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Converting Heaters to DC: worth it?

1/17/2000 5:41 PM
Rob W.
Converting Heaters to DC: worth it?
In my constant pursuit of low hum, would it be worth it to convert my heaters to DC in my Bassman clone? AC heaters are beginning to give me a headache when hum-chasing.  
What does this necessarily entail (circuit wise), and has anyone had good luck with it.  
Let the opinions fly! I want to get some good opinions on this.  
Rob W. :-)
1/17/2000 6:04 PM

Hey, it's been a while since the last DC/AC heater debate...  
Here's what I got out of the last one:  
- it'll help, ain't rally too complex, and ya might as well do it if you want to, since that's what building your own amps is all about.  
- if it's done correctly, it will eliminate the heater hum from the list of things to check when you're "hum chasin'"  
- proper AC heater implementation is good enough for most folks, especially if the AC is DC biased up a few volts, which is real easy on a cathode biased amp.  
- don't waste DC heatering the output tubes in a PP amp, it isn't necessary.  
- it may shorten tube life a bit.  
- if you're using relays, you can kill two birds with one stone.  
- you can get as easy as a big cap, and resistor and maybe another cap, or more complicated with some sillycone regulation.  
For the latter, check out the power supply section here  
for a good example...(fig. 3)  
... have fun...  
... Whit  
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1/17/2000 7:03 PM
Scott Swartz

So you're going to try DC heaters for the hum problem? It might help, but note that heater noise is usually more of a buzzing rather than a hum. Unsolder the 100 ohm resistors or the filament CT to hear it.  
A important issue on DC filaments is the increased load put on the power trans by the current peaks caused by the rectification to DC. The rule of thumb is derate the trans to 70%, so if you had a filament winding rated for 3 amps, you would only want to draw 2 amps DC on it. You don't say if there is reserve capacity in your power trans. Use 10,000 uF per filament amp for filtering.
1/17/2000 8:32 PM
Rob W.

Hi Scott!  
Well.. this is another project other than the Twin.  
(Gotta keep that soldering iron busy)  
The power tranny on this baby is the standard 50W Bassman type from New Sensor. #NSC022814  
690V/CT @ 140mA  
6.3V Heaters @ 4A  
I will be running only 4-5 tubes off of this configurations. 2x6L6GC, and 3x12AX7. Would this overstress the tranny?  
Suggestions would be awesome.  
The page/picture that got me thinking of this is right here:  
Would this be the way to lay out the DC heaters?  
Sorry about so many generic questions, but I am unfamiliar with DC heater construction.  
Thanks again!  
Rob W. :-)  
1/17/2000 10:56 PM
Scott Swartz

That trans should be OK, as long as you never put EL-34s in there.  
The strain2.gif looks fine, but shielded cable for DC might be overkill.
1/18/2000 8:02 AM
Steve A.

    I agree with you about the shielded cable. Shouldn't the heater leads be at least 18 gauge wire (most shielded cable isn't that heavy and the shield and jacket would tend to hold in any heat).  
    I believe that Rob would also want to isolate the negative on the bridge from ground with something like a 0.1uF cap.  
Steve Ahola
1/18/2000 2:25 PM
Rob W.
Re: More Questions :-)
Thank you all very much for all of the wonderful replies to this question.  
I believe that I will go ahead with this conversion, but I have a few more questions first :-)  
First: what is the purpose of having a resistor before the cap that comes off of the + side of the bridge? I every schematic that i have seen it tends to be a small value (ei.e. .33ohm)  
Second: i have heard about leaving the poweramp with AC heaters and running the preamp on DC... Is this more desireable or is this not a good idea with just one heater winding on the tranny?  
Third: since i got in trouble for giving advice on heaters the other day ;-) how would i want to hook up the heater pins for 6L6GC's and 12AX7's? All schems so far show connecting the heater pins together on the 12AX7's, connecting the heater wire to pins 4&5, and grounding pin (. (these pins are all connected together respectively)  
but what about the 6L6GC?  
Thanks Again!  
Rob W.
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