JCM800 / 2204 style build hum / buzz plea for help

[paul525715]

Hi everyone,
I have a hum and a slight buzz in my amp build that I need to sort out and I am hoping that someone here can help me source the problem. I've looked at another thread here Marshall JCM 800 noise which sounds similar so I'm hoping some of my problem won't be too hard to fault find.

The amp itself is based loosely around a 2204 but with a PPIMV and a switchable boost (relay controlled voltage divider to ground). The presence is fixed and I have instead installed a reverse cut control across the PI output. I have implemented a "larry" grounding scheme where possible (see the pics for as much detail as possible) but I still have buzz and hum.

Ok... problem one (I'll try to solve the buzz first)... if I remove all of the preamp valves except the PI (which gets rid of the other problem, which I'll deal with after solving this problem!), I get a slight buzz when the MV is dimed... actually, it's more like a a slight clicking hum. It gets louder as the MV increases in volume. For reference, the wires going from the 5k6 grid resistors to the PPIMV are shielded (wire shielding connected to the bias supply). The wires going to the 0.1uf PI output coupling caps are also shielded (wire shielding connected to the bias supply). The reverse cut (presence) pot is connected to the 6k8 and 500pf cap using shielded wire (shield to the resistor and inner core to the 500pf cap).



Can someone try to point me in the direction of where to start troubleshooting. All help is most gratefully received.

[Loudthud]

Originally Posted by paul525715

I get a slight buzz when the MV is dimed. It gets louder as the MV increases in volume.

This does not make any sense. Does your Master Volume go to 20? "Dimed" means it's on 10, all the way up. How do you turn it up from there?

1 Meg is too big for a PPI Master Volume. It should be 100K or 200K at most.

What happends when you turn the standby switch off (meaning from play mode to mute mode)? Does the hum fade out over a period of a couple of seconds or cease immediately?

The hum (buzz) that ceases immediately is from bad grounding in the power supply. The low organ note type hum that fades out comes from lead dress mostly.

Describe how all the filter caps are grounded and transformer connections to ground. Are the yellow wires used to make a power supply for the relay? Why do you have virtual ground resistors on both input and output of the bridge?

Why two bias supplys?

[paul525715]

Originally Posted by Loudthud

This does not make any sense. Does your Master Volume go to 20? "Dimed" means it's on 10, all the way up. How do you turn it up from there?

Bad wording on my part. It gets louder as it's turned from zero (no hum) to 10 (hum).

Originally Posted by Loudthud

1 Meg is too big for a PPI Master Volume. It should be 100K or 200K at most.

It's a dual 250k log pot (lar/mar style) as per the metro amp wiki.

Originally Posted by Loudthud

What happens when you turn the standby switch off (meaning from play mode to mute mode)? Does the hum fade out over a period of a couple of seconds or cease immediately?

(Shakes head sadly...) there's both. Just checked. A low hum with a buzz on top. Buzz cuts out instantly, hum fades out.

Originally Posted by Loudthud

Describe how all the filter caps are grounded and transformer connections to ground.

I based the grounding on a scheme from novosibir at the metroamp forum called the larry grounding scheme. It's as follows...

ground point #1 (near input jack): input jack, HT CT, V1 cathodes.
ground point #2 (near treble pot of tone stack): V2 cathodes, gain pot, preamp filter can cap, boost switch potential divider
ground point #3 (near bass pot of tone stack): speaker jack ground, OT ground
ground point #4 (output transformer mounting bolt): bias circuit ground
ground point #5 (along back edge of chassis): main HT and screen filter caps
ground point #6 (below iec socket): iec socket earth

Originally Posted by Loudthud

Are the yellow wires used to make a power supply for the relay?

Yes

Originally Posted by Loudthud

Why do you have virtual ground resistors on both input and output of the bridge?

I don't... that part of circuit was taken from another website. I only have the 100r resistors installed on the relay power supply. The relay ground is floating, not connected using the chassis.

Originally Posted by Loudthud

Why two bias supplys?

Independent bias control for the output valves

I hope this helps.

Paul

[paul525715]

Would you suggest following this grounding schematic instead?
http://www.joepopp.net/jcmtwins/jcmtwins003big.jpg

[Loudthud]

Originally Posted by paul525715

Would you suggest following this grounding schematic instead?
http://www.joepopp.net/jcmtwins/jcmtwins003big.jpg

The one problem I see with that grounding scheme is the connection of the preamp filter cap ground to the transformer bolt star. Moving that ground to the bus near the volume controls would reduce buzz several dB. I check the Metroamp forum shortly.

I don't like the "CT to transformer bolt" schemes. They are wrong from the get-go. The above suggestion will get the amp fairly quiet, until you try to connect a Fender Reverb unit. You'll have to lift the line ground on the reverb to get the hum out. For really low hum, read Kevin O'Connor's TUT books, not sure which one.

[jjman]

HT CT at the input jack doesn't seem right to me. On my 1st build about 15 years ago I put the HT CT far away from the filters ground point. It hummed for almost a year until I realized that current flows thru the HT CT, so it's important.

[paul525715]

Thanks both of you. I'll give these things a go.

[paul525715]

Thanks for all your help last week. I removed the whole board and rewired it using the grounding scheme above with the exception of the preamp filter cap which I grounded halfway along the bus wire. I also put the input 68k resistors right on the V1 valve socket.You should be able to see the bus wire on the photo below. The main ground point is just below the PT. The OT and output valve ground point is separate, as per the grounding scheme. Sadly, the problems still exist. If I remove the PI it's pretty silent., even if I max the PPIMV. Can anyone suggest where I go from here? I do have access to a signal generator and an oscilloscope should that help.

Thanks.

[paul525715]

For what it's worth...
Removing the PI valve = almost silent.
With the PI valve in place and shorting the valve input to ground using a 0.022uf cap = a slight increase in hum/buzz but still very acceptable.
With the PI valve in place and shorting the input to the tone stack at the start of the cap/resistor network to ground (0.022uf) = hum / buzz back again.

[Loudthud]

Any change to the standby switch test with respect to the amount of hum vs buzz?

Where is the speaker grounded? The best place would be where the Mid control grounds to the buss. The ground wire from the OT should still go to the speaker jacks and a ground wire from there to the buss.

BTW, the build looks good. Nice lead dress and components on the board are neatly installed.