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Marshall 800 Hum/Buzz - Reverb Control

Vincec811

Member
Messages
13
Hi! I have a Marshall 800 50W head that produces hum/buzz when you turn up the REVERB control. It is less at minimum, then increases, then reduces a little at mid level, then is much louder at full rotation. Other volume and tone controls do not affect the buzz. The reverb sounds fine except for the buzz. The buzz remains if I pull the reverb connections off the pan input and output. Tried grounding the reverb pan to the chassis--no change. I just replaced the power supply filters with JJ 50uf/50 uf 500V caps. Replaced all pre-amp tubes. Power stage has 6550s which I rebiased for 21 Watts. All voltages read fine and everything seems to be grounded per the schematic. If I pull V3 12AX7A out the buzz is gone (but so is the signal of course). Anybody have a problem like this and fix it?
 

ohmslaw

Member
Messages
208
Hi! I have a Marshall 800 50W head that produces hum/buzz when you turn up the REVERB control. It is less at minimum, then increases, then reduces a little at mid level, then is much louder at full rotation. Other volume and tone controls do not affect the buzz. The reverb sounds fine except for the buzz. The buzz remains if I pull the reverb connections off the pan input and output. Tried grounding the reverb pan to the chassis--no change. I just replaced the power supply filters with JJ 50uf/50 uf 500V caps. Replaced all pre-amp tubes. Power stage has 6550s which I rebiased for 21 Watts. All voltages read fine and everything seems to be grounded per the schematic. If I pull V3 12AX7A out the buzz is gone (but so is the signal of course). Anybody have a problem like this and fix it?
My SuperSonic also has a hum if you crank the reverb up all the way but that was greatly reduced by proper biasing of the power tubes and replacement of the reverb drive tubes. Make sure the power tubes are biased correctly; this makes a difference. You should try using a high-quality 12AT7 in the reverb circuit. Replacing all the stock tubes in my SuperSonic with high-quality tubes and biasing the amp reduced the noise floor tremendously.
 

Vincec811

Member
Messages
13
I did replace all of the tubes and had re-biased the amp making sure the 6550's were only dissipating 60% of max at idle = 21 watts. The amp sounds great except for this horrible hum/buzz from the reverb volume. Even the footswitch does not kill the buzz. I have read so many posts from people with this same problem--is this just a flaw in the Marshall 800 2205 design? There must be a ground loop in the design. I also just noticed that the reverb circuit is actually wired to V4A and not V3B as the schematic shows. The last pre-amp stagfe before the Phase Inverter is wired to V3B instead of V4A. Theroetically this works but could this be a source of ground loops or the wrong stages being coupled within the tubes?
 

Vincec811

Member
Messages
13
C'mon! All these super techs out there and no one has an idea where this hum might be coming from?? Or is this so simple I am just missing it??
 

SatelliteAmps

Member
Messages
6,182
I would check a few things. The cables going to and from the reverb pan, and also the connections and the ground inside the reverb tank. Those are mostly likely where hum could be coming from.
 

Vincec811

Member
Messages
13
Satellite - I did check those cables before and replaced them and saw/heard no change. Thanks!

Frankie - I will try that. I have been reading more and more articles about this and I agree this must be a common problem on JCM 800 2205. If I figure out exactly what is causing this I will definitely post it! Thanks!
 

Ronsonic

Member
Messages
3,305
Seems like every time I get one of those amps in the customer requests that I not fix the reverb. My experience with noise in that reverb circuit is that it's a matter of working the problem, no standard failure mode. Except that it isn't a good sounding reverb when it is working.
 

phsyconoodler

Member
Messages
4,315
If the reverb tank has corrosion on the connections the reverb circuit has to work hard to pass a signal.The tank itself could need some cleaning and a resistance check to make sure it is grounded properly.
A sketchy ground can cause hum in reverb circuits.
 

Vincec811

Member
Messages
13
I can remove the I/O connections from the pan altogether and ground the shields to the chassis. Noise stays exactly the way it was when they were connected to the pan. The pot acts like it is not grounded but I measure the low end to chassis and sure enough, 0 ohms. I replaced the power supply filter caps with JJ parts and no change. I am going to try resoldering all joints on the PC board and see what that does. Maybe a cold solder joint on a tube wire??
 

Vincec811

Member
Messages
13
Actually when I received the amp for repair the Input/Output cables were backwards. But I was still getting reverb out of it. The Inout /Output connections make no difference in the hum however when I put them in correctly. The reverb tank is an Accutronics 4DB3C1D. Is this the correct unit for the Marshall 800 2205?
 

Vincec811

Member
Messages
13
FrankieSixxxgun - You were absolutely right on Reverb tube- Put a 12AT7 in V4 and the Reverb hum was gone. Thank you much!

But- I still have 120Hz hum left meaning power supply hum. I see the 1-2V sawtooth on the diode output to the first stage 50uf filter. I tried new diodes, had already replaced the filter caps, lifted as many wires as I could making sure I still had a signal path, grounded the inputs, disconnected the Direct pot, lifted the SEND and RETURN jacks, added a series 15 ohm resistor after the diodes in the power supply, added a parallel 220K ohm across the filter cap, replaced the 6550s, etc., etc., etc. The only thing that helped MAYBE 3dB was looping a ground wire around the interstage connection to V2 (green wire). I don't have a clue what to try next. Many folks on the internet say this is just a bad designed amp. The 900 I have in for repair is QUIET. What could Marshall have screwed up so badly with these amps?
 




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