New mark iv combo owner with some huge concerns.

[lessarti]

I have recently acquired a beautiful mark iv combo that i play through my 4x12 cab. Sounds really awesome but i am noticing a hum which is pretty loud. It came with 6l6s a.k.a kt66/5881 tubes by groovetube and i tried swapping the outside two for a pair of mesa str447 el34s and a pair of str440 6l6s.. to my surprise the hum is much louder and just is ridiculous. Any ideas or help here guys?? This is killing me to get a nice amp only to have a hum noise problem. Secondly, im noticing from the back looking at the rear the left and middle left tubes seem to be red plating as well. :x

ANY responses with ideas or knowledge from experience are HUGE and greatly appreciated!

 

[lessarti]

Minor update, i tried even replacing the v1 preamp tube to see maybe if it was old and causing the hum, nothing changed. Still a loud hum that is unavoidable + serious red plating on both left tubes from the rear..

Oh and a NEW update now for those that are reading, the amp is no longer working and no longer powers up, i assume i blew the fuse. Checked the fuse and it doesn't have black burnt marks but i would still say the filament is not straight through and looks disconnected.. this is horse$hit... i've had the amp for literally 2 days and nothing but problems.

 

[Restless Rocks]

If you had red plating tubes you should have shut it down immediatly and find a tech. Could've been a bad tube, blown screen resistor, or failed bias supply. Simply replacing the fuse will not fix the problem. And also please place blame on the previous owner as these are very fine and reliable amplifers but unfortunatly many have been abused and dumped on fleabay. Find a tech before trying anymore user remedies as it is now it may be a simple repair for a qualifed electronics tech. Replacing fuses etc. can make it worse than it is now.

 

[stephen sawall]

If they are red platting ..... it could be number of problems. As said you do not want to run it at all if it has this problem. It well lead to more problems.

 

[Turumbar82]

If you bought the amp used from someone then you are inheriting whatever problems that person had or caused and weren't willing to fix.

 

[lessarti]

thanks for the replies thus far guys, I have recently taken it in to get checked out by a tech, i got so upset previously because i realise how good an amp this really truely is.. i just was getting somekiller sounds so fast.. but alas this is how it is meant to be.. i think it may be saved and running on all cylinders once again! I just have to make it waiting lol

 

[stephen sawall]

Glad things are looking like it well be positive. The IV is a great amp.

 

[dodger916]

Good luck, and please let us know what the tech finds.

 

[lessarti]

I will do that, it may take a while though like the end of next week to even find out if it is fixed or what the problem was/is. I'll keep you guys posted eventually.. some amp prayers that it is fixed and back home fast would be extra special! :mrgreen:

 

[Force]

I hope it gets fixed soon. Beyond any serious problems/issues, I was going to say that on the lead channel, if the lead drive and the lead channel gain are both up high past "8" at the same time, the amp can get a bit noisy, which may be enhanced further by other things in the signal chain. All the best to you with your Mark IV, it is still one of the best amps on the planet today imho.

 

[Boogiebabies]

The bias supply feed is the two left tubes (Blue Wire) and the right two tubes (Brown Wire).
From the terminal strips the voltage is sent to the 150K for the outer sockets and the 2.2K for the inner.
It's not too hard to see if you have a bias supply problem at these two points. Like RR said, if you have a
resistor feeding the screens go down, which red plating loves to eat them, then voltage to the screens may be way
up and drawing current on the left bank of tubes.

Good Luck.

 

[lessarti]

BB i think you are on to something, while i dont have the amp with me and it is in the shop awaiting a tech to check it out and fix by next week, i still did some reading about red plating and im expecting or anticipating a bill of fixed resistors or cap in the two left tube sockets as they were the main culprits for red plating. The loud hum could also be a result of this too, it was not because of high gain i know that hum as well.. this was almost a grounding hum. We shall see, im optimistic as it is in good hands.

On a side note, i just scored a mesa mark iii red stripe with simul class / eq, this thing is AMAZING!! A bit diff flavour than the iv and i love it! :shock: I am shocked at how good it is.. i tried a purplse stripe way back and it sounded way more stiff than this thing... are red stripes considered "good" or the "better" in any way to other mark iiis ? I read this thing is zeroed into the c+ lead mode.. hmmm interesting.

 

[Boogiebabies]

A loud hum is common when tube are red plating.

As for MK III's, the Red Stripe and Green are the best sounding to my ears.

 

[lessarti]

Ok, thanks bb, that is awesome to hear because that means chances are the humming + red plating is the same issue taken care of with one stone.

As for the red stripe, yes this thing is godly sounding, i cannot believe i didn't venture into the mark iii territory SOONER. Both simul and class a just rock, class a is balls to the wall though! Love this thing!! I played it today and tweaked an awesome tone, finished up turned the amp off and forgot about the presence on the back! lol i STILL could have been dialing.. much easier to dial in than the mark iv ofcourse :lol:

 

[lessarti]

Minor update for those reading; called the store i brought it to and the amp is on its way to some shop and will be 3-4 weeks until it is back. Bummer for the long wait as i am not one to be patient when it comes to amps.. but it sure is a good thing i picked up that mark iii now, so lots of time to tweak this thing perfect. 8)

 

[dodger916]

Minor update for those reading; called the store i brought it to and the amp is on its way to some shop and will be 3-4 weeks until it is back. Bummer for the long wait as i am not one to be patient when it comes to amps.. but it sure is a good thing i picked up that mark iii now, so lots of time to tweak this thing perfect. 8)

I'm still reading. Looking forward to hearing the resolution.

 

[lessarti]

Da lada da da da daaaaaaaaaaaaaa IT IS BACK!

Got a call today and picked up the amp!

The resistor for the fixed bias supply, fuse and some screws were replaced and she is good as new! Popped in some brand new mesa 6l6 440 and el34 447 str tubes and she is fired up and roaring again! I recently have just really dialed in my newly acquired red stripe mark iii so when i got to tweak the iv i immediately noticed a more mid range tone to what i had tweaked at least.. main differences noticed thus far between the red stripe and mark iv are the iv seems more compressed, mid rangey (harder to scoop with just 750slider down while the mark iii 750slider is more "effective") and the iv seems to have more of a harmonic lead tone to it! Either way the III is still a more aggressive heavy thick tone so both amps really are unique.. i was afraid they would both be pretty much the same deal tone wise but alas they are quite different when you fine tweak.. i'll be tweaking both still for days i bet.. or months

SO happy to have the IV back! thank you amp tech and tone gods!!

On a side note: Is there anyone that can recommend a good high gain replacement preamp tube if one was to become microphonic to replace v1 or v2 with? Groove tubes 12ax7c is currently in v1 as well as all other spots and i find it works best compared to a tung sol and ecc83 jj, the tung sol i tried in v1 was REALLY noisy and crackly and it is a new tube so im kind of puzzled since i read so much about them being good :? )

 

[stephen sawall]

With all new tubes there well be some bad ones ..... it is true of everything. Glad you have it back. :mrgreen:

 

[dodger916]

Great news! Glad it worked out well.

 

[mrd]

Try and score a NOS Mullard for V1 :twisted: