. I actually got a 12at7 from my spares that was a little better to replace the one that was in there. If it is solely for the reverb send and return then I will leave it as is because it won't affect anything in the gain. Can you tell me what V1-V3 do? I assume V1 is primary gain and that V5 is the PI. Are all the gain stages in V1-V3? Which tube handles the loop or is it even tube driven? I guess I could just take the chassis out and look for myself but if I don't have to I would like to avoid doing this.

 

[RussB]

Note that running an amp (with reverb) with the cables not attached to a reverb tank (the load) is rough on the reverb transformer. You'd be better off to REMOVE the tube completely until you can replace the reverb tank...

If I might ask, what did Mesa charge you for thr reverb tank?

the other Russ

 

[Boogiebabies]

Thanks Boogiebabies.

I guess the change in design made a change in the V4 tube. I hadn't changed to a 12ax7 yet from the 12at7. I actually got a 12at7 from my spares that was a little better to replace the one that was in there. If it is solely for the reverb send and return then I will leave it as is because it won't affect anything in the gain. Can you tell me what V1-V3 do? I assume V1 is primary gain and that V5 is the PI. Are all the gain stages in V1-V3? Which tube handles the loop or is it even tube driven? I guess I could just take the chassis out and look for myself but if I don't have to I would like to avoid doing this.

Send me a PM with your e-mail and I will send you a block diagram that shows you the sequence.

 

[Boogiebabies]

Note that running an amp (with reverb) with the cables not attached to a reverb tank (the load) is rough on the reverb transformer. You'd be better off to REMOVE the tube completely until you can replace the reverb tank...

If I might ask, what did Mesa charge you for thr reverb tank?

the other Russ

Good call.

I was thinking of you when I pulled the chassis on a Matchless Phoenix 35.
I was thinking of sending a link to ACP.

Ed

 

[Russ]

I think they charged me $39+tax (I live in CA) + shipping for the tank. They said it was the same tank as the Mark III. It didn't make a whole lot of difference to me though. I just wanted the tank. I saw a Mark III combo cab on ebay for $170/best. I thought about using it because it still had the tube holder, fan and tank in it. It looked really clean just missing the front logo. I would have to fab some footswitch mounting tabs but I have to already as mine is in a Mark III head cab so that wasn't a big deal. My total bill was about $320 because I ordered a footswitch. I hope that thing senses when you want to change and does it by itself...

 

[Russ]

Thanks for the info on V4 too Boogiebabies.

I do have the original Mark IV without the satellite.

I might just get a Mark IV cabinet to restore it. Any ideas where I could get one??? ok, besides Mesa/Boogie.

I have the rackmount too... and I really don't mind it in the III case for now, so if it takes a while that is fine. If it would have to be built that would be fine, but if I would have to wait I may as well get the real deal.

 

[Boogiebabies]

You have mail.

There is only one place I know for a Mesa MK IV cab replacement. Mesa.
Dr. Hardwood may be able to do one for you, but the notch and hardware for the footswitch are Mesa only items.

 

[Russ]

Thanks again man.

You are definitely an asset to these boards and those of us that you help.

 

[Boogiebabies]

Not a problem. This is a great group of Boogie fans.

 

[Russ]

OK I have a Mark IV case coming. I have hooked up the reverb tank now and find the reverb a little weak sounding. It is functioning properly. Is it just because the reverb is weak in a Mark IV? Will a 12ax7 make a difference compared to the 12at7 or will it be hard on the sound or circuit?

 

[Russ]

HI Britney,

Nice to have you here....

would you like to say something?

 

[screengrid]

Hi Russ,

how are you, I hope all is well. Regarding the reverb quality of the MK IV. My opinion is, that the reverb of the MK IV isn't weak at all! I would say, it is still one of the best tube reverbs, one can get on board of a tube amp, never had problems with the quality and you know, I own still my MK IV combo from 1990, which should be a MK IVA and has the 12AT 7 for the reverb circuit! I use it predominantly, also in combination with a stereo setup, to fatten up the sound a little. Knob setting mostly around 4-5. But also with higher setting around 6 -8, it sounds good, very 3-D like, deep clear and rich. I oftener have the opportunity, to compare the reverb quality of the MK IV with other tube amps, due to my electronic
activities ( restaurating / repairing other tube amps )
IMO, there must be something wrong with the reverb circuit of your MK IV, if it sounds weak as you say, perhaps the 12AT7 itself.

There are some differences in specs between a 12 AT 7 and a 12 AX 7, not only the amplification factor as you surely know.

Here are some characteristics and typical operations of both tubes by a given plate voltage of 250 volts for example:

12 AT 7

amp. factor 60
transconductance 5500 micromhos
plate current 10 mA

12 AX 7

amp. factor 100
transconductance 1600
1,2 mA


But you surely could try a 12AX7, just for knowing the difference. Ther should be no risk of damage, if carefully operating.


regards
Arno

 

[Russ]

I think my main problem was trying to get decent reverb and still use high gain. I am going to be trying some lower gain and a different guitar soon. If it is still not as prominent as I would liek it to be then I may try a 12ax7. I just don't want it to be too much. I guess I could always try a 5751 also. Thanks for your response though. There hasn't been much action on this topic in a while.

 

[Shep]

i just looked in my mk iv a and insteed of the 12AT7 there was a 12ax7 ...thing it does is make the reverbs to loud for high gain guitar...so the clean would sound great...but the rhy2/lead wouldn't..

 

[Russ]

Ok, that sounds like a real world answer. Thanks Shep. I know that reverb kind of disappears anyway with high gain or makes it sound terrible. I was just hoping to figure out a way to make the reverb a little more prominent in the clean.

I once played in a volcano in Italy. The natural reverb was incredible. It was like having delay but better. In Naples, there is a park in a volcano. When I was in the Navy we went there for a ship's BBQ. We played for hours. There were a few bands on the ship and of course there was a dJ or 2. I remember during setup and sound check listening to the slap back echo thinking it was really cool. Had I had more time I would have done more with it. I just used it a couple times like an effect during the set. I hit an open chord and then muted allowing the natural effect carry the sound playing back to myself. I am not sure what it sounded like below the stage but it sounded cool where I was standing. The notes coming back during the solos were a little distracting except when I could get them to harmonize a few measures back to me.

 

[Shep]

thats cool..yeah echo like that can be really good for guitar sounds...