Marshall 2204 build - slight hum

[andyhardy]

If disconnecting that cap stops the hum clip lead it to the preamp ground wire on the pots.
I would bet the hum will still be gone.

[Joe Popp]

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.

Thanks Phil! Will try.

[Joe Popp]

If disconnecting that cap stops the hum clip lead it to the preamp ground wire on the pots.
I would bet the hum will still be gone.

I currently have it to a lug on the filter cap itself. But that lug also shares the Preamp bus. It is currently like the below link.

Jana mentioned moving it which I have not tried yet.

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

[Joe Popp]

So logged all of the voltages on the amp and compared them to the voltages Mark Huss recorded.

Mark Huss Voltages:
[img:798:121]http://www.joepopp.net/JCM%20800%20Comb ... ltages.png[/img]

My voltages all seem relatively ok (My PT puts out 425 volts while Mark's is in the 460 range) with the exception of V2 pin 6 which reads way high and V2 pin 7 which reads way low. Pin 6 comes right off of one half of the cap can which I am guessing is bad.

[img:507:85]http://www.joepopp.net/JCM%20800%20Comb ... ltages.png[/img]

Jana (Forum Member) also sent an alternative grounding scheme which is much like the grounding scheme on my previous builds and I may incorporate that as well. I let everybody know how it goes.

Thanks again for all of your help!

[Jana]

Also, the voltage on pin 8 of V2 is quite low. Have you tried a different tube? Checked all the wiring? Resistor values (actual values, not the printed value on the resistor)? Maybe a leaky cap in the tone stack? Something is not right in that part of the circuit.

[Joe Popp]

Jana: Typo! 177.5 it should read. Updating now. DOH!

[Jana]

Okay.

Something is still weird. Pin 7 of V2 should read the same voltage as pin 1 of V2--they are (should be) connected with a wire.

[Joe Popp]

Okay.

Something is still weird. Pin 7 of V2 should read the same voltage as pin 1 of V2--they are (should be) connected with a wire.

Let me recheck Monday. The 100k resistor would have no effect?

[passfan]

I think you have transposed pins 7 & 8 of V2 on your voltage chart. It looks correct in the photo

[Jana]

It does look correct on the photo, but transposing the readings for 7 and 8 would still be weird voltages.

[Joe Popp]

OK, so I'm a goofball. After checking again my voltages are indeed in the ballpark of where they need to be. I found another voltage chart from a JCM 800 and mine seem to be in the right range for the transformers I am using. Here are my correct readings:
[img:506:88]http://www.joepopp.net/JCM%20800%20Comb ... ltages.png[/img]

Here is a JCM800 voltage chart I found.
[img:690:799]http://www.joepopp.net/JCM%20800%20Comb ... echart.jpg[/img]

I pulled the board and I am going to redo the grounding. I am installing new F&T filter caps and I will probably swap out the top hat diodes with UF4007s. I am retesting all components as well.

Jana sent me some grounding tips which are more in line with my previous amps but I am tempted to try the "Larry Grounding" since many have had success with it.

Anybody else have any ideas? Another JCM800 I built has no hum at all. And that amp incorporates a buss bar on the board like the older Ceriatone layout. It also has Vishay resistors and Brand new Sozo Caps. Maybe I'm chasing a dragon I can't catch but I am going to keep trying!

[martin manning]

Another ground scheme: https://tubeamparchive.com/viewtopic.ph ... 106#297106 Untested, but I am confident it will work.

[gktamps]

Larry grounding scheme worked very well on my recent build. Amp is extremely quiet.

[xtian]

*rant* GROUNDING SCHEMES ARE A BIG FREAKING MYTH!!! *end rant*

Sorry. Spent hours yesterday trying to eliminate hum from my YBA2a build by adjusting the grounding scheme. Hum comes in at the PI. Thought it might be ground loop, but I spent some time doing my best to CREATE ground loops, and I could not get the hum to increase at all, no matter what ridiculous thing I tried.

Then I connected my scope, and could see the hum coming from the B+1 node to the plates of the PI. I tried tacking in an additional reservoir cap, but this reduced the ripple only 20%. I tried inserting a choke in series, but this made no improvement. Next, I guess I'd have to add a complete node (RC), but didn't do this yet.

[Joe Popp]

[quote="xtian"]*rant* GROUNDING SCHEMES ARE A BIG FREAKING MYTH!!! *end rant*

Ha Ha!

Let me know how it goes.