Mark IIC changes in tone

[Eddy Scheepers]

After playing / practicising about an hour there is a serious change of tone quality in my mark II c. From transparent and warm with nice bass and sweet highs it becomes muddy even distorted, loses the "life" in it, has an extreme "mid" accent and doesn't cut through any more. I mainly (almost exclusively) use the CLEAN channel with my pedal steel. Turning it off for some time (so it cools down) helps, but only for a short time. What could be the cause of this problem? I changed all preamp and poweramp tubes (original boogie). I live in Belgium, Europe. Does anyone in the region here know qualified technicians with an ear for tube amps?

Eddy Scheepers

 

[stokes]

Could be time for new filter caps.Could also be a heat related problem,some component drifting after it gets hot.I would do the caps first,tho.Best to find a tech in either case.

 

[Eddy Scheepers]

Thank you for your reply. I suggested this to my amp techician, but he assured had been over this before and all values stayed well withing tolerances....

Eddy

 

[Elpelotero]

Is it possible your ears just get overloaded and you lose the highest frequencies?

I obviously don't know how severe the tone change is. If it were your ears, it's a subtle thing.

 

[Eddy Scheepers]

I've been thinking about this and it could very well be. I am often practising for hours -though on a very reasonable volume - but what about the distortion coming up, the loss of headroom and what's more - I haven't mentioned this yet- : the noise in the master volume, that is NOT THERE THE FIRST HOUR OR SO, with jumps in volume and cracks around settings 3 or 4?
I reported all this to the repair man, but he is convinced that "there is nothing wrong with this amp" A month ago - while my mark IIc was at the repair shop - I used a friend's mark I instead: no problems at all! Well... I think there is nothing mysterious about a guitar tube amp. Only: who can help me? Anyone over here in Belgium, Holland, Germany, France?

Desperately hoping way over the Atlantic.

Eddy

 

[mrd]

Have you done the easy stuff, like rule out a possible grounding issue, faulty cables, connections, etc?

Try every combination of guitars, cables, cabs, whatever that you can, to see if you can rule out anything. If your amp is plugged into a power conditioner, take everything else out of it and just run your amp into it, and run your guitar straight into your amp.

Also, be mindful of other electromagnetic interferences like lights, TVs, even the amp itself (it can cause the pups to produce a disturbing hum if you're within close range).

It may be something else, perhaps your output tranny. I don't know much about those at all, but you said it's fine at first then it gradually gets worse and turning it off to cool down will fix it for a while. It sounds to me like the heat of the amp is causing something to progressively falter. The main things that I know of that produce heat are the tubes and the output tranny.

 

[stokes]

The output tranny doesnt "cause" heat.If it gets hot you have a problem.The only component that creates heat in an amp and should be considered normal are the tubes.The PT may get warm,but if it gets hot to the touch it is being over taxed.I love when a tech tells you that your filter caps are okay because "values are within tolerance".I have seen caps oozing their guts out and still measure "within tolerance".Your amp is over 20 years old.There has been too much debate over this topic to go into the whole story again.After ten years or so the electrolyte in the filter caps begin to dry out.This occurs faster if the amp is left sitting for long periods without being used.This begins to cause many different symptoms,some more noticeable than others.The filters are the heart and soul of your amp.As I have said here before,when I get an amp in for a problem,that is as old as yours is,I insist on new caps.I give the guarantee that if the customer doesnt see the improvement,I will put the old ones back in and he doesnt pay a dime.In over 20 years I have never had to put the old caps back in.The symptoms you describe,loss of bass,life,doesnt cut through all point to old filter caps.You play clean with a pedal steel,new caps are definately in order.If you do change them dont use any of the Asian brand caps,they suck.I use Sprague or F&T's only.

 

[Eddy Scheepers]

Thanks for all the help so far. Now, which capacitors are most likely to cause the problem: the 30UF-500VDC (the one in the preamp or those in the poweramp section?)or the 220UF-300VDC (in the power amp section)? Should they all be replaced?

Eddy

 

[sundaypunch]

Thanks for all the help so far. Now, which capacitors are most likely to cause the problem: the 30UF-500VDC (the one in the preamp or those in the poweramp section?)or the 220UF-300VDC (in the power amp section)? Should they all be replaced?

Eddy

Yes, replace them all. It is like having tires on your car with all of the tread worn off. Replacing just one doesn't make any sense.

 

[stokes]

Yes change them all.Including the smaller bias supply caps,and any other electrolytics in that amp,I dont recall offhand,but there are a couple besides the power supply caps.