Troubleshooting volume fluctuation problem in Mark IV

[TFC]

Hi,

I've got a Mark IV version A 1x12 combo that I got new in 1991. Recently the volume of the amp will drop either suddenly or gradually, and then suddenly/gradually come back up and drop again, etc. It has happened with different guitars and cables, so I believe it is the amp. The last time it happened, I switched through all 3 channels on the amp and also switched between Class A and Simulclass. I tried tapping on each of the power and preamp tubes while I was playing and didn't notice any strange sounds or sparks or anything, but I was playing at low volume.

So, if it is a tube issue, I think it's a problem with one of the outer power tubes (A or D), or any of the preamp tubes except V3. Have I narrowed that down correctly? Any thoughts on which tube is the most likely culprit? I don't have any spare pre-amp tubes to test it with, and unfortunately the problem comes and goes, so it won't necessarily be so obvious when I replace the faulty tube with a good one.

Could it be something besides a tube? Any other troubleshooting I could do?

The preamp tubes are the originals. So they're almost 20 years old, but for about 13 of those years the amp was mostly unused. The power tubes I last changed 6 mos to a year ago. After all this time, this is the first issue of any kind I've had with the amp, so after almost 20 years I've got a noob question.

Thanks for any advice!

 

[Restless Rocks]

As you say your IVa has spent long periods of neglect er,um uh I mean non-use it is entirely possible the control pots, jacks and esp. loop jacks may have some oxidation. This will sometimes have the volume problems you describe. Also the "pull silent record" switch is known to be "ify" on these amps as well. A good contact cleaner/lubricant and a few rotations of the pots may do the trick. This requires pulling the chassis so if you aren't comfortable with this a good tech could give it a go over in a few minutes time.

 

[TFC]

Thanks for the tip. I think I'll give it a try myself. What do I need to do to protect myself from getting a nice shock, other than unplugging it and not touching anything other than the pots & jacks? Thanks again.

 

[MesaDCLP]

First thing I would do is change the tubes. The preamp tubes may be ok, but I change mine yearly regardless of their condition. That gives me a stash if one goes out. Also, I would change the power tubes too if you use the amp regularly within the past year. Of course, the first thing that comes to mind about your issue is not the tubes. It could be the caps, and they may need to be changed. I would suggest taking the amp to a tech to get them checked out if new tubes don't clear up the problem. Hope that helps.

 

[stokes]

Any amp that sat unused for 13 years definately needs a complete service,especially new filter caps and any other electrolytic caps.Your tubes may very well be okay,but those caps will have to go,they degrade much faster when they arent used for long periods.I would not even turn an amp on that sat that long without bringing the volts up with a variac at least.

 

[TFC]

Thanks everyone for your input. Based on Restless' advice I got myself some contact cleaner and cleaned out the FX send & return jacks. That seemed to do the trick. At least, I haven't had that problem return so far, and all channels seem to sound better. Much appreciated!

Looks like I should seek out a decent tech and get a long overdue tune-up before anything fails. Approx. what would a decent tech charge for this kind of service?

Thanks.

 

[GianBulls23]

Hi,

very few times also my MK IV short head is used to lose some volume and then restore it.
But my head has new Mesa pre tubes and very new STR440 power tubes.

Yesterday, for example, happened in Tweed power, the last 2-3 months ago.

Thanks for tip, I suppose was not a tube issue.