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anyone for another hum riddle?

2/13/2001 7:54 AM
Tim Gagan
anyone for another hum riddle?
This amp I'm building here, basically an EL84 Plexi with a Fender Blackface Reverb circuit, has way too much 60 cycle hum, about 40mv with the volume turned down. It gets progressively more ridiculous as the volume gets turned up, so I'm thinking the cause of the hum must be pervasive (except that there's no hum from the power amp). I've swapped tubes, biased the filament circuit, checked my grounds, shielded key signal path runs, grounded those things in the middle of the tube sockets, all to no avail. The grounding scheme is what I've used with fine results in several other amps: star grounding the AC and power supply (including all filter caps) near the pt, and a ground buss that ties to the chassis near the input for all signal path grounds.  
This amp is built inside a stripped out Mesa .22, with the Mesa transformers. Because the chassis has so many ventilation holes, I had to screw down a plate to mount the tube sockets to. I could go into details about how the transformers are mounted if someone thinks transformer placement could be the culprit, but the pt is horizontal mount and the ot vertical. The reverb transformer and the ot are sitting 90 degrees from each other, about 3" apart.  
Anyone have any ideas as to what's causing this hum?  
Tim Gagan  
The World's Only Double Doubleneck Lefthand Band
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2/13/2001 3:23 PM

First off:  
how do you know it is 60Hz hum?  
if the volume control is after the first stage and it worse when you turn it up then the hum is before the volume pot or at the pot.  
if you still hear it with the volume pot down, then it must be bypassing the pot too.  
Which makes me think it's right there.  
Of course not seeing or hearing it puts us at a tremendous disadvantage. It could be one simple little wire not quite at the right spot in the ground scheme.  
I'd suspect your ground buss/chassis/input thing is not right.  
Most amps have at least a 15mvac to 20mvac hum when measured at the speaker jack.  
This is not too much.  
2/13/2001 3:36 PM
Knut Tore

I know the plexi circiut but not the fender reverb circ - so I'll just have to do some guessings...  
Do you use DC or AC heating? I have had some bad AC heating exp. Normally I've found that the biggest hum problems are in the first one or two tubes, because of the high gain (all the rest of the amp. stages) and the low signal in this stages (high noise to signal ratio) Anyway the first thought that comes to mind is the reverb tank wich serves as a hum antenna. Do you have a true bypass (both reverb input and output switched)? If not try to earth/short the reverb tank output and read the scope for changes. I have experienced that the direction and the position (still inside cabinet) of the rev.tank can change hum radically. In some cases I have made an own housing for the reverb tank in steel plates for magnetic shielding and put it in the oposite side of the transformers in combo cabinets with good results (use shielded cable).  
If you in anyway overload the pwr. transformer into core saturation (to small transformer or to high bias current etc), I've found this to give excessive magnetic noise. Another way to reduse hum is to kill the source, using a magnetic shielded toroid pwr transformer (custom made to resonable cost ~$100 for one - and short delivery time) available from
2/13/2001 5:29 PM

An oscilloscope is handy for finding hum. You can trace the circuit from one end to the other. Look at the plate and signal grid pins of each tube. Start at the power stage and move backwards toward the input jack. You should be able to find in what stage the hum originates.  
Without a scope, you could try unplugging each tube, starting with the first preamp. Since 12AX7's are dual tubes, it will be at least narrowed down to a stage-pair location.  
Make sure the bias supply is well filtered DC. If it were a power stage problem, the AC riding the bias voltage will act like an AC signal input.  
Since you did mention the hum level can be varied with the volume control's setting, I'd suspect that the hum source was somewhere between the volume pot's output and the input jack, and that the PI/power amp section was probably not contributing anything but amplification.  
Don't overlook the pots. Sometimes you get a factory defect, or extra heat input from soldering annoys the mechanical connection between the lug & the carbon trace.
2/13/2001 5:37 PM

could you describe a little more in depth how to check for hum (or squeling in my case) with a scope?? i have a Eico Dual Trace (a nice one) and i have no real clue how to make it useful. if it ist too much trouble, could you include the settings used on the scope so i dont hurt mine when pooking around in high voltage areas!! thanks!  
mike z
2/13/2001 8:50 PM
Tim Gagan

Thanks for the suggestions. I haven't quite worked it all out, but after taking the amp for a car ride the volume-down hum is now between 7 and 12 mv, which doesn't bother me too much. I do think it's still excessive with the volume up, though, so I'll investigate the pot.
2/14/2001 9:33 AM
Hey Tim,  
I knew that taking a noisy baby for a drive can quiten 'em down a bit.  
Didn't know it worked for amps too :D  
Did you have the stereo turned up as well?  
PeteR :)

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