I don't have bias voltage

[skyboltone]

Greetings:
Well I'm done wiring! Yaaa! That's the good news. Now for the bad. The green wire coming out of the power transformer turned out not to be a bias winding after all. Strange, cause I measured it when I started and had 60 volts on it. Must have been some sort of very low level inductive artifact because a thorough ring out indicates it's related to the filament windings. Like 50%. Ah, what does the word duh mean?

Anyway, so I made a voltage divider off the lower impedance of the two high voltage windings (one is 74ohms to center the other 68ohms. I figured the 68ohm was the more robust winding) with a 270K and a 47K resistor. This is kinda light I think, it drops about a mil but should make about 56 volts unloaded (375-0-375 windings). Nope, I got 35 VAC and about 14VDC with the half wave bias supply.

So, those of you who have made a bias supply off the ac high side of the transformer give me the scoop. Everything else seems ok to start adding tubes. No smoke etc.

It's been real and it's been happy but it hasn't been real happy if you know what I mean.

Thanks gents
Dan

[Normster]

On my 6V6 amp I ended up with 180K feeding the diode, 15K range resistor, and a 10K pot with 33K tail to ground (coming off of a 300VAC leg on the PT). Since you've got 375VAC, I'd say start with a 220K. Marshall JMP50 uses 180K with a 15K balance resistor and 68K range resistor.

Here's a link to my layout so you can see what I ended up with:
http://www.bad-domain/6V6_ODS/6V6_ODS_124.JPG

[heisthl]

http://www.freeinfosociety.com/electron ... ahat50.pdf is a quick and easy bias circuit to copy if you don't have a bias tap.

[Bob-I]

On my 6V6 amp I ended up with 180K feeding the diode, 15K range resistor, and a 10K pot with 33K tail to ground (coming off of a 300VAC leg on the PT). Since you've got 375VAC, I'd say start with a 220K.

That's about what I have too. I was shooting for a total resistance of about 250K to keep the loading down.

[skyboltone]

http://www.freeinfosociety.com/electron ... ahat50.pdf is a quick and easy bias circuit to copy if you don't have a bias tap.

Heisthl, I'm going to go with Normster's suggestion because I have the parts. I am curious though. I assume that the .05 cap in the aforementioned scheme keeps the bias supply from sucking the whole power supply down? What sort of cap would that be? Is it safe to put a .05 orange drop in a 375 VAC environment? Also, I've never seen this sort of thing in a power supply. It looks like a voltage divider grounded at both ends and fed/tapped from the middle. I suppose a couple of RN65 56K resistors might be hefty enough for this duty, but really they should be 1 watt for a healthy lifespan. Because current is constant I suppose one needs a 1 watt pot too. Am I seeing this right?

Thanks
Dan

[heisthl]

The cap that feeds it is just a DC blocker. I use the crappiest 600V polys I have laying around for this purpose as no tone is involved (I'm currently doing a project incorporating this bias circuit using one of those old "brown" .068 600v ones common in the CBS SilverFace Fender amps Yuck! Tone destroyers). You are seeing the voltage divider aspect right but the current flow in this circuit is very minimal. Think of it this way: Even if you were feeding the first part of the circuit 500v it is over 50,000 ohms to gound. So the current through the first dropping resistor would be <.01A. In the second part after the diode you have less than 100v and it is over 75,000 ohms to ground so the current flow would be waaay below .01A (the grid current is nothing). The trimmer is a 25K so a 1/4 watt trimmer is fine. Next time its convenient I could stick an ampmeter in circuit to see what real world readings would be.