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Running AC to Phase Inverter (in a Fargen Blackbird)

coyoteblue

Silver Supporting Member
Messages
242
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? I'd appreciate any clarification you can offer.
 

andrekp

Member
Messages
5,711
I can't answer your question, but maybe I can clarify what you're talking about.

Every tube has two things going on:
1). DC (from the rectifier/power supply is being used to bias the tube and amplify the signal.
2). AC or DC is being used to heat the tube to operating temp.

I think what you must be talking about is #2. Most tubes in an amp require 5-6.5 volts of heater voltage that can be in the form of AC, or DC. It comes off a separate tap on the power transformer and either goes direct to the tube heaters (AC), or is rectified into DC first. The reason for rectifying it is that some feel it is less prone to create hum in the amps output than AC. Done correctly, AC heaters are not a problem.

As an added complication, sometimes AC heaters can be made even more quiet by "grounding" the heater's real or virtual center tap on one of the power tube's cathodes at about 9v (in a cathode biased amp).

So maybe this short explanation will help clarify what you know, but I have no idea what you are refering to. Personally, I don't see how running the PI's tube on AC would improve anything. Done correctly, I doubt you'd hear a change at all. But again, I don't know of the mod, so...

These functions are separate from each other.
 

phsyconoodler

Member
Messages
4,305
Unless the Fargen Blackbird is a high gain amp it likely has AC filaments already.The rest of the phase inverter cannot operate on AC.
You need to be a little more clear on what this 'mod' is all about.
Even high-gain marshall and many other amps use AC filaments on the PI(normal) and DC filaments on the rest of the preamp tubes to reduce noise.
A normal tube amp uses AC filaments for all tubes.

I looked at the Fargen site and the Blackbird looks to me to me to be like a Princeton Reverb.
I'm thinking you mean to do the PI mod where you increase the DC voltage to the PI. for more clean headroom.??
 

coyoteblue

Silver Supporting Member
Messages
242
Thanks for the response. My understanding is that older Blackbirds like mine use DC filaments for the PI, and that in recent BBs the PI draws on AC filaments. So my question is, is it worthwhile change the use of DC to AC filaments for the PI? I hope I'm wording this correctly.
 

phsyconoodler

Member
Messages
4,305
I really doubt there would be any advantage to swapping the filament voltage from a performance standpoint.All the filament does is heat the tube.DC filtered voltage is usually quieter than AC.That's about it.
 

andrekp

Member
Messages
5,711
You can certainly pay someone the money to change the filiments if you want, but it's NOT going to change the sound of the amp. If you currently have a hum problem that needs to be addressed, then by all means address it, but it won't affect anything else - and if it ain't broke...
 

Mike Fleming

Member
Messages
1,228
Ben Fargen used to post here occasionally, maybe he'll see this thread and chime in himself. Or you could even PM him to hear what he has to say about it.
 

coyoteblue

Silver Supporting Member
Messages
242
Ben suggested the mod though he made no claims about tone being improved. Another GP member he said this change made a significant difference. Perhaps something else affected the member's amp other than this filament change. I'm going to stick with the way it is.
 




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