How does this look for a bias supply?

[SILVERGUN]

I have a bench power supply that I am able to tap 64 VAC off of for a bias supply and I just want to make sure I am seeing this correctly....
I came up with this schematic to use a bridge rectifier, and I just wanted to make sure that this looks OK to you guys...

Thanks in advance for taking a good look at this one for me 

[HotBluePlates]

Put another cap to ground right after your rectifier.

64vac * 1.414 = 90.5v, 90.5v - 1.4v = 89.1vdc (the 1.4v is two diode drops through the bridge).

You have 37.2kΩ from that 89v to ground. 89v/37.2kΩ = ~2.4mA

Taking I²R for each resistor shows you will not come close to overheating those resistors, so their power rating is fine.

2.4mA * 10kΩ = 24vdc
2.4mA * 35kΩ = 84vdc
= 60v of bias voltage adjustment, from -24vdc to -84vdc.

With a single-turn 25kΩ pot, that may be too twitchy to get fine control of the bias voltage. Also, I doubt you will need -84vdc for bias (but maybe you will if you get adventuresome and build a triode output stage or something).

I'd think you'll want a multi-turn pot for the 25kΩ and/or increase the size of the 2.2kΩ resistor.

[zendragon63]

Most older Fenders schematics show the pot wired this way though it might be a consideration to short the one side to the 10K; if for any rare reason the pot were to lose contact, you won't have your bias floating out into never never land. Changes the span a little me thinks. I actually had this happen--but just like college, found out it was just bad pot.    Regards

dennis

[kagliostro]

If I can suggest an architecture for a Bias Power Supply circuit I'll suggest this

of course you must arrange the values of the voltage divider as to suit your needs

The presence of the diode permit a quick charge of the 10uF capacitor and a slow discharge if there is a problem due to fuse blow, the purpose of the 470k resistor is to avoid problems if the wiper of the pot fails

K

EDIT: Now the schematic is correct, previously, as noted by Steve, there was a great mistake and the bridge was draw backward  

[eleventeen]

On Zen's dwg, (good idea) I would have the wiper shorted to the OTHER (more negative) end terminal so that if the pot wiper failed, the bias would snap to the NEGATIVE side and tend to shut off the output tubes.

[SILVERGUN]

With a single-turn 25kΩ pot, that may be too twitchy to get fine control of the bias voltage. Also, I doubt you will need -84vdc for bias (but maybe you will if you get adventuresome and build a triode output stage or something).

I'd think you'll want a multi-turn pot for the 25kΩ and/or increase the size of the 2.2kΩ resistor.

Thanks guys,,, and HBP, that's a great idea I didn't think of, for a multi turn pot...
I hadn't seen a bias tap with 2 legs so I just wanted to make sure I had the polaity of the bridge and cap correct

Also,,,I left one thing out.....the xfmr is being controlled by a variac so when my HT goes down, my bias V will drop too,,,so that upper range of -84vdc will come down quickly
With the Variac at max, I've got 473vac, with 64vac bias tap.....dialed back to 300vac, the bias tap drops to 42vac

Listening to K's other thread and your response about separate xfmr supplies really has me thinking  

Thanks eleven,,,you posted while I was typing.......that's something else I wouldn't have thought of

[eleventeen]

Well if you're stuck with the bias winding on the mains transformer output and you're putting that tranny on a variac.....and you want to get all tooby and everything.....then you should use a voltage doubler on the bias supply (using silicon diodes) and feed either an 0C3  (octal) or an 0A3 (7-pin) 75 volt gas regulator tube.  These will start/fire on the inrush current and require about 10 mils to keep running. Then place a voltage divider downstream from the reg tube that pulls 2-3 mils, including a pot in the middle of two limiting resistors, and your bias will stay steady even with changes in the B+ supply as selected by the variac. Fun!

[SILVERGUN]

OK, I adjusted the schematic to reflect some of the changes suggested...

Well if you're stuck with the bias winding on the mains transformer output and you're putting that tranny on a variac.....and you want to get all tooby and everything.....then you should use a voltage doubler on the bias supply (using silicon diodes) and feed either an 0C3  (octal) or an 0A3 (7-pin) 75 volt gas regulator tube.  These will start/fire on the inrush current and require about 10 mils to keep running. Then place a voltage divider downstream from the reg tube that pulls 2-3 mils, including a pot in the middle of two limiting resistors, and your bias will stay steady even with changes in the B+ supply as selected by the variac. Fun!

Thanks 11, but I'm trying to get this thing (breadboard) up and running,,,,,for now I'll deal with re-biasing with each HT change....and the next amp on the list is cath. biased........I'll have to get back to this whole infinitely adjustable bench supply idea 
I'm glad you don't help my wife with the Honey-do list......I'd never sit down 

[eleventeen]

You shun the opportunity to add another tube to your project? Especially such a cheap one that uses no fil current and lasts forever and glows pink/purple??  Shame on you!



Ooopps...I got the reg tube upside down. Now fixed. Dot down, plate up.

[SILVERGUN]

You shun the opportunity to add another tube to your project? Especially such a cheap one that uses no fil current and lasts forever and glows pink/purple??  Shame on you!

.....didn't mean to shun
That is pretty cool...being fairly new here, I've never heard of such a thing,,,,,but a regulated bias supply sure does sound intriguing 
Hmmmmm......
Thanks for taking the time to put it out here....I'll have to "get back to it"....sure does look simple (I didn't need 1 more thing to think about)

I'm currently cutting my 1/2 acre of lawn every 8-10 days thanks to the "perfect" spring were having here in Pa.(just to keep up with my crazy lawn obsessed neighbors).....and that's seriously cutting into my available golf time....plus I'm getting the house "ready" for a graduation party....so that's why i'm trying to keep the short list short 

I'll throw this out there, just so someone can confirm my simple thinking...if i'm adjusting the B+ with the variac anyway, wouldn't I have to be re-adjusting the bias voltage anyway?.....so,,, whether it stays constant or varies, it would still need to be adjusted with HT changes

[eleventeen]

"..if i'm adjusting the B+ with the variac anyway, wouldn't I have to be re-adjusting the bias voltage anyway?.....so,,, whether it stays constant or varies, it would still need to be adjusted with HT changes"

Yup. But there's no easy method of knowing or assuring they (B+ volts & bias volts) would track each other on any kind of predictable basis and you'd *want* them independently adjustable if you are really in the breadboard mode. The idea of the reg tube is you would have a way of them NOT moving together and then of course diddle the pot to control output tube current. You could of course use a 75 volt zener but......what fun would that be?

Actually, I believe the 75 volt reg tubes glow ORANGE (neon) vs Purple (argon)

[kagliostro]

Usually who decide to build a supply for that purpose, uses for filament and bias a separated transformer that isn't under the Variac control

However there are many way to do things and you can adopt the way is more comfortable to you and with the materials you can have in the easy way you can

if the bias supply varies with B+ because you have it from a transformer that is under Variac control, the way is to regulate the DC bias voltage you obtain at the output and to remember that if you do this way, changing B+ via the Variac as to test the amp at different voltage at the same time change the bias AC and you must readjust the bias setting

consider also that a small (bias PS requires few current) transformer, say 60v-80v, can solve the problem with a small waste of space and weight

---

However the idea of using a gas stabilizer is attractive 

K

[sluckey]

If I can suggest an architecture for a Bias Power Supply circuit I'll suggest this...

K, your bridge is backwards and will produce a positive bias voltage with reference to ground.

[kagliostro]

Yes Steve you are right, I apologize  

I've redraw the schematic from Jschem to FidocadJ and I've done that BIG mistake  

thanks, I'll correct the schematic

Franco