# AC Power to Phase Inverter Question



## coyoteblue (Feb 8, 2006)

I'm trying to understand a suggested modification to my amp, a Fargen Blackbird. I've been told that running the AC to the Phase Inverter filament will improve the sound quality of the amp. But I thought that the Preamp section already ran on AC, and the power tubes, via the rectifier, on DC. I'm not very knowledgeable about how the power supply in an amp works, but want to understand what I'm asking my tech to do when I ask him to make this change. Is this a simple mod, or would it take some time? Any clarification would be appreciated.


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## madkatb (May 14, 2009)

There's three voltages running through any tube amp.
First, DC voltage. Created through the rectifier (tube or solid state) and filtered to provide DC, this is the voltage that "runs" the tubes ( preamp, phase inverter and power) and causes them to amplify.
Second, AC voltage. This is your signal and should be the only AC inside the amplification circuit. Which is why the caps in the power circuit are so important as they filter out AC from the power supply circuit.
Third, voltage to run the filaments (or heaters). This is the one you are talking about. The filament is what heats up the cathode inside the tube allowing it to emit electrons and therefore allows the tube to work. No electrons emitted, no signal amplification. The heaters are, however, not part of the amplification circuit. They can be run on AC or DC. If AC there is a possibility that the alternating current (AC) can create a signal in the cathode (ie. 60 hz hum). This should not be a problem if the heaters are run on DC or if care is taken to ensure proper wiring layout.
Unless you are experiencing large amounts of hum in your amplifier there is no need to have the circuit changed to run on DC. It's not mentioned on Fargen's site but they are probably AC. In a hand made point-to-point amp like that I'm sure that the wiring layout was done very carefully to make sure that hum could not occur.


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## keeperofthegood (Apr 30, 2008)

To run 6.3 volts DC to the fillaments you need to have a 12.6 volt transformer, bridge rectifier (SS), two [email protected] caps, and a [email protected] choke (or 1A, depends on your tubes, one of the ones I have in my simulation has a 2.5 amp filament). Using resistances instead of the choke the voltage loss becomes big (I.E. 12.6volts-(voltage drop of 2 diodes + the voltage drop of the choke/resistor) = 6.3V), the choke from what I was seeing today was well over 100 dollars >.< but this would give you smooth DC rather than rough DC.

If you go rough, you are not really removing the hum causing alterations in voltage, but you can go rough for as little as 7vac and a single diode.



Though if it is already DC going to that tube (need a schematic to see how it is wired) then just pulling the AC to it could be dangerous as higher AC is needed due to the drops of the rectifier and filter.


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## nonreverb (Sep 19, 2006)

keeperofthegood said:


> To run 6.3 volts DC to the fillaments you need to have a 12.6 volt transformer, bridge rectifier (SS), two [email protected] caps, and a [email protected] choke (or 1A, depends on your tubes, one of the ones I have in my simulation has a 2.5 amp filament). Using resistances instead of the choke the voltage loss becomes big (I.E. 12.6volts-(voltage drop of 2 diodes + the voltage drop of the choke/resistor) = 6.3V), the choke from what I was seeing today was well over 100 dollars >.< but this would give you smooth DC rather than rough DC.
> 
> 
> 
> ...


1.True. The cost and build factors into DC filament circuits
2.Well, not really..remember the AC filament supply has more AC swing than the rectified DC voltage...even if it's rough
3.Not nearly enough to cause problems..if I'm reading your statement correctly


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## keeperofthegood (Apr 30, 2008)

nonreverb said:


> 1.True. The cost and build factors into DC filament circuits
> 2.Well, not really..remember the AC filament supply has more AC swing than the rectified DC voltage...even if it's rough
> 3.Not nearly enough to cause problems..if I'm reading your statement correctly


I am thinking a variable voltage regulator IC would be better for putting DC to the filaments. Less system loss overall. Not sure, just started today plotting out what would be needed to get my voltage/current values.


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## JMC Amps (Dec 19, 2009)

coyoteblue said:


> I'm trying to understand a suggested modification to my amp, a Fargen Blackbird. I've been told that running the AC to the Phase Inverter filament will improve the sound quality of the amp. But I thought that the Preamp section already ran on AC, and the power tubes, via the rectifier, on DC. I'm not very knowledgeable about how the power supply in an amp works, but want to understand what I'm asking my tech to do when I ask him to make this change. Is this a simple mod, or would it take some time? Any clarification would be appreciated.


G'day

I believe you are referring to the DC Stand-off method for reduced filament noise.

The mod consists of taking DC voltage off the plate supply, usually at the Phase Inverter B+ node (filter cap), adding a voltage divider and capacitor, then sending about 60-70 volts DC to the center tap of the AC heater winding. This simply refers ground "further away" from zero, reducing hum considerably on the AC filaments. If no center tap exists, a virtual center tap using two 100 ohm resistors is used in a similar way.

The book "The Ultimate Tone 3" by Kevin O'Connor explains this method in detail. 

Ben Fargen uses and licences some of Kevin's methods in his amps (Power Scaling), so this mod sounds like a natural. Why not just e-mail Fargen, see what he has to say? I hear He's a really nice guy! :smile:

-JMc


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## JMC Amps (Dec 19, 2009)

keeperofthegood said:


> To run 6.3 volts DC to the fillaments you need to have a 12.6 volt transformer, bridge rectifier (SS), two [email protected] caps, and a [email protected] choke (or 1A, depends on your tubes, one of the ones I have in my simulation has a 2.5 amp filament). Using resistances instead of the choke the voltage loss becomes big (I.E. 12.6volts-(voltage drop of 2 diodes + the voltage drop of the choke/resistor) = 6.3V), the choke from what I was seeing today was well over 100 dollars >.< but this would give you smooth DC rather than rough DC.


Hey Keeps,

I simply bridge a 6.3 volt winding with four 1N4001 diodes and add a 10000uf 25v cap, you'll end up with a filtered heater supply of about 6.8 VDC @ 1A. This is what Marshall does (or used to do) for their preamp heater supply, esp. in high gain amps, except they use a cheapo BR that always burns out. Simple, inexpensive.

-JMc


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## coyoteblue (Feb 8, 2006)

Thanks for the info, though I have to say much is beyond my understanding at this point. I did email Ben and he says that running AC into the PI filaments is the preferred way to run the Blackbird. He does this now but did not do it in earlier versions of the BB, such as in my 2005 version. I guess this means the PI was run off DC in earlier versions of the amp. I could have asked him for more explanation but the amp was used when I bought it, he answered a few of my other questions about the amp, and I'm just trying to get some other input on this mod. I didn't realize that the main issue is ground hum. My BB does have some hum though it's not excessive, and it's hard to tell how much is my basement outlet and how much the amp. I've only had the amp for a few days. A member of Gearpage.net told me that this mod improved the tone noticeably, so because I'm taking the amp in for biasing anyways, I thought I'd do this mod also. Thanks for your explanation, JMc, I'll pass it on to my amp tech here in Toronto when I take the amp in.


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## JMC Amps (Dec 19, 2009)

coyoteblue said:


> A member of Gearpage.net told me that this mod improved the tone noticeably...


Switching the filaments from DC to AC improves tone? Hmm, never heard that one before...

Mabey just converting the amp to the current schematic Ben uses now might show an improvement, but then again, great amps like his are very hard to improve upon! 

Good Luck

-JMc


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## WCGill (Mar 27, 2009)

"Switching the filaments from DC to AC improves tone? Hmm, never heard that one before..."

The stereo guys swear by it.


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## keeperofthegood (Apr 30, 2008)

JMC Amps said:


> Hey Keeps,
> 
> I simply bridge a 6.3 volt winding with four 1N4001 diodes and add a 10000uf 25v cap, you'll end up with a filtered heater supply of about 6.8 VDC @ 1A. This is what Marshall does (or used to do) for their preamp heater supply, esp. in high gain amps, except they use a cheapo BR that always burns out. Simple, inexpensive.
> 
> -JMc


Did this up in a simulation, and using 6.3 as my starting AC, I got 3.6 volts out after the bridge and cap. I moved the supply up in voltage as I smoothed the DC. 12.6, bridge, two 8uF and 1H choke got me 6.35 volts on my filament resistances in the simulation. I will need to hard wire build it yes, that is next  and see real world application against the simulation. The only hold up for the moment are the caps which I am waiting on next clear pay to purchase 

If you are interested, when I am more awake, I can get you the simulation and information and you can see what I am seeing and maybe show what I am missing


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## coyoteblue (Feb 8, 2006)

Is this AC to PI filament mod something any competent amp tech could do? Any suggestions for someone in Toronto who would do this mod right the first time? Thanks.


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## nonreverb (Sep 19, 2006)

WCGill said:


> "Switching the filaments from DC to AC improves tone? Hmm, never heard that one before..."
> 
> The stereo guys swear by it.


Reduce hum? Yes. Improve tone? No. I don't buy that. I think that's just more audiophile lore.


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## keeperofthegood (Apr 30, 2008)

WCGill said:


> "Switching the filaments from DC to AC improves tone? Hmm, never heard that one before..."
> 
> The stereo guys swear by it.





nonreverb said:


> Reduce hum? Yes. Improve tone? No. I don't buy that. I think that's just more audiophile lore.





JMC Amps said:


> G'day
> 
> I believe you are referring to the DC Stand-off method for reduced filament noise.
> 
> ...




Hummm and Tone aside, isn't this the way to reference the cathode in relation to the filament? I know that having the cathode and filament not too far apart is important, especially in any cathode follower stages, usually tagged to not exceed 60 to 90 volts depending on tube. I thought this center-line filament voltage was to ensure the filament and cathode stay together? Or have I misinterpreted that?


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## Wild Bill (May 3, 2006)

Could any competent tech do these filament DC mods? I would think so! Otherwise, he or she would not be a competent tech!

Running filaments on DC or elevating the filament by putting a DC bias on the centre tap is an OLD idea! I've seen it done in equipment from the 1930's!

How well does it work? If you have a well-designed amp that already has very little hum these DC filament tricks might remove the last little bit. For ultra hifi amps or for a studio guitar amp it can be worth it.

However, it will NOT fix hum in an amp that is poorly designed or screwed up in the first place! If you've made a lot of mistakes then you will have to fix them first. DC on the filaments is NOT a magic wand!

That's why most amps don't bother. If you build things properly, the extra expense and labour is not really worth it, most of the time.

Most of the 5E3 Fender Tweed Deluxe clones copy a trick pioneered by Bruce Collins of Mission Amps, where he puts the filament centre tap onto the cathode of the 6V6, above the 250 ohm cathode resistor. This elevates the filaments by the amount of the cathode bias. Doesn't always seem to be necessary but hey, it's an easy thing to do!

:food-smiley-004:


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