Tone controls - loss of range?

[Machine Gun]

Got a question for any of you amp techs. It seems like I've lost the range on several of my tone controls on my Mark IV. Could this be a tube issue? Any help would be greatly appreciated.

 

[lerxst88]

perhaps a cracked solder joint? The tone controls on my mark iv are very subtle especially on the lead channel

 

[tim6string]

If you have the treble turned way down it will greatly affect the range of the other tone controls.

 

[dodger916]

Got a question for any of you amp techs. It seems like I've lost the range on several of my tone controls on my Mark IV. Could this be a tube issue? Any help would be greatly appreciated.

Is this on all channels?

 

[Machine Gun]

Yes, it seems to be on all channels. I actually run my treble on the high side too. 6 or so. One thing I like about the Mark IV is how responsive the tone controls are, and for the last few weeks it's lost it's range. Usually you can get so much treble out of it that it will make your ears bleed. So I'm at a loss at the moment. I've never changed the tubes. I think I got it in 2008, but it doesn't exactly get over used either.

Any ideas? I may give Mesa a call.

 

[dodger916]

Mesa might give you the canned answer, namely "buy new tubes from us", and it could be a tube issue, but not necessarily. 12AX7s last a long time, and they typically get noisy or microphonic before they lose there amplifying ability. Power tubes don't last nearly as long as preamps, and their death is usually preceeded by a gradual degradation of tone and volume, and a loss of dynamic range. If you have no spare tubes, you should get at least one pair of power tubes and one 12AX7. (There might be a spare 12AX7 in the plastic retaining clip inside the amp cab.)

I assume you don't have spare power tubes, so run the amp in Class A. If no change, power down and swap the inner pair with the outer pair and power up again. If no change, it might not be power tubes, but you should always have at least one pair of power tubes and one 12AX7 on hand anyway.

If that produces no results move to preamp tubes. If you have a 12AX7 tube that you know is good, install it in V1 and see what happens. Because the issue is global, I would try V5 next, then try the other sockets. If you don't have tubes, I would swap V4 and V1, then V4 and V5. etc.

 

[Machine Gun]

I did try swapping V1 with the spare 12AX7. No change, but that's as far as I got. I will try the others.

Thanks.

 

[chipaudette]

Do the tone controls for the other channel have any effect? Like, if you're in the clean channel, do the tone controls for the lead channel have any effect at all? They should have none. If they have *any* effect, it sounds like you've got some weird channel switching going on. LDRs or relays gone bad.

Alternatively, within a given channel, do all the tone controls (treble, bass, and mid) seem less effective, or just the treble?

Note that R2 shares some tone controls with R1. Do any of them work better in R2 than in R1?

Chip
BTW, I don't think it's tubes at all.

 

[dodger916]

BTW, I don't think it's tubes at all.

+1, but at least it's east to test.

 

[seth155]

i lost the function of my mid knob but only on the gain channel. this was with my 50-cal+
i took it to 4 people before sending it to Monsta-Tone. i think among other things he had to replace some LDRs (what ever those are)

 

[chipaudette]

LDRs are one of the more common items to fail. My questions to the OP were aimed at determined which LDRs might have failed. It's also possible that the circuit elements that create the LDR voltage rail might have failed, which means that none of the LDRs are working properly. Fixing that might only require some caps, diodes, or resistors, which might be cheaper than replacing a bunch of the LDRs themselves.

BTW, LDRs are the type of electronic switches used in certain places within Mesa pre-amps to do channel switching and to do switching of some of the tone control elements. They're "Light Dependent Resistors".

Chip

 

[Machine Gun]

Well, I had been running my amp in tweed, so I did swap the pairs of 6L6's for the heck of it. It seemed like I regained some range on my tone controls, but then when I switched then back, I did not notice a difference. I guess it seems like my tone controls are working again, but maybe not quite the range as it had brand new.