no difference between clean and fat

[rick16v]

Hello all, my name's Rick. Great forum, sorry my first post is a question.

I just bought a second hand Mark v head, stock tubes, no mods. On channel 1 there is no audible difference when I switch between clean and fat mode. Tweed sounds different, but clean and fat are identical. Can anyone advise please? Have I bought a broken amp, has anyone seen this problem before?

 

[bandit2013]

Rick16V, Welcome to the forums. I started out much the same way, with a question...

There is a slight difference between the two voice modes clean and fat but the essential tone and gain structure is almost the same. Fat will provide more gain than clean. You should also notice more bass and lower mid boost in the fat setting compared to clean. It is more noticeable in 90W mode compared to 45W (if using tube rectification) and 10W mode. There is also a notable difference when using the full power compared to variac power mode. Clean should be bold but yet offer shimmering highs for a piano like tone on the low frequencies and crisp notes on the upper frequencies. More of a dry acoustic guitar character (especially with an open back cab, not so much with a sealed 212 or 412). Also makes a difference if CH1 is set to bold compared to normal. (switch between the gain and master control). It may also depend on type of guitar you are using. A Fender Strat for instance, does not have as much amplitude in the guitar signal compared some other guitars that have humbucker pups. I have compared my friend's strat though all of my amps to my guitars including variations of single coil to humbuckers (on full or tapped), most of my guitars have higher output capacity than my friend's Fender. Generally I find when using CH1 with humbucker equipped guitars in the neck position, I have to use the coil tap due to the excess low end (unless I roll off on the volume control of the guitar). I do not believe you have an issue.

 

[rick16v]

Thank you for your reply. I have it on 90w, pretty loud, bold switch on, I cannot hear a difference at all. There isn't a pop or anything either, to hint that something is changing. Les Paul standard with Doug Aldrich humbuckers.

Crappy phone video, but surely there should be a noticeable difference?

 

[bandit2013]

I did not see any image but in the sound clip I can hear the switch being changed. It did not sound like Fat was even being enabled. Character should sound thicker and a bit darker in tone, sort of bluesy character if that describes it.

Has the amp chassis ever been removed from the shell ( if used that may be unknown). It is possible one of the ribbon jumper cables may be loose or not seated properly which is an easy fix, use extreme caution if you decide to pull the amp chassis out (unplug the amp, and leave the set the stand by switch to on to discharge stored energy in the power supply caps. It is tricky if you never pulled the chassis out before. It could be something unexpected too or too complex to repair. solder joint on the switch for CH1 voice, relay not switching, or JFET issue... etc. I would inquire about time, cost for service with a certified and qualified Mesa tech before taking on any repair on your own if it is more than just a loose connection. Does the amp have any warranty? It should be transferrable if the amp is less than 5 years old.

 

[rick16v]

It arrived through the post. Its second hand so I don't know if it has had the chassis out previously. All i have done is fit the output valves which were removed when it was posted. The seller seems reasonable, he suggested maybe something happened during transit. Not sure where to go from here. How tricky is pulling the chassis. If the switch needs soldering or a,connector needs pushing on, that's easy. What's the shock risk on these things?

 

[bandit2013]

Removal of the chassis is not too difficult. First start with the amp un-plugged. Then turn off the standby switch as if you were going to play though the amp. This will discharge any stored energy in the large power supply capacitors. There are bleed resistors in parallel to the very large caps, but there are non on the various high voltage capacitors after the stand-by switch. Setting the standby switch to operate the amp will discharge nearly everything that stores energy. You can leave the power switch in the off position as this will not do anything to discharge the caps.

Since this is your first time to actually remove the chassis, best to remove all of the power valves (all 4 6L6 or EL34 tubes) as well as the Rectifier 5U4GB. Keep note where the Rectifier tube is installed. If you forget, it installs closest to the power chord.

First you must loosen the 1/2inch nut with a 1/2inch (13mm) box wrench that secures the support post to the power transformer. Once free, spin the nut so it moves up towards the amp chassis. Next would be to turn in the opposite direction the support post, use a 5/16 inch box wrench (8mm) to loosen the post. Once it is loose, you can turn the amp on its side for a better grasp to rotate the mounting post by hand. It is not necessary to lower the post more than 3/8 of an inch (10mm).

Next step is to loosen the top mounting screws. Most of the Tolex and or leather covered amps will have two metal straps (hard wood versions will not.) Note: only loosen all 4 mounting screws but do not remove them until all 4 screws are loose (the metal strap may end up turning if you take one of the screws completely out which may tear the covering.) Remove the screws starting from the front of the amp to the rear. You may need to support the chassis with one hand while doing this, especially with once one side has been freed from the mounting screws.

You will need a small flab blade screw driver for the next step. Carefully pull the amp chassis out part way. There should be two wooden side bars at the back of the shell that will support the chassis. Use the small screw driver to pry out the white reverb return plug from the jack, try not to damage the jack. Fit is usually tight. You will also need to support the chassis momentarily while removing the one plug. The molded plug is the reverb send which can be removed by hand without any tools. Both use RCA type plugs.

Once the reverb cables are free, carefully remove the chassis completely out. You can set it on the transformers only if you removed the large bottle power valves and rectifier, if not, you will need something to support the chassis to prevent damage to the power valves (tubes).

The below picture shows the inside view of the Mark V (amp chassis on the bottom of the photo, one on the top is the Mark IV.) You can see the several gray ribbon jumper cables that connect the front panel to the main amp board. If you need to solder a switch, good luck as the disassembly process is extremely difficult. If you inspect the area closest to the CH1 section you may find what is causing the issue. If it is just the ribbon jumper cable, that may be easy to fix. You may want to check the wires on the ribbon cable since they use insulation displacement type terminals, one wire may just be loose in the cable (or broken). Good luck with your investigation. Special note: Do not leave the standby switch in off position when you re-assemble the amp. It won't hurt but when you turn the amp back on it will take a long time to reach operating voltage if the standby switch is not set to standby.

 

[OldTelecasterMan]

Shock risk... the CAPS can kill you. One hand at a time don't give current a path through your heart...ie don't put both hands in at the sametime.

Listened to the Video. There should be a change and I heard no change. I live on the west coast and Mesa Hollywood guys are awesome. The technician John Is very good and listens to you. He has fixed my amps every time I had an issue. Great guy, very knowledgeable well all the guys at the Hollywood store are great.

I just take my amps in. It's never very expensive for repairs and you know it was done right.

 

[bandit2013]

Regardless if you believe the caps are discharged, consider the advice provided by OldTelecasterMan. He is absolutely correct. The DC voltage potentials stored in the capacitors, including some of the smaller caps (does not need to be a large can electrolytic either) have the potential to cause cardiac arrest (worse case) or permanent nerve/tissue damage. You cannot make the assumption that the bleed resistors are sufficient. If you are not experienced working with similar equipment, lack the proper PPE (personal proection equipment), and suitable skills to work on potentially live circuits, it is best handled by qualified technicians (certified by Mesa). My mistake is the assumption that you may have the same skills that I have. I apologize if I have posted this in error due to my negligence. Be SAFE !!!! One mistake is all it takes.

 

[sduck]

Thank you for your reply. I have it on 90w, pretty loud, bold switch on, I cannot hear a difference at all. There isn't a pop or anything either, to hint that something is changing. Les Paul standard with Doug Aldrich humbuckers.

Crappy phone video, but surely there should be a noticeable difference?

I can hear a definite difference, even with this lousy video. There's not going to be much difference with the gain that low. Try turning the gain up past noon and seeing if there's more difference - it should be fairly obvious past a certain point of the gain knob.

Don't take it apart just for this - take it to a tech if you still can't hear any difference.

 

[bandit2013]

I took another look why Fat and Clean may sound the same.
Set the tone controls for CH1 to noon (treble, Mid, and Bass). You should at least notice Fat voice will increase the low end and make it sound on the loose side, clean voice will keep the low end tight and crisp. However, if your power tubes are lacking the bottom end you may not notice a difference between clean and fat, same would apply if you dialed out the bass on CH1, there will be no difference between the two. It could be as easy as having old tube (preamp or power). As for a popping noise, it will make the same amount of noise as changing from normal to bold (there will not be any noticeable pop from changing the switch with the exception of selecting the TWEED voice which makes many changes to the gain structure of CH1 along with other things such as reverb attenuation.

 

[rick16v]

Thanks for the replies. There was no difference at all when flicking the switch, the amp was faulty. I bought a replacement circuit board complete with switches from america and fitted it, amp now works fine.