Hum increases with chassis top installed vs open?

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figuringstuffout

Well-known member
Joined
Dec 10, 2011
Messages
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So I have a tube preamp design that I'm goofing around with (just a basic 12ax7 based design with a gain knob in between the two gain stages with a mosfet source follower installed on the output of the second gain stage before hitting the output transformer going to the XLR out.
Heaters are AC, dressed well, elevated to 70v, filtered well (100uf) and even run through a 500R humdinger pot for good measure.
Power transformer is mounted horizontally hanging out the back of the chassis.

It sounds great.

The problem is this:  When the chassis top is off the hum from the heaters (60hz) is VERY low...as in basically not there.  However, when I put the chassis top on the 60hz hum jumps quite a bit - maybe 10db.

Any ideas as to why this would be happening?  I thought that maybe there was some kind of ground loop or something but I have a single star ground connection connecting to the chassis at a single point.

Thanks for any help!
 
Sometimes a steel chassis can act as a magnetic conductor and couple magnetic fields from a mains transformer to other transformers in the box. Without the lid this coupling may be decreased.

Cheers

Ian
 
The chassis cover can both distort the magnetic field, and alter chassis ground path resistance.  The cover is not the problem but aggravating problems elsewhere.

JR

PS: This topic has been discussed here before.
 
Separate power supply ground to near the AC inlet and the remaining grounds to a point very near the mic input works in many cases for me.

Regards

/John
 
ruffrecords said:
Sometimes a steel chassis can act as a magnetic conductor and couple magnetic fields from a mains transformer to other transformers in the box. Without the lid this coupling may be decreased.

Cheers

Ian
Click to expand...

Interesting.  And aluminum doesn't do this?


johnheath said:
Separate power supply ground to near the AC inlet and the remaining grounds to a point very near the mic input works in many cases for me.

Regards

/John
Click to expand...
So having the power supply ground separated like you said might fix the issue with the cover?  Wouldn't that cause more issues since you would have separate ground connections from the circuit to the chassis?

Sorry for the ignorance on my part
 
Well, I work almost solely on tube amps and tube preamps and I also use steel chassis. I found out by trial and error that separating the power supply ground from the "rest" works fine on the possible grounding issues… just this week I finished a couple of LA2A clones and I did just that on those two and they are practically noiseless… and the LA2A can be a bit noisy... often caused by the ac on the heater supply.  But as mentioned the grouding can also create noise if done wrong.

Much humming noise in guitar amps are often caused by grounding issues due to a "quick fix" in grounding reducing costs I guess? :)

Try it it might reduce the hum or just keep it the way you have if it is not the problem :)

Also preamps with a none canned output transformer can also be susceptible for magnetic stray from the power transformer… I have found this particularly common with Hammond power transformers for some reason.

A good toroid power transformer is the best and I use power transformers from don-audio.com with no problems. Otherwise I like input and output transformers from sowter.

Best of luck

/John
 
johnheath said:
Well, I work almost solely on tube amps and tube preamps and I also use steel chassis. I found out by trial and error that separating the power supply ground from the "rest" works fine on the possible grounding issues… just this week I finished a couple of LA2A clones and I did just that on those two and they are practically noiseless… and the LA2A can be a bit noisy... often caused by the ac on the heater supply.  But as mentioned the grouding can also create noise if done wrong.

Much humming noise in guitar amps are often caused by grounding issues due to a "quick fix" in grounding reducing costs I guess? :)

Try it it might reduce the hum or just keep it the way you have if it is not the problem :)

Also preamps with a none canned output transformer can also be susceptible for magnetic stray from the power transformer… I have found this particularly common with Hammond power transformers for some reason.

A good toroid power transformer is the best and I use power transformers from don-audio.com with no problems. Otherwise I like input and output transformers from sowter.

Best of luck

/John
Click to expand...

Playing around V1 can help a lot. I made a couple of LA2A, ac heatered, and tried a bunch of tubes at V1. Some helped a lot and the unit met the manufaturer noise spec, some other surpassed it, while other tubes where Noisy as hell. Feeding V1 with DC heater can help a lot. I had a trouble one time with the output transformer polluted by the power transformer, Inside a steel chassis. I lifted the output transformer on a thick piece of aluminium and it cured it. 
 
Yes I have been wrestling a couple of LA2A's before… I switched to Sowter power transformer and a canned Sowter, mounted on the outside, output transformer and the ac were dressed properly so this time it was a great success.

Regards

/John
 
johnheath said:
Yes I have been wrestling a couple of LA2A's before… I switched to Sowter power transformer and a canned Sowter, mounted on the outside, output transformer and the ac were dressed properly so this time it was a great success.

Regards

/John
Click to expand...

Never used the Sowter power transformer. Had lot of "trouble", hmmm no sorry, not trouble, but unimpressed by the performance of the popular Allied 6K88 power transformer, and Hammond power transformer in general. I found nearly all of the Allied were mechanically humming a little (ok, you have to put your ears close to it but hum is there). This is why I switched to use Par-Metal chassis. Serie 10 has the front and rear panel aluminium, which help for noise propagation across teh chassis. And now, I think of switching to toroidal power transformer for all my la2a build. I think the biggest challange is to find the right tube for V1. The one who will humbucking efficacely and will provide good noise performance.
 

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