Mark IVa Noise in R2 and Lead

[dunkonu23]

Hi,

I've got a 1990 Mark IVa that has what sounds to be white noise at varying levels in the R2 and Lead channels. I've isolated it to the effects loop. It doesn't matter if an effect is in the loop or not; if the loop is engaged, the noise is there. Dunno what is causing it. I have the schematics and V2 seems to be a good candidate except it was replaced along with all the other tubes about six months ago (it's a Mesa 12AX7SP--yeah, I know ).

Another problem is a nasty overtone (almost an octave lower) when playing an open high E string and the fretted unwound strings above the seventeenth fret. It is not the guitar, it happens on all my guitars, but is most noticeable on my JP6 BFR. This overtone happens plugged straight into the amp with no effects connected at all.

Any ideas?

Thanks in advance,

Scott

 

[ANIMATED SUSPENSION]

Using the fx loop adds another tube stage so on high gain settings may increase noise slightly, but are you saying it's only there with the fx loop engaged and goes away if it's not, even with nothing in the loop ? My MKIV is pretty noisy on R2 and Lead too actually, but only with high treble and gain settings at volume. There is a sweet spot where you can get the tone/gain without the excess noise, but it takes a bit of tweaking !!

 

[ANIMATED SUSPENSION]

The overtones you describe sound like very old worn out strings or pups that are too close to the strings or if you're unlucky a problem with the amp or maybe just power tubes that are out of spec ?

 

[dunkonu23]

Using the fx loop adds another tube stage so on high gain settings may increase noise slightly, but are you saying it's only there with the fx loop engaged and goes away if it's not, even with nothing in the loop ? My MKIV is pretty noisy on R2 and Lead too actually, but only with high treble and gain settings at volume. There is a sweet spot where you can get the tone/gain without the excess noise, but it takes a bit of tweaking !!

That's exactly what I'm saying. The noise is there with effects in the loop and nothing in the loop--cables unplugged. It is there only when the loop is switched on.

Tweaking... yep. I've got it at a minimum and it's still too much.

Scott

 

[dunkonu23]

The overtones you describe sound like very old worn out strings or pups that are too close to the strings or if you're unlucky a problem with the amp or maybe just power tubes that are out of spec ?

I know it's not the guitars. I sent my BFR back to EBMM with this specific complaint. Since, I've used other guitars and it's there with them--brand new strings, pups properly adjusted, too.

I'm pretty sure this is the amp, as well as the other problem because neither problem happens on any other of my amps. I'd just like to fix it myself or at least get close. Failing that, does anyone know of any authorized repair centers in Michigan, preferrably near Detroit?

Scott

 

[Xqzdust]

http://www.mesaboogie.com/US/Dealers/rep_select_2.php

Right off the Mesa website. Go to this link!

 

[dunkonu23]

http://www.mesaboogie.com/US/Dealers/rep_select_2.php

Right off the Mesa website. Go to this link!

Thanks! I missed that.

The one in Highland is less than 6 miles away!

Scott

 

[dunkonu23]

Hi,

I've got a 1990 Mark IVa that has what sounds to be white noise at varying levels in the R2 and Lead channels. I've isolated it to the effects loop. It doesn't matter if an effect is in the loop or not; if the loop is engaged, the noise is there. Dunno what is causing it. I have the schematics and V2 seems to be a good candidate except it was replaced along with all the other tubes about six months ago (it's a Mesa 12AX7SP--yeah, I know ).

Another problem is a nasty overtone (almost an octave lower) when playing an open high E string and the fretted unwound strings above the seventeenth fret. It is not the guitar, it happens on all my guitars, but is most noticeable on my JP6 BFR. This overtone happens plugged straight into the amp with no effects connected at all.

Any ideas?

Thanks in advance,

Scott

I just had something similar...

Burned up winged C :x

MIck

Mind if I ask which tube? Thanks!

Scott

 

[trem]

Ghost note is usually caused by either speaker problem, filter caps or power tubes. I had that happened, and it went away with power tube change. Try it with different speaker cabs. If that fails then try replacing the power tubes. The noise could also disappear with tube change. Try V2 with other tube.

 

[dunkonu23]

Ghost note is usually caused by either speaker problem, filter caps or power tubes. I had that happened, and it went away with power tube change. Try it with different speaker cabs. If that fails then try replacing the power tubes. The noise could also disappear with tube change. Try V2 with other tube.

Thanks. I tried my 5150 cabinet and no problem. Then, I tried my 5150 into the cabinet I normally use with the Mark IV -- a Grendel Sound Dead Room loaded with a Mesa MC-90, no problem. Back to the normal setup, harmonic problem reappears. I can switch out the speaker to an English made Vintage 30, but that will have to wait until tomorrow. Interesting stuff.

At any rate, I'm going to be ordering new tubes as soon as Doug Tubes gets back to me. Failing everything the amp goes to the shop.

Thanks!

Scott

 

[marklyn]

Scott,

There are a couple things you can try to rule out some potential problems.

Try switching to Class A, if the problem goes away, the problem is in one of the two inner power tubes. If it doesn't go away, let the tubes cool (power off of course) and switch from inner to outer slots. Try again.

If the problem is still there try switching out an extra preamp tube for the 2nd position. If the noise is still put the 2nd position tube back and then try the 3rd position.

These should hopefully let you find where the problem is, or where it is not.

Good luck!

 

[dunkonu23]

Thanks for the suggestions! My new tubes arrived yesterday but I haven't had time to really go at it, yet--that won't happen until tomorrow or Tuesday. I'll be certain to do as suggested.

Here's the tube lineup, from Doug's Tubes:

Winged =C= 6L6 and EL34
V1 - Tung-Sol reissue 12AX7
V2 - Mullard 12AX7 reissue
V3 - Penta Labs 12AX7
V4 - JAN Philips 12AT7WC
V5 - Sovtek 12AX7LPS

Currently the amp has all Mesa tubes. As suggested, they will be swapped in a manner to identify the failure.

Thanks again!

Scott

 

[dunkonu23]

No joy. The amp goes to the shop. Thanks to everyone who replied. I tried all suggestions, nothing changed. :-(

EDIT: I'll qualify "...nothing changed". The white/static noise on the loop is gone, but the overtone remains. R2 has never sounded better--extremely tight and aggressive. The overtone on the open high E remains with all guitars.

Scott

 

[jamme61]

The low E problem sounds like bad caps. you said it's a 1990 and caps should be changed after 10 years (depends on how much the amp is used) so your over do for a cap change. Those caps are a little expensive, i would say about $50 to 60 bucks for all new caps. Bringing it to a tech, is a good idea IMHO. let us know what happens. Good luck

 

[dunkonu23]

Today, I tried swapping the MC-90 speaker in the Dead Room Iso cabinet with a Vintage 30. No real progress there, but the frequency of the overtone changed. Interesting to me, probably to nobody else. With either speaker I can get the overtone to go away if I turn the volume down--it starts at just before 2. One would think it would be feedback, but it's more of a sympathetic vibration--you can feel it when you touch the Iso cabinet, but it's not the cabinet, nor is it the speaker. I wish I had an o-scope so I could look at the output of the amplifier, but that's something for the shop. As I said it's going back to the shop, but right now, at lower volume it's quite usable. In fact, at the lower volume, the V30 sounds very tight and focused. Oh, well.

Scott