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running preamp filaments on dc


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9/29/1997 3:03 PM
robert hinson
running preamp filaments on dc
this one's for R.G., mark, or any other experienced tube amp guys. could you please post a good tried and true design for running tube amp filaments (preamp tubes) on dc? i have tried the setup in torres's book and weber's idea of floating dc on the ac voltage and both of these ideas seem to make matters worse.  
it seems like a simple process but wish you guys could post your ideas or possible schematic/description of things you have actually used in the past that have worked to reduce hum in preamp tubes. i would be using this mainly on octal 6volt preamp tube types (6sl7, etc.) since their filaments are nonhumbucking and seem to hum quite a bit more than the 12v type. anyway could one of you guys address this and post it? thanks
 
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9/29/1997 6:43 PM
R.G.
I just used brute force - I used a separate AC supply, rectified it to DC, regulated it or dropped it with resistors to the right DC value, then tied it to the filaments.  
 
It's critical that the resulting DC be ground referenced to the amp's signal ground somehow, whether one end (positive or negative) tied to signal ground, a resistive center tap on the DC tied to ground, or the whole mess tied to some other voltage that is then tied to ground. If it's not ground referenced somehow, it will hum.  
 
I forgot this on the first version of the tube univibe, and spent quite a bit of time chasing hum.  
 
I'm not familiar with the Torres concept you state about floating DC on the AC, but I wouldn't expect that to help. If by this you mean floating an AC filament voltage on a DC level, it was a common dodge at one time to tie the center tap of an AC filament voltage to some positive DC voltage to make the filaments suck back in any electrons emitted from the filament, which was a source of hum also. It only helps if the source of hum is the emission from the filament.  
 
There are a number of items on reducing hum listed in the Tube Amp FAQ, which I assume you've already read.  
 
Are you sure it's filament hum you're chasing? Could it be grounding hum, or input signal ground hum? Or possibly magnetically induced hum from power components or AC filament wiring?
 
10/6/1997 11:39 PM
Steve Ahola

According to Dan Torres, a DC filament supply is most effective in reducing the hum for the preamp tubes, while biasing an AC filament supply with 45 to 50 VDC helps reduce the hum in the output tubes. (His book says that biased filament supplies were used by Ampeg, Sovtec, Dynaco, EICO, MacIntosh and Scott).  
..I opted to go with the DC filament supply by adding a 25A bridge rectifier to the AC filament taps and removing the center tap, filtering the dc output with something like a 1000mfd cap. I measured the dc volts to make sure it wasn't over 6.3 volts. It was actually a bit low (5.9v, I think) but it has worked just fine.  
..With a DC filament supply, the positive (+) side should be connected to pins 4 & 5 of the preamp tubes and to the same pins on the output tubes (I chose pin #7 for the pair of 6L6's). On a '65 Pro Reverb, the filament leads alternated from tube to tube so I had to reverse the leads at every other socket.  
..The crystal clean normal channel on the Torres Super Texan kit had a noticeable 60hz hum that would not go away until I rewired the filaments for DC. (I had first replaced all of the off-board signal leads with shielded cable and still had that residual 60hz hum...)  
..I later read that Peavy used a dc filament supply for their Classic 20/30/50 amps to reduce noise. (Those are nice little amps, but with their printed circuit boards they are not very conducive to DIY mods and experimentation.)
 
9/29/1997 6:47 PM
Dave H

Dear Robert,  
 
Glass Audio, issue number 1,  
volume 9 1997 has a circuit  
that might fit your needs in  
the section entitled Glass Shards.  
Back issues are available. Check  
out their website at audioxpress.com  
for further info.  
 
D.H.
 
9/30/1997 12:01 AM
Bruce

Robert I made a comment a while back about DC  
filaments and stated that I was sitting on  
the fence about.  
I use the star ground/buss ground combo method  
and tie most all the power supply caps together &  
connect them to the same place on the chassis  
where the high voltage center tap is grounded.  
The main supply filter caps have their own  
seperate ground wire to the main chassis ground.  
 
Twist those filament wires togther pretty tight  
and make sure you use two different color  
wires and put one color on all the same  
pin numbers of like tubes.  
Reference the 6v wires to ground through a  
couple 100ohm resistors.  
Go back to AC. It is so much simpler.  
I have NEVER built a homebrew amp that needed  
DC filaments to get it quiet enuff.  
However, I recently tried a DC filament again  
and it still hummed away merrily until  
I discovered that if I disconnected the input  
jack from the chassis (oh boy did it hum then!!)  
and used an insulating washer, then took a  
homerun to ground from the ground side of the  
jack to the center tap of the power transformer.  
WOW!! The hum and alot of other strange nosies  
quite simply...went away.  
I mean so much so that I thought my preamp tube  
went away instead of the noise.  
Weird!  
I have read Torres mods too and I just have not  
had the need to get a guitar amp background hum  
down to stereogearhead levels.  
Bruce
 
10/6/1997 10:54 PM
Steve Ahola

Sounds like a great idea to isolate the ground on the input jack from the chassis and run it straight to the star ground! Does anyone make insulated jacks, or do I just look for insulating washers? (I was hoping I wouldn't have to ream out the hole on the chassis because those metal shards end up in the darndest places!)  
Steve Ahola
 
10/6/1997 11:58 PM
Bruce

Steve I think the stock Marshall type jacks are  
insulated from chassis gnd, aren't they?  
Any how Mouser sells the shouldered washers  
and the plastic Marshall types are available  
from just about everyone too.  
I get mine from Tom at Magic Parts. (RUBY TUBE)  
Bruce
 

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