Mark IV Tube Task Chart, Description of tube functions

[Irongiant14]

Hi. Can someone please help me understand the "Description of tube functions" on page 17 of the Mark IV Revision B manual?

I'm trying to diagnose why my combo is blowing fuses. It's got four new tubes in slots A,B,C,D; they're about 8 months old. None of them glow red hot when I look back there. I called Mesa Boogie and spoke with a tech and he suggested I swap out the 12AX7 tubes with the one replacement I have until I identify the culprit.

I'm trying to understand the amp and decipher how they described the tube functions but it's confusing.

V1A=Input stage. Does that mean that the V1 slot of the 12AX7 and the A slot for the 6V6s together make the Input stage?

Other descriptions make me think this is the key to understanding it, like V2B=FX Return. But there is no mention of slot C or D, i.e. no V4C=something, so I wonder if I am thinking about it wrong?

 

[hugsy41]

Hi Irongiant14,

A 12AX7 tube is an 8-pin tube which contains 2 separate triodes. Each triode has 4 pins (one pin each for the heater, anode, cathode and grid) which manipulate the gain of the signal. Since there are two gain stages in each V(#) socket, to be able to distinguish these triodes from each other, the nomenclature 'A' and 'B' are used.

As you mention below, V1A is the input stage to your Mark IV. That means, one of the two triodes in the 12AX7 in position V1 is used for the first amplification stage of your amp. What this also means is, if either one of the triodes is faulty in a particular position, the whole tube needs to be replaced.

Finding out the roles of each "half" of the 12AX7 also helps with fault finding. Determining what works (audibly, from your sound) and what doesn't to the roles each 12AX7 is designed to do will help you narrow down potential problems.

Having said all that, a power tube is generally the more likely culprit for blowing fuses. Don't forget that with Mark IV amps, to run 6V6 tubes, the amp must be in the Tweed power mode, simul-class setting and the speaker plugged into the 4 ohm jack (regardless of the speaker ohm-rating).


Chris

 

[woodbutcher65]

Preamp tubes don't draw enough current to be likely suspects when the amp is blowing fuses.

More likely it's a power tube.

First and easiest test: Yank out all the power tubes and turn the amp on. If it still pops the fuse, it needs to see the amp doctor. Likely candidates at this point are filter caps (you can expect to replace them at some point if they're more than 10 years old, and you SHOULD replace them every ten years to maintain performance) or, hopefully not, a transformer. If either transformer is bad it can blow fuses even with no tubes installed in the amp.

If the fuse does not blow, turn it back off and install one pair of power tubes. Turn the amp back on and see if it works. If it blows the fuse, take out that pair of tubes and put a different pair of tubes in, hopefully a pair that is known to be good.

A bad tube can cause the fuse to blow. You will have to isolate which is the bad one.

 

[Irongiant14]

Hi Irongiant14,

A 12AX7 tube is an 8-pin tube which contains 2 separate triodes. Each triode has 4 pins (one pin each for the heater, anode, cathode and grid) which manipulate the gain of the signal. Since there are two gain stages in each V(#) socket, to be able to distinguish these triodes from each other, the nomenclature 'A' and 'B' are used.

As you mention below, V1A is the input stage to your Mark IV. That means, one of the two triodes in the 12AX7 in position V1 is used for the first amplification stage of your amp. What this also means is, if either one of the triodes is faulty in a particular position, the whole tube needs to be replaced.

Finding out the roles of each "half" of the 12AX7 also helps with fault finding. Determining what works (audibly, from your sound) and what doesn't to the roles each 12AX7 is designed to do will help you narrow down potential problems.

Having said all that, a power tube is generally the more likely culprit for blowing fuses. Don't forget that with Mark IV amps, to run 6V6 tubes, the amp must be in the Tweed power mode, simul-class setting and the speaker plugged into the 4 ohm jack (regardless of the speaker ohm-rating).


Chris

Thank you for explaining this, Chris and Woodbutcher65. I tried replacing my new tubes with the very old tubes that they replaced and made it through a couple hours of playing without blowing a fuse, so it seems, as you both wrote, that the new power tubes are the source of the problem after all. It's disappointing because they're past the 6 month warranty. Do I need to get another matched pair? Or if there's only one problem tube is it advisable to just order one? I have not yet isolated what tube(s) are problematic, so I actually don't know how many I'll need. But there's another wrinkle.

When I studied the back of my amp, in my last troubleshooting session, I noticed that some of the settings on the back of the amp had been changed. My toddler got to it, basically, and I didn't realize how fast he was, the little tweaker! What changed was the Ground rocker was switched from OFF to B and SIMUL-CLASS was on CLASS A. I always have used TWEED power and SIMUL-CLASS before but was not aware that Tweed was a must with 6L6s. So, NEVER do FULL power or CLASS A with 6L6s? May I ask why? I don't remember reading about this before and I've read the manual a few times, although I confess I am still learning about this wonderful machine, so maybe I didn't comprehend what I was reading.

I reset the above settings to Ground OFF and SIMUL-CLASS and tried the new, problem tubes and still got a lot of dangerous sounding hum. As I said before, the old tubes had much less hum and didn't blow a fuse. Do you think I may have accidentally 'fried' these tubes by running them in CLASS A or GROUND B? It couldn't have been more than 3-6 hours of playing time.

Likely candidates at this point are filter caps (you can expect to replace them at some point if they're more than 10 years old, and you SHOULD replace them every ten years to maintain performance) or, hopefully not, a transformer. If either transformer is bad it can blow fuses even with no tubes installed in the amp.

I had not heard of a FILTER CAP before. I found a thread on here and read through it, When to replace filter caps? http://www.grailtone.com/forum/viewtopic.php?f=3&t=12138&start=105 My amp is early 1990s so I guess it's past time for preventative maintenance, at least. I don't like the idea of them exploding on me!

I built my own instrument cables and rewired my Treble/Rhythm switch but don't know if I'm equal to the task of replacing these filter caps myself. Is it a recommended self repair? If not, can anyone recommend a good person to do the job in Boston, MA? I'm right up the road from Berklee School of Music, so I hope there is someone close to the School. I asked an acquaintance that works there to ask around but haven't heard back yet. The Mesa website lists official repair shops but they are all outside the city.

Thanks again to you both for your replies!

 

[woodbutcher65]

If you know how to run a voltmeter, a soldering iron, and a screwdriver, you can change out the filter capacitors yourself.

They'll be the big blue cans inside the amp. Two very large ones on the power supply board and three big blue ones on the main board.

Actually there may be more. I haven't actually been inside a Mark IV. If there are more big blue caps on the bottom side of the power supply board, replace them too.

There are also some smaller (usually brown) caps that you might want to replace too, but they're more difficult as the PC board has to be removed to get to their solder connections. Radial lead capacitors always have to be changed from the bottom. Axial lead caps (one lead sticking out each end of the can) can be replaced from the top if you're careful. Be sure to trim the leads to be long enough but not TOO long. You would not want the lead ends to contact the metal chassis underneath!



Confirm the values and order replacements from Mesa. Mesa's caps are as good as any made today and their price is as good as anybody's so just get them from Mesa and be done with it.

You should need 2 220 uF 300 volt caps and 3 30 uF 500 or 450 volt caps. Plus the smaller ones.


Make sure the amp is off, unplugged, and all voltage has drained down out of the blue caps on the boards, using your voltmeter from cap positive leg to ground. There needs to be no voltage in the caps when you do the work.

Just replace the old caps with the new ones and do pay careful attention to getting the polarity markings right. Caps that are installed backwards will blow their guts out in a few minutes of operation and the amp won't run with the caps in backwards anyway.

 

[Tuna141]

I always have used TWEED power and SIMUL-CLASS before but was not aware that Tweed was a must with 6L6s. So, NEVER do FULL power or CLASS A with 6L6s? May I ask why?

I noticed you misinterpreted Chris' post. He stated that Tweed must be used with 6V6 tubes not 6L6. You can use all modes/settings with 6L6 tubes.

 

[woodbutcher65]

Tweed reduces the voltage to the tubes. The 6V6 REQUIRES this. Try to run a 6V6 at the 6L6 position and it will die rather quickly, and maybe with an impressive display of lightning bolts inside the tube.

A 6V6 just can't handle the higher voltages.

 

[Irongiant14]

Thanks Tuna141 & woodbutcher65, for these responses. I did read them at the time but must not have replied.