Hand induced hum on JLTi copy, any ideas??

I am building a copy of the JLTi using all the usual sites and search features of this great forum. I am at the troubleshooting stage and trying to find a source of a hum. I also can see a slight movement of the speaker cone back and forth slowly with a pattern, four times and pause, four times and pause, etc.

Any ways, during ground arrangement testing I found that if my hand gets near the circuit from pin 2 on the valve through 3k9 resistor to the POT, the hum gets louder. I don't have a scope right now but I'd say it is 120hz or so. it changes a little with the volume knob but it definately still there with it all the way down.

The chips and rectifiers are not hot at all. Music sounds great. removing input source makes the hum slightly louder but not much. The 3k9 resistors are right at the tube socket. The trannys and rectifiers are in a seperate shielded case. cases are grounded to earth.

Any ideas??? If I could get the cone movement to stop I'd live with a little hum and just keep my hands away from the input:)
 
THe joy of trouble shooting or wish I had a bigger gun

FIrst thing is the fluctuation you are describing is most likely an oscillation in the amp.

Second I would reheat all your solder joints. You never know and a cold joint sneaks up on all of us from time to time. Be very methodical and trace through your circuit making sure it follows the design. Better yet do the trace with someone else. An extra set of eyes never hurts. You may find that there is a fault somewhere. After that you really need a scope.

MArk
 

costiss

Member
2002-12-26 1:27 pm
greece
you probably need some shielding.. use some copper foil to guard your high impedance nodes.. its not a fault.. had the same effect putting my finger on my 549s input twisted pair.. shielding this, i have no problem.. the high impedance of the input is prone to pick up noise
 
That's what I was thinking about trying, I went through all my joints to no avail. I did reduce the cone movement with resistors across the input + and - but I am not sure it didn't affect the sound quality a bit. Something about the sound stage is different.

A question for COSTISS, I ground the copper shields to the system right? Thanks for the replies people! JS
 
The shields helped quite a bit I think I can live with the remaining hum as it is only audible with my ear at the driver. the cone movement is gone. I do have some distortion at higher levels that I didn't have with my straight IGC, but I am going to remove the 100k resistors from across the inputs and see if that changes anything. Does it take a while for tubes to break in or warm up? I know about the first 10-12 seconds it makes noise, but how about distortion levels after that?
 

Shoog

Member
2002-08-15 10:16 pm
Eire
I had similar issues with one of my valve buffers. Moving a hand near to the valve made it buzz. If I remember correctly it turned out to be a dodgy input connection on the valve base. Valves will go looking for signal if they are not correctly referenced to ground.

Shoog