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First, it's not an all valve amp - it's 1/2 solid state. The preamp EQ and driver are all transistors...combined with tubes. We call this a "Hybrid" amplifier. Second - volume fluctuation I suspect the 2 MOSFET transistors in the phase inverter driver stage need replacing. OR clean / replace...

I'm not sure that the problem is gone. People who reported this said the problem was intermittent. Other people brought their amp to a Mesa tech... the tech told them there was no problem...but then later, the problem came back. So, I am not really convinced.

Well... the loop bypass test did not help the situation. Although it is worth a try. (effects jacks fail frequently, causing similar noises) From the recording, it reminds me of a break in the signal path, the power amp input being an open circuit...possibly causing the power amp to oscillate...

Try the bypass cable first, that eliminates the possibility. The buzzing may be the power amp oscillating, because the input (to the PA) path is an open circuit. We don't know anything for certain yet. That's why we're trying the most common failure point first. (bad switch contacts on effects...

You MUST use a professional isolated effects power supply. Yes, it's probably a ground loop. Ground loop: also caused by three prong power cables. Only ONE of the power cables should be grounded. (at the amp usually) The rest of the grounds should be lifted (with this)...

The high voltage attracts certain type of contamination from the atmosphere. It's normal. It happens in TV sets too. The contamination can be cleaned off with alcohol. The alcohol should be 91% or higher. (you can use Q tips) The circuit board will not be damaged. Allow the cleaner to dry...

Martin, Plug a short guitar cable between effects send jack and effects return jack. No effects for this test please. Now, re-test the amp. Did the noise stop? Let us know if this test stopped the noise.

The same symptoms are showing up in three amps in three locations... Two in Canada and one in USA. + the amp mentioned above by bottle rocket. (That's FOUR amps total) Yes I have a tester, I'm an electrician. The noise comes and goes intermittently. Shutting off the amp and turning it back on...

3 reports of same problem, amp makes intermittent :roll: buzzing noise, resembles ground loop, etc... Sounded like a broken input jack or a broken input wire, broken ground connection...but have not found anything broken yet. If anybody knows anything, please let us know. No schematic...no...

Boogie has international transformer, with multi voltage taps. Unless, it is already installed... many times the power taps are already there. A technician should change it over, internally. Check the Mesa service center nearest you.

The coupling caps are the orange caps (orange drops) originally made by Sprague. https://www.tubesandmore.com/products/C-P-600V The blue caps are the filter caps.

a bad socket or a broken bias voltage path... The audio coupling capacitor between the phase inverter output and the power tube grids is shorted. I'm pretty sure. The original capacitor is rated 400 volt, replace it with 600 volt. These are the 2 coupling capacitors between pins 1 and 6 of the...

You probably are asking questions about this, so i am going to post some answers... The capacitors are put in parallel, to achieve the desired values, because two caps in parallel will handle twice as much ripple current. The little radial cap can only filter "so much" current, from the ripple...

Change the 22 uF capacitors in the preamp high voltage supply to 47 uF, change them all. This will eliminate the hum, buzzing noise, and will get rid of the muddy sound. Importantly, inexperienced people should not work on this amplifier. The circuit board is very easily damaged by improper...

A lot of these mods do not address the real problem... the power supply is not stiff enough to start with. If you would just double the value of the capacitors in the B+ supplies, then most of the problems would be solved. For example, the preamp uses 22uF caps in the high voltage supply...

It's probably not really fixed.

I have seen this happen many times. The problem is caused by a design defect that triggers high frequency oscillation and motor boating in the power amp. This oscillation burns up the power tubes. When techs claim that this problem is "fixed," typically the real problem is NOT fixed...

The switch contacts at the effects loop jacks have failed. That's the crackle sound. Replace the jacks or clean the switch contacts in the jacks. That type of jack fails pretty often. So, I use CLIFF UK jacks instead. Muffled Sound in the loop: Typically it's a failed FET transistor. The FET is...

5881 is a wimp version of 6L6. Not as heavy duty. I would not recommend 5881 at all. The bias adjustment is a real problem, since it's non-adjustable. No matter what tubes you install, the bias "should" be adjusted. But you can't do that.

You can't adjust the bias circuit. Not without modifying it anyway. The power supply for the bias is shared by other circuits. If you change the bias voltage you might disable the other circuits. But as for the bias voltage on pin 5, of the two inner tubes: It's not the sockets. The bias comes...