Separate Bias Winding Question

[mauiboy81]

I am building an amp using an old PT I had sitting around. It has a separate 38-0-38 center tapped secondary winding that I want to use for the bias supply. I have not used a separate winding for bias before and was wondering how to go about it. Can I just do the standard full wave rect except use a reversed diode off each secondary wire? Would i need more filtering than in the standard d style #124 bias supply?

Mucho Mahalos!

[martin manning]

Sure no problem. Full-wave supply with diodes and caps reversed. You really could get away with less filtering since it's FW, but you can keep it as-is if you don't need the room. The 38 VAC might be cutting it close, but you can always do a FWB or HW and ignore the CT.

[iknowjohnny]

Is there an advantage to doing it that way? More stable bias voltage maybe?

[Firestorm]

Is there an advantage to doing it that way? More stable bias voltage maybe?

Full wave bias is quieter. If you have enough juice. You didn't say what the output tube complement was.

[tubeswell]

There isn't really that much of an advantage. The bias supply draws almost zip current, so it not as if the ripple for a 1/2-wave rectifier is going to be very exaggerated.

[Firestorm]

There isn't really that much of an advantage. The bias supply draws almost zip current, so it not as if the ripple for a 1/2-wave rectifier is going to be very exaggerated.

True dat.

[Bob-I]

You can always use a voltage doubler, very quiet and will give more than enough range.

[Firestorm]

You can always use a voltage doubler, very quiet and will give more than enough range.

True dat, too.

Very little current required. You can make that voltage whatever you need.

[mauiboy81]

Hey guys I appreciate the help! I'm going to forget about the center tap and go with the HW or possibly FWB. Have any of you really noticed any difference in noise between the two? Btw It'll be running either a pair of 6v6's or 5881's.

[Leo_Gnardo]

I'm going to forget about the center tap and go with the HW or possibly FWB. Have any of you really noticed any difference in noise between the two?

You can filter a HW (just a single rectifier, like Fender) with 100uF or a pi filter say 22uF-10K-22uF and have no hum/buzz from the bias supply show up in your signal.

Full bridge - 4 rectifiers - can't say I've ever seen that done in a bias supply. I'd stick with ol' fashioned HW and keep the parts count down.

Where are you in Maui, one of my favorite places on the planet?

[mauiboy81]

Thanks! I was leaning towards the HW for that very reason. Hopefully I can get to wiring the amp up today!

I'm from Makawao / Olinda. It is a magical place for sure!

[tubeswell]

You also can tap the 1/2-wave bias supply from one side of the HT winding (like a brown or BF/SF princeton)

[Leo_Gnardo]

I'm from Makawao / Olinda. It is a magical place for sure!

Great location, got to visit Makawao @ 12 years ago, on the way down from "the top of the hill." If I ever hit the lottery, I'm movin' in!

Couple of years ago there was a rumor of a wild black cat haunting the neighborhood. Not a house cat, somethin' bigger. Locals were worried on behalf of the wildlife.

FWIW theres a hi fi builder somewhere on the island, he posts on Audio Asylum with the name Poindexter. His real gig is building custom yachts apparently (gotta make a living somehow), but his audio ideas are top notch. Simple, straightforward, quality builds - he published a schemo for a prime looking stereo amp, pair of 6V6 per channel. IIRC he uses 6AQ5 to drive them, a bit unusual but those who have built 'em, like 'em.