Mark III clean channel volume fade-in.

[Jona]

I've recently noticed that if I constantly strum a note/chord on the lead channel then switch to clean (R1), the volume comes in quietly, then 'blooms' up to its correct volume within about half a second. It's quite subtle but now I've noticed it it's bugging the hell out of me.

Is this an issue with my specific amp, or an idiosyncracy of all mark IIIs? I know some amps (Marshall JCM2000 springs to mind) have a circuit that prevents relay popping by momentarily dropping signal; is it a version of that? In either case, is there any way to stop this happening?

If it helps, mine is a blue stripe combo.

Thanks

 

[jpdennis]

I noticed something similar with mine but never though anything of it since it was extremely slight with my amp. For other reasons I drove to Petaluma a couple weeks ago and Mike Bendinelli did and end-to-end service & repair. Mike said he will use his "fine tooth comb" to go over it! :lol: I pick it up tomorrow so I see if it is "normal" and let you know.

Dennis

 

[Neptical]

I don't have this problem with either of mine. Have you tried switching out the V3 ( lead mode) tube to see if it goes away? or V1/V2 even? Could be as simple as a faulty tube.

 

[Jona]

I'll try swapping valves when I get home, although it seems like an unlikely cause... As I understand it, V3 is engaged when the lead channel is selected; all others are active constantly. The volume drop is only apparent when switching from lead to clean, not the other way round. Also there's no evidence of noise, microphonics or wavering output which I would expect from a faulty preamp valve.

I haven't tried switching between R2 and lead, or using the push/pull to change channels instead of the footswitch so I'll see if that solves the problem.

 

[MrMarkIII]

Check the LDRs. These Light Dependent Resistors handle the channel-changing. My guess.

 

[boogiemon]

i think this is a feature designed to keep the reverb circuit from flooding the clean channel w/signal from the gain channel when switching from ch3 to either ch2 or ch1.

 

[CoG]

What boogiemon said-- this is a feature, not a bug.

 

[dgr888]

LDR's are slow to change which is what causes the lag in volume you are noticing. Mesa used/uses this type of LDR switching in almost all their amps to eliminate audible pops (especially in high gain situations) caused when switching mechanical relays in and out of their circuits. As a side note LDR's are solid state devices and running a signal through to another stage defeats the purpose of an all tube amp. Mark 4's have somewhere between 20-30 of these switching different functions!! .Luckily the earlier Marks have only a few of these in the circuit.

 

[>Photi G<]

As a side note LDR's are solid state devices and running a signal through to another stage defeats the purpose of an all tube amp. Mark 5's have somewhere between 20-30 of these switching different functions!! .Luckily the earlier Marks have only a few of these in the circuit.

Actually, no. And, no. LDR's are quite simple devices. You could make one yourself if you wanted. It's basically an infrared LED pointed at a photocell (light dependent resistor) all cast in plastic. It causes a negligible amount of signal degredation, and no noise when switching, which is why they were used in mark IIC's to mark IV's. The mark V uses no LDRs. It uses around 49 relays for channel/mode switching, as the "pop" made by relays during switching has been fixed by better quality relays, and improvements to the supply circuits. Still, when switching channels on a MKV, you can hear a bit of a pop, since these devices are after all, mechanical.

 

[Jona]

Hmmm....

Neptical, any chance you could recheck your mk. IIIs to make sure this isn't happening? It's just that it's a VERY subtle effect so it could easily be missed, and I'm getting mixed messages as to whether this is a bug or a feature.

TBH my instinct is that it's deliberate- it seems too subtle to be a fault, and I have experienced similar effects in other amps. In the Marshall I mentioned I found it intolerable (there was actually a momentary silence!), whereas in the mk. III I think I could live with it.

As a secondary question, how do other manufacturers get around this problem? My old Peavey 5150II switched instantaneously and with no audible pops, why can't Mesa and Marshall use whatever Peavey use??

 

[Neptical]

Hmmm....

Neptical, any chance you could recheck your mk. IIIs to make sure this isn't happening? It's just that it's a VERY subtle effect so it could easily be missed, and I'm getting mixed messages as to whether this is a bug or a feature.

Sure, when I get back to the studio tomorrow I'll crank it and check it out.

 

[CoG]

The Roadster does a more advanced version of this, from the manual:

"When switching Channels with Reverb applied or switching the Reverb itself in and out of the signal path with the Footcontroller,
it is normal to experience approximately 1-2 seconds of delay before the Reverb effect is mixed back in with the dry signal."

"It is normal to experience approximately 1-2 seconds of delay before the REVERB is mixed with the dry signal after switching
the REVERB or Channels. This intentional built-in delay prevents both switching noise from the hard bypass relays used to isolate
the pure dry signal from the effected REVERB signal, and the prior Channels sound coming through the REVERB in the current
Channel when switching. At first this may seem strange, but after a couple minutes in real world situations you will see that it is prefer-able to the embarrassing alternative of a loud overdriven crunch chord drowning out your beautiful clean passage in the middle of a breakdown."

I'm 90 per cent certain that someone got the "official word" on this from Mike B. on this re: the III, but I can't narrow down the search terms enough lol

 

[dgr888]

Sorry my mistake and I stand corrected concerning the Mark 5 as I meant to say it's the Mark 4 that has 20-30 LDR's and less in earlier models. An LDR is not a true(carbon based) resistor as the photocell which passes the signal is a high resistance semiconductor (cadmium sulfide) that changes when light is applied. And as with any semiconductor they have the ability to create noise/signal degradation and frequency response changes when a signal is passed through it. The LDR's slower change from an on/off state and vice versa allows the coupling caps connecting the circuits to discharge slower therefore creating no audible pops. The other side effect of an LDR is if the supply circuit feeding the led is unstable in the least it will cause fluctuations in the signal being passed.
It it actually should be called a variable semiconductor..

 

[Jona]

Out of interest, does this effect occur in the mark IV and V amps?

It would be very strange if they did not fix this glitch with the available technology, especially in very expensive flagship models.

 

[dgr888]

photo g's post says they are using relays in the mark5 so it should be resolved in that model but the mark 4 still uses LDR's and could develop the same kind of issues over time.

 

[Soundside]

Hi all,
My MkIII has a pretty pronounced fade-in. It actually "gets in the way" of things sometimes (first lick or two after the channel switch gets lost). Is there a way to eliminate this "feature" or are there any mods that could be done to minimize the effect? Could it be that the LDR is just "weak" and needs to be replaced?

Thanks!

 

[boogiemon]

Hi all,
My MkIII has a pretty pronounced fade-in. It actually "gets in the way" of things sometimes (first lick or two after the channel switch gets lost). Is there a way to eliminate this "feature" or are there any mods that could be done to minimize the effect? Could it be that the LDR is just "weak" and needs to be replaced?

Thanks!

i'd say give Mesa a call & ask them.

 

[dgr888]

Yes it can be replaced. Your tech will have to try a few of these because of the variances some will perform better/quicker than others.