bias supply

[phsyconoodler]

OK I've got a transformer I salvaged out of a fender super 60. It gives perfect plate voltages but it uses a bridge rectifier. I need to get a bias supply figured out for it. I have it cathode biased but want a different sound.
  It has two brown ac wires that are used for bias and low voltage supplies but it has a large parts count.
 Can I do a bias supply like in this diagram?

[sluckey]

You need to turn that diode around for negative bias voltage. Your simple circuit will give you about -70V. Probably need a little extra circuitry to drop that and also make it adjustable for your needs. Expect your B+ to increase a bit when you switch to fixed bias.

[phsyconoodler]

OK wired it up on a breadboard. Does indeed make -78v. But inserting any resistor where the diagram shows it burns the resistor immediately.
  Adding a pot in series with the output negative voltage does nothing.
I'm confused here.

[sluckey]

Just copy the supply from the Super 60.

[tubeswell]

... inserting any resistor where the diagram shows it burns the resistor immediately.

I'm picking that you probably have a low resistance that isn't rated highly enough for power handling? (Did you measure the resistor?)

[eleventeen]

Sluckey has the right approach, or you could copy a Fender blackface bias rectifier ckt. 


You say "But inserting any resistor where the diagram shows it burns the resistor immediately.


Shouldn't. You have let's call it 80 volts. a 22K resistor in there (across the + and - ) should pass 80/22,000 = 3.6mils. Power = .0036*.0036 * 22,000 = .287 watts. But you don't especially want a resistor there anyway.



" Adding a pot in series with the output negative voltage does nothing." <<--it won't unless there is a load on the supply. On your breadboard, you are (probably) applying no load. No load = no current flows. No current = no volts will be dropped across a series resistor.


A typical bias circuit in a tube amp draws tiny, miniscule current. What you want to do is to set up a voltage divider; 2 or 3 series resistors, across the filtered output of your bias supply. Let's say 2 resistors. If those resistors are equal value, then the midpoint will be 1/2 your full 80 volts = 40 volts. The trick, usually, is to set up such a voltage divider using THREE resistors, the middle one being a pot such that you can adjust the voltage that appears on the wiper of the pot from let's say -30 volts to -60 volts. This way, you cannot accidentally adjust the bias too low (which would redplate the output tubes) but you CAN apply too much negative bias so as to almost shut down the output tubes. The good setting of the bias pot is roughly in the middle. If your pot is 25K and the resistor above is 25K and the resistor below is 25K, then your pot should cover the middle third of your 80 volts. From 27 to 54 volts. That's pretty close and perfectly usable right there. I like making the one "above" (from the negative filtered output on the bias supply) a bit lower, say 15K. Not necessary. All 3 equal would work fine. And you don't specifically need a 25K pot. Almost any linear pot will do (prefer not 1 MEG or higher)

[phsyconoodler]

OK I can't copy a blackface bias circuit cause it uses a bridge rectifier. Won't work. Been there done that.
 I used the actual taps of the power transformer to test my circuit. And it immediately burns any resistor even two watt ones in that location.
  I am going to use the super 60 bias circuit as drawn.
I see one leg grounded as soon as it comes out of the transformer.  Is that correct?

[sluckey]

I used the actual taps of the power transformer to test my circuit. And it immediately burns any resistor even two watt ones in that location.

What value resistor is burning? Be specific! 78v will not burn a 22K 1w resistor. There are thousands of other value resistors that will also not burn.

[eleventeen]

Wait, I thought you had a dedicated brown-brown winding for your bias? It (that winding) should not care what you have going on with the main B+ winding.  (which is probably red-red)


For grins, please check with your ohmmeter with power off to see that there is no internal connection (or short) in the transformer between the bias (browns) and B+ (reds?) windings. There should not be. If that brown bias winding is indeed independent, you should be able to use either a bridge (4-diode) rectifier or a single (half-wave) diode to change that AC to your DC bias voltage. Doesn't much matter.


If those browns are fully independent (which is how they show on the amp schematic) then you should be able to use either type (bridge or HW) rectifier to make your bias. Now remember, you are working to get a NEGATIVE voltage for your bias. So your job is to make the B+ rectifier > filter to get your 350-400 volts and then you will connect the POSITIVE lead of your completely independent bias supply to the ground of your B+ supply. Within the bias supply, it is the "+" terminal(s) of the filter caps that go to ground. Get that right! At that point the NEG lead of your bias supply should furnish your raw -40 to -80 volts of bias suitable for use in the amp. The POS lead will be at circuit ground.

[phsyconoodler]

Thanks yeah I know that. What I meant to say is you can't take bias fr.one of the secondary red wires like a Marshall does or Princeton reverb and do a blackface style bias supply. In this case the two brown wires are for the bias supply and any other low voltage needs the amp has like opamp plus and minus voltage.
  The diagram I pulled off the Internet with the resistor between the two taps does not work as drawn. With the resistor removed and wired as shown with diode and cap reversed it produces -78v DC.
   I am going to do it the way it is in the diagram Sluckey provided. Should be fine. I will let you know soon.

[sluckey]

That diagram I posted came straight from of the Super 60 schematic. I just eliminated the clutter that had nothing to do with the bias circuit. The circuit is just like a typical Marshall bias supply. But really, with a totally separate bias winding like you have, you can build almost any bias circuit you want. I think the Marshall is the best.

Look at page 5 of this pdf...

     http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf

You can build either the Fender or the Marshall bias supplies at the top of the page. Just connect one brown wire to ground and the other to the left side of either circuit. Omit the 220K for the Marshall. You may have to experiment with the value of resistors a bit to get whatever voltage you need. And that reminds me, what power tubes will this circuit be connected to?

[phsyconoodler]

Thanks Sluckey. I built it exactly as shown in the diagram and it works 100%. Dead on -58v before the bias pot.
  I didn't quite understand that two taps like that could be used with one end grounded. It's basic I soppose but I skipped basic training and jumped right into the masters class.
  I learned a lot here in the last few days for sure thanks to you guys on the forum.
Who said you're too old to learn?
   I appreciate you removing the clutter around that basic bias supply for me. Clear as a bell once you understand it better.
   Oh yeah its using 6L6 power tubes.  ☺