Marshall 2204 build - slight hum

[Joe Popp]

I have built a few 2204 circuits and they all seem to have a slight hum. I built this one recently and I plan to keep it. I've tried two different grounding attempts with the same results. The current grounding is the "SDM" grounding. The transformers are Merren 1202 -118 Power and a 784 -139 Output. Any thoughts on this or do they all just hum a little? The hum is the same no matter where the master volume is. It does not matter is a guitar is plugged in or not. Again, it's a slight hum but trying to make it as quiet as I can. Thanks guys. Coming over here from the MetroAmp forum.

SDM grounding:
http://home.comcast.net/~jbjdav26/1959/ ... ubbers.JPG

[img1367]http://www.joepopp.net/JCM%20800%20Comb ... G_2380.jpg[/img]

[xtian]

Well, welcome to TAG, and wow, that's a beauty.

First question: do you hear the hum when all the tubes are removed? Just wondering if the PT is radiating hum into the OT.

[Joe Popp]

Xtian,
Thanks for the quick reply and the complement!
If I pull out the phase inverter tube (V3) the hum stops. I've tried a few different tubes in there with not much change. The hum is soooooo slight. I think I am worrying about something I can't fix. I do have Vintage ARS Filter cap cans (reformed) that I got from Valvestorm and I am wondering if they might be the culprit.

[xtian]

That last bit of hum can be a bitch to tame. That's what master volumes are for!

[ToneMerc]

Welcome to TAG Joe, as usual all your builds are killer.

Mike

[matt h]

(deleted)

[ampgeek]

Welcome Joe! Great looking build..for sure!

The fact that the hum doesn't change when fiddling with the MV (Pre-PI MV I assume) but totally abates when the PI tube is removed indicates to me that the hum is sneaking in somewhere after the MV pot but before, or at, the PI tube. Can't overlook the NFB loop either.

Can you tell if it is 60 hz (probably from heater supply) or 120 hz (probably from B+ supply)?

Consider chopsticking around the PI grid and plate wires to see if you get any changes in hum amplitude. Also consider disconnecting the NFB connection at the tube socket just to eliminate that from the equation.

Elevating the heater supply as previously mentioned can't hurt either!

Finally, consider whipping up a signal tracer and checking the PI socket pins for the offending noise.

Good luck,
Dave O.

[Jana]

Yes, great looking build--it's drool central for me!

It sounds like the hum is from a grounding issue. I took a look at the link you posted and I would not be comfortable with that grounding scheme. I really hate to tell you to start moving wires around--such a beautiful build. But, if it was me, I would want the ground for the PI, the speaker secondary ground, and the filter cap for the PI node all in one point. I would want the grounds for the entire preamp connected (with solder, not a screw lug) very close by as well.

[sepulchre]

Could it be how the OT is oriented in relation to the PT? When I start a build one of the first things I do is the headphone trick. I find that the best position for the OT may be only at a slight angle to the PT but is (so far) never perfectly square to it.

I've only built one 2204 type but it has no hum at all until you plug a guitar into it. And then it's not bad and depends, of course, on the guitar.

[martin manning]

Could it be how the OT is oriented in relation to the PT?

The fact that the hum stops when the PI tube is pulled says no.

[sepulchre]

Yep. That's what I get for skimming while at work.

[Joe Popp]

Thanks for the replies everybody. I did try chopsticking around with no results. I will try a few other things and report back. The elevated heater idea sounds interesting. I've looked at a few articles but have not really dug in yet. I would like to keep the amp as close to stock as I can.

[Joe Popp]

I am thinking it is the filter cap can under the board. When I disconnect the ground for that cap the noise is gone. I ordered some new F&T's that I will swap in for the ARS. I also did a ton of research and it seems that a filter cap is the problem with this kind of hum. I am not sure if it is 60hz or 120hz. Is there an easy test for that? I will have a complete list of voltages by the end of the day.

[Joe Popp]

Yes, great looking build--it's drool central for me!

It sounds like the hum is from a grounding issue. I took a look at the link you posted and I would not be comfortable with that grounding scheme. I really hate to tell you to start moving wires around--such a beautiful build. But, if it was me, I would want the ground for the PI, the speaker secondary ground, and the filter cap for the PI node all in one point. I would want the grounds for the entire preamp connected (with solder, not a screw lug) very close by as well.

I'm not against moving wires at all! I will try some different grounding schemes if the cap can does not solve the issue.

[Phil_S]

I am not sure if it is 60hz or 120hz. Is there an easy test for that?

Very simple. The A (5th) string is 110Hz. 120Hz is between A# and B. 60Hz is one octave lower between A# and B below the low E string. Just plug your guitar in and tune up the A string a little and see if you can match the tone frequency. Right away, your ears should tell you if it is that or an octave below.