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Homebrew Hum

3/18/2003 8:08 PM
Homebrew Hum
5e3 hoffman  
TD is laced up, but not buttoned down. I fired it up and it works, havnt took any measurements yet (pulled all nighter and ditched my am class's) there is some serious hum issues:  
All jacks sound the same:  
1) both vol's increase in hum when turned up  
2) tone pot increases higher freq crackle/fizz "zzzzzzzzzzz" sound  
the controls seem to have a sweet spot early in their adjustment where the hum is somewhat muted, I poked around w/a wood spoon moving wire especially around the pots and filaments, changed nothing.  
some considerations  
I put a stdby spst off the recto  
I didnt use the 2 100ohm resistors off the lamp/heaters  
I didnt use a ground buss off the pot bodies  
I used 2 point star ground :  
1) pots/preamp grounded to inputjacks  
2) power amp/PTCT/earth ground/OT2ndary ground to PT leg  
Any Ideas?  
Gotta make it to my pm classes and wont have any time to F-around w/this thing till the WE. Now it's starting to get fun!!  
3/18/2003 8:40 PM
Pulling V2 (12ax7) cancells all hum  
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3/20/2003 2:00 AM
A tidbit till the real amphounds show up..
Hi Wayne,  
I'll throw a thought or two out and then let the pro's dice me up..  
The amps I've built that experienced hum didn't amplify the hum, that is, the hum didn't increase with gain, but since yours does that will probably be a good smoking gun to finding out what's causing it.  
You mentioned that the hum stops when you pull V2, so the noise source is somewhere in the pre-amp, or phase inverter. One thing to ask is, what does the noise sound like? Is it a low pitch hum, or does it have any higher frequency abnormalities, like a fast ticking sort of noise on top of a lower freq. hum? I find that sound with poor grounds, but again, they usually aren't gain dependant, I just asked the question so your answer would give the other folks a better picture of what your amp is doing.  
You failed to mention it, and I don't want all of to assume so, but did the noise not stop after pulling only V1?  
Gotta scoot, hope these questions help guide you to the solution..  
3/20/2003 4:23 AM

theanks bill  
killed the hum, dropped 2 100R's off the lamps = no hum  
3/25/2003 1:45 AM
Matthew Springer

It sounds like the hum is almost certainly in the very early stages of the amp. Did you shield the input from the jack to the first gain stages grid? Definately shield the first stage. Next thing is to make sure the preamp is sufficiently decoupled from the rest of the amp. Try increasing the amount of capacitance on the firts stage and see if the hum goes down. Also try switching tubes around. Sometimes, you just get a bad tube.  
Also, try putting the two 100ohm resistors back in and lifting the heater ground center tap.  
3/25/2003 10:13 AM
I am going to try this with my Bandmaster which i wired into a super reverb..  
but just to clarify..  
the heater ground CT should be disconnected from ground and just left insulated hanging in mid air?  
(currently it is grounded..the green-yellow wire)  
this is if i were to also drop 2 x 100R's off the heater lamps and onto ground..(currently no 100R's in place)  
thanks for your clarification  
3/25/2003 4:23 PM

Yes, cut the 6.3 CT then shrink tube the end, coil it up and put out of way. Put 2 100ohm R's off the legs of the lamp,then to ground. My application didnt have a filament center tap, so the two balancing R's helped hum tremendously. Ive also heard that its good practice to measure the value of the 2 R's and make sure their close as possible.  
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