Check my layout please......

[C Moore]

This is an Ampeg M-12.
http://www.webphix.com/schematic%20heav ... 12_6v6.pdf

I had been working pretty diligently on this amp, but then got interrupted for a few months. Anyway.....it is "done" now. The trem is kind of funny sounding, but it works. Definitely sounds way different than the trem of a Fender AB763.
The amp has a fair amount of headroom, and really sounds pretty good. My biggest concern is the "noise floor". It is not bad, but would show up in the studio if the volume was turned up at all. Noise increases with both Tone and Volume. No hiss at all, just some extra/un-wanted hum. I cannot chop-stick the noise away, so I am wondering if you guys see anything in my ground or potentiometer layout that you think I should improve. This was an old PA Head. I did the best I could to make everything fit. The trem circuit is located on the same side of the chassis as the tubes, transformers, and pots.
Do you guys see anything that looks like it may be a noise/hum issue.?
Thank You

http://i111.photobucket.com/albums/n143 ... -12007.jpg
http://i111.photobucket.com/albums/n143 ... -12006.jpg
http://i111.photobucket.com/albums/n143 ... -12005.jpg
http://i111.photobucket.com/albums/n143 ... -12004.jpg
http://i111.photobucket.com/albums/n143 ... -12003.jpg
http://i111.photobucket.com/albums/n143 ... -12002.jpg
http://i111.photobucket.com/albums/n143 ... 2001-1.jpg

[selloutrr]

did you make sure your metal was clean and making good contact as a ground for the pots?

are all nuts and bolts tight?

The inputs are directly crossing the heater wires. - are they touching?

Have you tried a grounding bus on the pots?

[C Moore]

did you make sure your metal was clean and making good contact as a ground for the pots?

are all nuts and bolts tight?

The inputs are directly crossing the heater wires. - are they touching?

Have you tried a grounding bus on the pots?

Yeah, I used a Dremel to make sure I had good ground contact with the chassis. Everything is good and tight.
It is hard to tell from my pics, but yes, the heater and grid wires have a fair amount of air between them.
You ask about a "pot grounding buss". I have not tried it and not sure I even know what it is. Would it just be across the pot body's and then to ground at some point.? Does that stay separate from the pot lugs that get grounded.?
Thanks

[jjman]

I worked on a friend's M12 with 6v6s. Only NOS 6SL7's eliminated the hum. It had newer Sovteks in it during the hum. Even DC heaters didn't help the Sovteks. His trem is fantastic and I'm thinking of switching one of my amps to it's approach.

[C Moore]

I have NOS in the pre. I had a couple and then I bought 2 more from Doug's Tubes.
My 6V6 are modern day Tung-Sol.

[selloutrr]

you can experiment with the grounding of the pots with alligator clips
clip to a common ground point and then jumper to the pots. if it works make it perminat if not no solder to remove.

(how are you grounding your audio and power)

are you tubes tested for noise? , microphonics?

[C Moore]

I did not know you can test tubes for noise.....I bought the SL7 from Doug and the 6V6 from KCA.
My mains is grounded to a PT bolt.
The speaker jack is grounded to a un-used tube socket.
Regarding the pots, I am still not clear what you are saying to do. Do I leave the pot lugs grounded just as they are, and also ground the pot body's.?
Thanks

[dynaman]

I used to own a couple of Bogens like yours and the sheet metal construction is pretty complicated. There are lots of different parts that are spot welded or just screwed together. Screwed connections like those holding your front panel on suffer from rust/oxidation after 50+ years. If you're relying on just the front panel for "ground" you should at least make sure the panel connections offer no resistance to the main chassis and rest of the circuit. Do your pots have at least one ground wire running to a known clean connection like the input jack or somewhere on the main chassis?

[C Moore]

Yeah.....if you look at pic number 3 and 4 you will see that.
Thanks

[selloutrr]

My amplitrex and VTV can test for noise in tubes.

yes you can leave the pots just how they are and add a new ground bus to the back of the pots terminating to the main chassis. as stated the panels can suffer over the years and loose a solid contact.

another fun test is after you add the ground bus to the pots. if it's not better try floating the pots from the front panel while still using the ground bus... any better?

[C Moore]

My amplitrex and VTV can test for noise in tubes.

yes you can leave the pots just how they are and add a new ground bus to the back of the pots terminating to the main chassis. as stated the panels can suffer over the years and loose a solid contact.

another fun test is after you add the ground bus to the pots. if it's not better try floating the pots from the front panel while still using the ground bus... any better?

OK.....all I can do is apologize for my lack of comprehension. Physically, I understand what you are telling me to do, but I do not get the theory. There is no continuity between pot lugs and pot body....what is accomplished by grounding the body of the potentiometer to the chassis.?
Thank You

[selloutrr]

the body of the pot is being grounded to the chassis via the threaded shaft. if you are not grounding and legs of the pots to the body as a ground, the body still acts as a shield.

any grounds you have coming off the pots should go to the same piece of metal as the other grounds.

I'm guessing, since i can't physically test it but the material of the chassis may also play a part in your grounding issue.

you may also want to meter (ohm) to all grounds and possible resolder them to get the lowest reading possible.

grounded power cord - short isolated (it's own lug) to ground?

[Alexo]

Wonder if it's due to the PT being so close to the pots.

[Structo]

on your input jack do you have the part that contacts the shaft of the plug grounded to your ground buss?

With point to point it is hard to tell whats going on there but your grounds, such as the cathode bypass for your preamp tubes and the grounds from the other tube cathodes should all be joined together at some point.

Since the chassis is painted where you have your terminal strips, I would not count on any of that being grounded simply by their being mounted to the chassis with screws.

When you look at a schematic and you see the grounds below the preamp tubes, that ground should be common to all those points that are represented by the ground symbols.