1976 MK 1 reverb circuit

[James]

Hoping to find some advice on this issue with a 76 MK 1 head.

This amp was missing the reverb tank when I bought it (the smaller OC electronics type - not sure of the spec) so it now has the standard replacement from Mesa. Measures around 1 ohm (input) and 230 ohms (output)

It works and is actually very strong, but there’s this unwanted white noise (oscillation?) as more gain and treble is introduced. The problem is that this happens when the reverb control is on zero. If I turn the master down, I can also hear the springs physically rattling when this sound occurs.

Any ideas what causes this? Years ago I had a 50 Caliber + that had the exact same issue, but I never figured out what it was.

Chassis photo: https://imgur.com/a/nTCblFd

I can post more if necessary. It has already been serviced by Mesa with the reverb circuit update.

 

[user 21540]

It could be a bad or microphonic reverb driver tube. I would try there first. I don't know the tube assignments for the old Mark Is but the driver tube may be a 12AT7 (though I'm not certain and someone may have put a diffenet 12A in that slot). All the info I can quickly find about that year is in this combo model here which does have 12AT7 penned on the chassis although that could be a phase inverter -- I don't know:
https://www.scottfrench.com/journal/2019/04/14/padauk-mesa-boogie-mark-i-1976-drg/

In a head too you are always going to have problems with the proximity of the tank to the transformers which can lead to a lot of noise issues that can be hard to fully remedy.

 

[James]

It could be a bad or microphonic reverb driver tube. I would try there first. I don't know the tube assignments for the old Mark Is but the driver tube may be a 12AT7 (though I'm not certain and someone may have put a diffenet 12A in that slot). All the info I can quickly find about that year is in this combo model here which does have 12AT7 penned on the chassis although that could be a phase inverter -- I don't know:
https://www.scottfrench.com/journal/2019/04/14/padauk-mesa-boogie-mark-i-1976-drg/

In a head too you are always going to have problems with the proximity of the tank to the transformers which can lead to a lot of noise issues that can be hard to fully remedy.

Thanks - I did try a different tube (I think it's all 12AX7s except for the phase inverter) but no change. I'll have to try it with the tank removed from the head cabinet.

I asked Mesa after I got the amp back and was told that it shouldn't be sending signal through the reverb tank when the control is on zero, but there is still some kind of interaction based on the spring noise. If I unplug the tank, it sounds much better.

My only other thought is that the earlier amps used a different spec tank. I see replacements are generally listed at 8 Ohm (input) and 2250 Ohm (output)

 

[user 21540]

Bummer it wasn't the simple fix. Unless you're confident working on high voltage gear it sounds like you proably need to get it to a tech. I'm not sure why signal would be bleeding through if the control is on zero. It could be a bad pot but it could be something else somewhere in the circuit that is leaky or drifted.

 

[mark1406]

Where is the reverb tank mounted. I had a Mark 1 back in the day and the reverb tank was mounted on the back panel of the cab as opposed to being on the bottom of the cab in a bag. As a result the reverb tank interacted with the speaker and feed back. I did put the tank at right angles to the speaker and the problem went away. It was a design issue.

I believe the reverb valve should be a 12AX7, I recall that’s what was in my old Mark 1.

The reverb control should stop all reverb coming from this circuit, it is a volume control after all. You may still hear reverb if the pot doesn’t go down to zero or there is signal going through the power supply.

The springs will be heard if you give the amp a tap, but if the amp has been there for sometime then there should be no rattling though the speaker could cause trouble for the reverb tank. Try turning the amp off and remove the reverb valve and see if the noise is still there.

I wonder what the reverb circuit update was?

Regards

Mark

 

[James]

Where is the reverb tank mounted. I had a Mark 1 back in the day and the reverb tank was mounted on the back panel of the cab as opposed to being on the bottom of the cab in a bag. As a result the reverb tank interacted with the speaker and feed back. I did put the tank at right angles to the speaker and the problem went away. It was a design issue.

I believe the reverb valve should be a 12AX7, I recall that’s what was in my old Mark 1.

The reverb control should stop all reverb coming from this circuit, it is a volume control after all. You may still hear reverb if the pot doesn’t go down to zero or there is signal going through the power supply.

The springs will be heard if you give the amp a tap, but if the amp has been there for sometime then there should be no rattling though the speaker could cause trouble for the reverb tank. Try turning the amp off and remove the reverb valve and see if the noise is still there.

I wonder what the reverb circuit update was?

Regards

Mark

It's a head, so it's on the bottom of the cabinet. Open side up. I know Mike B said the tank (included with the new head cabinet from Mesa) was the wrong orientation and he corrected it.

Yes, I believe the amp is labeled for all 12AX7s.

The reverb effect is off when the pot is on zero - the problem is that there is still some kind of interaction, which affects the original/dry sound once the treble and gain are turned up. The rattling I describe isn't caused by vibration from sound or anything else - If I turn the master down, I can hear mechanical noise coming from the tank while playing. I can try to make a video if necessary.

I did try removing the reverb driver and the problem went away. I'd be fine running it like this, but wasn't sure if removing the driver could damage the reverb transformer

Only other thought was that it might be the wrong spec tank to begin with. It measured what Mesa suggested (which matches the one in my IIB), but I don't know if the early ones with the OC electronics tanks were different.

As for the update, it's mentioned in the MK 1 maintenance/repair manual. It seems to describe this exact problem, but I assume the amp already had these modifications.

A metallic sounding buzz in the tone of amps having Reverb may be traced to a parasitic oscillation in the Reverb driver stage under certain settings. To cure, perform the following modifications:
• Install minimum 220K resistance in series with the capacitor (.005) coupling signal to the 12AT7 grid (pin 7, or pins 2 & 7 if amp also has Graphic EQ.) There may be approximately 50 K in series already, but it might not provide enough isolation from the adjacent circuitry. (If the buzz is greatly reduced but still persists, substitute 470K for 220K above, and reduce .005 coupling cap to .001.)
• Replace the 470K (yellow-purple-yellow) grid leak resistor (also leading to pin 7 or 2&7) with 100K.
• The cathode circuit of the 12AT7 (pins 8 or 3&8 if EQ) should have a 470 ohm to ground with no bypass capacitor.