Capacitive Divider Bias Supply Help

[Wilder Amplification]

I use the Antek transformers and for bias you can take a small (<6VA) 120v/6v PCB mount transformer and run it backwards from the heaters - or you could use a small isolation transformer - I've done both. I think they're like 3 bucks and about 1" square or something. I thought about going down the road you're going down and was really put off by the hoops you had to jump through to make it work. Seems so much easier to just a small transformer just for the bias in these sorts of situations.

That's very true. If that build ever happens, I'll probably end up using the 6.3VAC one backwards off the heaters. Doing it with an iso-xformer would require a dual tapped primary for use in other countries, but the backwards off the heaters trick would be better since heater voltage is always the same provided you configure the mains for the mains voltage you're running. And since it doesn't have to be very big it can fit under the chassis.

[Pedro Vecino]

Another alternative is the AC50 bias circuit. This example is for typical EL84 amps (-15 volts aprox.). Works great

Diode is 1N4007 (not 4001)

[defaced]

That's very true. If that build ever happens, I'll probably end up using the 6.3VAC one backwards off the heaters. Doing it with an iso-xformer would require a dual tapped primary for use in other countries, but the backwards off the heaters trick would be better since heater voltage is always the same provided you configure the mains for the mains voltage you're running. And since it doesn't have to be very big it can fit under the chassis.

Check out the Triad flat pack transformers (this is what I'm using now). They've got some small dual tapped primary/secondary isos.

[Merlinb]

It just makes me wonder...why do some companies make power trannys without a separate bias winding?

I don't get what the problem is with the cap coupled bias? Sure you don't need much more that 10% of the HT for your bias?? In any case, just use a bigger coupling cap!

[imaradiostar]

I'll ride Merlin's coattails...what's wrong with cap bias? Sure there is the occasional story about an Ampeg blowing up but that's way in the minority compared to all the cap biased amps out there that work just fine. If you're worried about it buy a surplus cap that's rated for jiggawatts worth of voltage and current and rest easy knowing it's a more stable bias supply than many other designs. Mouser.com sells 2000 volt rated Wima box caps for pretty low prices- they're fairly indestructible, right?

If you're worried about having a bias tap on an Antek TF why not just add a few turns to the transformer? It wouldn't look super professional (left uncovered or whatever) but it should work perfectly and you have total control over the voltage of your bias winding. There's no reason to unwrap the Antek TF- I'd just add a bunch of turns and then tape over them (and the whole TF) with self-vulcanizing tape. It'll look like a brand new custom made part!

jamie

[defaced]

A 470nF cap gets you 31% of the HT magnitude!! (Assuming a 20k load on the bias supply itself in both cases.)

The problem is (and it's a big problem), if the load on the HT drops below about 10mA the HT starts its inexorable rise! And at 400V, 10mA is a lot of current to dump with just a bleeder.

There must be an easy way to tame this beast....

I don't get what the problem is with the cap coupled bias? Sure you don't need much more that 10% of the HT for your bias?? In any case, just use a bigger coupling cap!

I'm confused. These two posts from Merlin sound contradictory to me.

For something like a bias supply, an engineered solution just to take advantage of an existing winding seems like a pain to me when a simple solution is so cheap/easy.

To make matters worse, none of the windings are center tapped.

I just caught this. Both the primary and secondary windings are doubled (parallel) so you have the option to configure them to act as center tapped or not.

[Steve Conner]

If you're worried about having a bias tap on an Antek TF why not just add a few turns to the transformer?

I do this quite often, but adding a lot of turns to a toroidal transformer can be tedious. Ones of the size we're looking at usually run about 0.2 volt per turn, so you could be there a while making a 40 volt winding.

If you want to do it, make a 10 turn test winding to get the volts/turn figure for your own transformer first.

I once added a winding with some old magnet wire that had damaged insulation. When I turned it on, the whole winding instantly went up in smoke. Now I just use hookup wire and don't tape over it: less insulation to catch fire.

[Merlinb]

I'm confused. These two posts from Merlin sound contradictory to me.

The cap charge transfer circuit is the dodgy one, that is a hard beast to tame. The regular cap-coupled bias that Wilder was trying out is the one I was referring to when I said 'mo cap!