Servicing my Mesa Mark 1 re-issue - can I do this myself?

SquireJapan

I have a well loved (in every sense) Mesa Mark 1 reissue.

It's got a couple of issues:

1. Pre-amp seems to occasionally get farty when turned up. It's not consistent behaviour, even from 1 min to the next (with no changes to the amp settings. I know it's the pre-amp as when I do fiddle with the two pre-amp controls it does affect the fartyness (it's definitely not just over saturated cascaded gain type flub)

2. There's a "scratchy pot" type noise, which isn't related to the pre-amp (I think), as even with all the controls down (barring the master volume) the noise is still there. Same when I plug a guitar directly into the effects return (and I assume bypass the pre-amp)

ICBM said for the second piece it could be plate resistors (?), I've tried swapping out V1 for one of two (different manufacturer) valves, but nothing has changed.

Is there more fault finding I should be doing, and should I entertain the idea of an "amp service", which I presume would be replacing caps and other things that age (like these plate resistors)?

Being handy with a soldering iron and a multimeter I'm happy with basic stuff, but given I don't know how valve amps really work I'm cautious of doing it myself (not shy of it though, if it's just de-soldering some caps/resistors and replacing, I'm happy with that).

Thoughts (and thanks in advance!)?

Also, what should a tech charge for something like this ... ballpark?

ICBM

First, replace each preamp valve in turn with a known good one, not just V1. Valves are always the easiest possible fault to eliminate.

The scratchy noise is in the preamp if turning the master volume down stops it, since it's the last component in the preamp.

Does turning Volume 2 down affect it? 

SquireJapan

Thanks - I'll get on the valve swapping shortly.

On the volume 2 question, the pre-amp issue seems to be more prominent when volume 1 is high rather than volume 2. If I plug directly into input 2 I don't really get any of that happening (but given it's cascaded I'm not sure how much that helps?)

ICBM

SquireJapan said:

On the volume 2 question, the pre-amp issue seems to be more prominent when volume 1 is high rather than volume 2. If I plug directly into input 2 I don't really get any of that happening (but given it's cascaded I'm not sure how much that helps?)

That tends to indicate the problem is in the Input 1 circuit and the first gain stage. If it’s not the valve itself, there are no more than half a dozen components it can be.

SquireJapan

Well, this is annoying. I moved the amp into my living room so I could get at everything properly. Quick test before I make any changes and now everything seems to be working better than ever.

All I can think is that I've accidentally reseated something.

Will report back once this starts again

ICBM

Move it back to the other room. If the problem starts again it’s external noise being picked up by the Input 1 circuit - which it will be more prone to exactly because they’re cascaded so the gain is far higher.

SquireJapan

Odd indeed. 

Have moved it back. Same socket, and tried with/without pedalboard / cable swaps etc. and all is now running better than ever. I can’t duplicate the issue now. I couldn’t _not_ make it happen before. 

Intermittent problems - gah!

welshboyo

dirty Vol 2 input?

SquireJapan

welshboyo said:

dirty Vol 2 input?

Unfortunately not; I can't get a single crackle out of either Volume control (or any of the controls).

DartmoorHedgehog

It seems the answer to the thread title question is "yes - just carry it around for a couple of minutes"

FWIW, I had some weird intermittent noises through my amp and it turned out to be caused by some power line network adaptors I was using in the house - the noise only happened when there was network traffic.  Took me ages to realise that.  It only happened with one amp I had (must have been more sensitive to noisy power).  Don't know if that's remotely relevant to your problem, but thought it was worth a mention.

SquireJapan

DartmoorHedgehog said:

It seems the answer to the thread title question is "yes - just carry it around for a couple of minutes"

FWIW, I had some weird intermittent noises through my amp and it turned out to be caused by some power line network adaptors I was using in the house - the noise only happened when there was network traffic.  Took me ages to realise that.  It only happened with one amp I had (must have been more sensitive to noisy power).  Don't know if that's remotely relevant to your problem, but thought it was worth a mention.

You might be onto something, the wiring in my flat is ancient and I’m not sure if I ever checked that socket for being earthed / wired properly. 

I will when I get home. 

SquireJapan

So, I took out the power tubes so I could get at the 12ax7 tubes to check them, and found one of the 6L6s has a definite rattle. 

It has a loose bit at the “top” (round end). That’s new. 

I’ve ordered a new pair of power tubes

Ossyrocks

SquireJapan said:

So, I took out the power tubes so I could get at the 12ax7 tubes to check them, and found one of the 6L6s has a definite rattle. 

It has a loose bit at the “top” (round end). That’s new. 

I’ve ordered a new pair of power tubes

The problem (yet it was designed this way) is that there is no bias adjustment in Mesa amps. So unless you bought a calibrated set of power valves specifically for Mesa amps, then the new ones will likely be drawing either too much or too little current. The only way to tell is to measure the current draw.

SquireJapan

So I've got two matching valves due to turn up tomorrow. What I can't find is which valves are paired; on other Mesa amps it seems to be outside pair, inside pair.

Can anyone confirm? My googling hasn't found anything

ICBM

Ossyrocks said:

The problem (yet it was designed this way) is that there is no bias adjustment in Mesa amps. So unless you bought a calibrated set of power valves specifically for Mesa amps, then the new ones will likely be drawing either too much or too little current. The only way to tell is to measure the current draw.

There are good reasons for that - it's only a problem if you use valves that are outside a central bias range. Mesa amps are biased quite conservatively and it's rare to find valves that won't bias correctly in them.

SquireJapan said:

So I've got two matching valves due to turn up tomorrow. What I can't find is which valves are paired; on other Mesa amps it seems to be outside pair, inside pair.

Can anyone confirm? My googling hasn't found anything

Yes.

I can't actually think of any amps where this isn't the case - it would be needlessly complicated to wire it up otherwise.

SquireJapan

ICBM said:

Ossyrocks said:

The problem (yet it was designed this way) is that there is no bias adjustment in Mesa amps. So unless you bought a calibrated set of power valves specifically for Mesa amps, then the new ones will likely be drawing either too much or too little current. The only way to tell is to measure the current draw.

There are good reasons for that - it's only a problem if you use valves that are outside a central bias range. Mesa amps are biased quite conservatively and it's rare to find valves that won't bias correctly in them.

SquireJapan said:

So I've got two matching valves due to turn up tomorrow. What I can't find is which valves are paired; on other Mesa amps it seems to be outside pair, inside pair.

Can anyone confirm? My googling hasn't found anything

Yes.

I can't actually think of any amps where this isn't the case - it would be needlessly complicated to wire it up otherwise.

@ICBM - you sir, are a legend.

Thank you!

SquireJapan

Little update for posterity. 

Replacing the broken valve (the pair) has fixed the crackly pot sound. 

It turns out the farty noises are as a result of having the treble up high and the effects send too high at the same time. Turn either down and it goes away. It’s actually spelled out in the manual, I think as a result of the EQ and effects loop sharing a valve. I was running an EQ in the loop. 

Quote from the manual:

” The EFFECTS LEVEL Control can serve another purpose that could be called a “juicy” secret. By simply connecting a short shielded cable to the SEND and RETURN jacks and increasing the FX LEVEL Control, more gain can be introduced into the pre-amp. This creates many possibilities for lead sounds in both channels. INPUT 1 becomes a high gain dream using this little control, while INPUT 2 boasts truly useable lead or crunch rhythm sounds. Using the FX LOOP in this manner combined with the TWEED position on the POWER switch transform a “vintage” circuit into a modern Gain machine - with sound and feel that must be experienced to be believed!

NOTE: WhenusingtheFXLOOPtoachieveadditionalgain,avoidhighsettingsoftheVOLUME,TREBLE,PRESENCEandFX LEVEL simultaneously. Extreme settings of all these controls at the same time puts undue stress on the pre-amp tubes and can cause the pre-amp to oscillate. An oscillation with these controls “maxed - out” is normal and is “setting - correctable”.”

So there you go ...

I do love this amp, it really feels like an instrument you have to learn to work with. 

wire_twerp

That's why I eventually gave up on Mesa's - too much time spent tweaking and not enough time playing. YMMV of course.