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DC filaments a good idea?

11/19/2005 8:16 AM
Chris G
DC filaments a good idea?
I was looking up the datasheets on 12A_7 and 6V6 and it says that you can run the 6.3V fils either AC or DC.  
The first Paia tube mic pre I built ran the tubes on AC filaments. But when I switched to DC filaments, the noise floor was reduced noticably.  
Could this work for my 5E3? I wonder, because it seems obvious, yet I haven't found any mentions of this while searching the forum ...  
Anyone done this?
11/19/2005 8:51 AM

I have what may be an unpopluar opinion on this.  
I build pedals and preamps in addition to amps and one would guess I use DC filaments in such applications. But, I have found that AC can actually be quieter if done very well. In fact, it is easier to to a really good job on AC fils than to mess with DC and trying to get the voltage and current right after it is rectified (a huge filter cap and voltage regulator helps), etc, etc.  
So after comparing the 2 extensivley (because I was a bonehead and WANTED to use DC), I only use AC fils and have no fil noise problems.  
Now maybe in a mic pre, you can really hear the difference, but I haven't found that to be true in guitar gizmos.
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11/19/2005 9:15 AM

DC filaments are a pain. DC lifted filaments are quiet. I always use this method.  
In a nutshell: Take the filament centertap and put 40 to 60 vdc on it.
11/19/2005 9:26 AM
Chris/CMW amps

Hi Chris  
Because of various reasons I hate rectifying filament-voltages :) :D  
Here's a cool hum-reduction tip:  
instead of grounding the centre of the filamenttap or the connection between the 100 ohms resistors: put that connection on the cathodes of your 6V6s in a 5E3 or another cathode biased amp.  
Hope this helps,  
11/19/2005 11:37 AM
Chris G

Hmmm. In the hours since I posted, I found a ton of info on why NOT to use DC filament voltages. (Might be OK in a mic pre or phono stage, but not in a guitar amp ...)  
As for the cathode-grounding tip, I'll give that a shot, thanks. In the meantime, I'll do a little more research on that as well.  
11/19/2005 12:02 PM
Al Lang

Lifting the ground is an old Radio repair trick to help with hum, it works well.  
11/19/2005 8:41 PM
Dai H.

check this link out:  
and find a chassis pic of a Soldano SLO100--a high gain amp with AC filaments.
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