Mark I reverb hum

[Shantiano]

I hear a slight hum that gets louder when I increase the reverb level even without any cables plugged to the amp.
Master 1-10, Reverb 0, almost no hum.
Master 1-10, Reverb 1-10 hum gets louder as reverb level inscreased.
I checked the cables of the reverb tank & everything seems to be connected.
Any idea? Is it normal?

 

[markchatwin]

I have a Mark II C+ that did this and there is a repair in which the two cables (where they connect up under the chassis) are connected by a piece of wire and soldered together. The reverb cables are kind of like RCA cables and the wire is soldered to the flange of each end connector. The grounding problem went away. But then a new grounding problem arose unrelated to reverb. It was present in the "Lead" mode. A loud "bassy" buzz when I cranked up the lead channel. I took it to my amp guy here in Syracuse, Jerry at FJA Mods, and he called Mike Bendinelli, since this was a difficult problem to solve. Mike told Jerry to change out the black oxide washers with regular ones and the problem went away - sorry to divert...

 

[Shantiano]

Thanks markchatwin. Was the loud bassy buzz caused by this wire that you soldered in the RCA cables?

By the way I just receive an email from Marcus Daniel at Mesa Boogie saying that this is a normal occurrence in any Mesa amp with reverb... he suggests to try swapping out the 12AT7 preamp tube in the V4 position that drives the reverb to see if this helps reduce the hum at all.

 

[markchatwin]

The "Bassy" buzz in lead channel was unrelated to my reverb problem, it was caused by the black oxide washers. My reverb problem was like yours, it increased as I turned up the reverb knob, kind of like a buzz as well, even without cables patched in...

SO the soldering job was a possible fix to my reverb problem. BUT come to think about when I got my amp there was a host of issues like a motor boat sputter in the circuit, the reverb buzz, the bassy buzz, etc... Since I don't have time in my life to change out tubes one by one and troubleshoot I just changed them all (Preamp tubes). This coincided with soldering the ends of my reverb cables. The sputter went away, the reverb buzz was gone... So I think replacing V4 is maybe even more of your cullprit...But the cable solder is not a bad thing to do.

Then I just had one problem, the bass buzz in lead mode. I figured it wasn't tubes since I changed them all and the problem was there before and after. I changed the power tubes as well and still had the bass buzz. I thought maybe filter caps and worse yet the power transformer. But Jerry at FJA Mods got with Mike Bendinelli and he suggested a changeout of the black oxide washers behind the guitar input jack. Mike said if that didn't work then try changing tubes out one by one. Well as soon as we cranked it up the bassy buzz was gone.

 

[zebpedersen]

My Mark I has a buzz the higher the reverb level gets, although it's not constant, only when you hit a note - like a metallic fizz added to the harmonics when you play.

If you go to the Boogie website, there is a Mark I maintenance manual which has a short chapter on combatting noisy reverb.

 

[markchatwin]

Zeb - sounds like just typical crap spring reverb noise...

 

[Shantiano]

Hello markchatwin,

does the soldering mod requires removing the chassis?

 

[CudBucket]

My Mark I has a buzz the higher the reverb level gets, although it's not constant, only when you hit a note - like a metallic fizz added to the harmonics when you play.

My Mark IV does this on the Lead channel (high-gain) when I increase reverb. See my "Does Your Mark IV Do This?" thread. When the reverb is off, no problems. Once I get the reverb to 5 or more, the notes get rattly and metallic, thin, etc.

Mesa and my authorized tech said that it's inherent in the reverb circuit.

 

[markchatwin]

No Shantiano - Just pull the reverb cables out from underneath the chassis and solder a 2 inch general purpose wire from the outside of one flange to the outside of the other. Be careful not to get solder on the inside of the flanges as this would affect how it slips over the female receptacle under the bottom of the chassis...

 

[zebpedersen]

ADDENDUM
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.

From the Mark I repair manual - hopefully your problem will be solved with a less involved process!

 

[CudBucket]

ADDENDUM
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.

From the Mark I repair manual - hopefully your problem will be solved with a less involved process!

This is great, thanks! I wonder if this would help with my Mark IV? I'm not sure it would though since the IV uses both tubes in V3 and V4 to feed the reverb send and return.

Dave

 

[Shantiano]

My problem is a constant hum once i increase the reverb level from 2 & up when the master volume is from 1-10. The fix mentioned in the repair book is for the metallic noise in the tone.

Can any of you guys with a Mark I reissue or any Mark series try to turn off all levels to zero, master on 3-4 & start increasing the reverb & see if there's hum?
I want to see if it's really what the guys at mesa boogie told me that it's a normal occurrence in all their amps with reverb.

Thanks

 

[55Tele]

Yes it is "normal"

My present Mark I does it

My previous Mark I did it

My Bandmate's Mark I does it....

 

[aaronlyon]

I installed my own reverb tank using "vintage" reverb wire, but now I notice in many photos there are TWO kinds of wire used for connecting the tank, one for from and one for to. Why different types of wire? Could this help reduce hum or buzz? Also, I soldered the grounding wire to all four RCA plugs. Perhaps I saw somewhere that one should NOT be connected to ground?

 

[aaronlyon]

Bump.

Can anyone answer my question, above, about the two types of reverb wire?

 

[Shantiano]

Thank you 55tele.
I'm keeping the Master volume on 2 & cranking up the input volume. That's the cure.