I need a bias voltage source

OK here is the issue I am building a 5f4 clone out of a chassis out of a hammond tone cabinet. The PT tranny is rated 360 0 360 and has a center tap which will go to ground. Now according to the schematic for the 5f4 there is a red blue wire going to the board which I am sure is a bias tap well I do not have that so is there a way I can get a bias voltage with the tranny I have some how? I have two red wires going to the rectifer tube for the B+ the center tap for ground two blue wires for the 5 V rectifier heater voltage and the 6.3 taps.

If this would not work then my only other choice is a PT I have out of a Peavey Duel amp that had 4 6l6s in it with a bridge rectifer. It reads 390V across the two B+ red wires the center tap is not used of course this is reading high as nothing is hooked up to it I do not know what the voltages would be normally. The orange wires read 21V with the center tap as ground and 40 V across both of them so the center tap reduces the voltage by half. I really would prefer to use the other tranny I have hooked up if I could get a voltage out of that somehow and is bias voltage positive or negative? the 21 V I get off of the Peavey PT does not read negative and if that 21 V sounds a bit high it is because the PT is not hooked up to anything.

I can't think of which models do this off the top of my head, but search some of the Fender blackface model schematics/layouts on Schematicheaven.com (Princeton Reverb maybe?) and look for an example where they take a hot feed of the high voltage of the rectifier and feed it to the bias circuit through a resistor or two.

I'm not sure how to do the math to get the correct step down resistor but the PR might be close. A Fender with a SS rectifier probably does the same thing (Bassman, Bandmaster).

The problem is that a bias voltage needs to be negative so you need a separate rectifier connected "backwards" to produce a negative voltage (with filter caps installed "upside down"--negative terminal connected to voltage buss).

You cant generate a negative voltage from a positive voltage supply without getting tricky. (DC-DC converter)


The easy way out is to get a separate transformer to generate the -40V. It can be small since it will not draw any current. Something in the range of 30-50V would work, using a single diode, a filter cap, and a pot between the negative voltage buss and ground to adjust the voltage, which will be taken from the center lug of the pot. It is good to have a filter cap at the take-off point, negative terminal connected to the cap.

Be sure to start with high bias voltage rather than zero bias, which would kill the tubes in a second.

Once the proper setting is determined to get the -40V, you can measure the resistance on each side of the center terminal of the pot and substitute fixed resistors--or make up most of the resistance with fixed resistors and put a low value pot between them to allow for a limited range of adjustment.

Make sense?

The Pro Junior generates its bias from the negative-going pulses on one of the high-voltage taps. Here's a snip of the PJ schematic.

http://www.fenderforum.com/userphotos/photo.php?id=53754

Making R29 a 25K or 50K trimpot gives you a nice adjustment range for the bias. However, a trimpot without limit resistors allows you to make the bias stupidly hot or cold.

The diode can be a conventional 1N400x.

That's right, the voltage out of the secondary is AC, and you just need to rectify and filter it. Voltage divider, half wave rect, and a cap.

I've seen in some Marshalls, that they use a series cap off the secondary as a reactance for the voltage divider. A 0.022uF is ~120K ohms at 60 hZ.

Bill.
This does the same thing, right? It's probably very similar to the 5F4 PT and I think the voltages are 360-0-360 on this one.
http://i78.photobucket.com/albums/j109/milocj/bias_rectifier.jpg

Heh, I speak schematic, not layout.

But yes, it does the same thing. The difference is that the PJ couples the AC with a capacitor, while the PR is directly connected to the secondary through a dropping resistor. The cap gives you a way to limit voltage/current without a big dropping resistor.

Eman is saying the same thing about the Marshall circuit.

I prefer two stages of filtering on the bias, as in the PJ. Hum on the grid is not cool.

Heh, I speak schematic, not layout.

http://www.ampwares.com/ffg/schem/princeton_6g2_schem.gif

OK so that brings up an other question since my PT is NOT the same one in that Fender used I would guess my Voltage is going to be off what that is My PT is rated at 360 0 360 according to the schematic I have of the origional Hammond Tone Cabinet. Or were some of the Fender PTs rated at 360?

You're going to make the bias adjustable, aren't you? If you use the PJ approach, just use a smaller capacitor, like the .02uF that Eman mentioned and jigger the dropping resistors accordingly.

If needed, I can run through some calc's and get some reasonable values.LMK

If all else fails, you can always make it cathode bias. :wink:

- Scott

BillM I talked to my tech he said to start out to leave it fixed. Eman that would be great if you could come up with some values for me.

You're going to make the bias adjustable, aren't you? If you use the PJ approach, just use a smaller capacitor, like the .02uF that Eman mentioned and jigger the dropping resistors accordingly.

BillM your talking to a rookie here so I copied the schematics you guys posted and will show them to a tech friend as he will get the parts for me. Since the Pro Jr has a voltage of 132 vac and I am working with 360 vac what wouldbe the bias voltage or would I have to substitute values and if so I have no clue what to use.
So your saying the princeton one would give you hum? If so could that one have a cap added and if so where in the schematic and what value?

Just breadboard the PJ circuit and test it with your transformer. Start with a .02uF 630V cap and use a 50K trimpot for R29. See what kind of voltage range you get at test point 23 and where the trimpot has to be set to get it. If the trimpot needs to be set below 5K ohms, decrease the cap to .01uF and test again.

Strictly seat-of-the-pants, but ultimately effective.

If Eman can throw a little science at it, so much the better.

OK your suggesting that I use a .02 mf instead of the ,047 mf right and then get a .01 too incase I need it. I called my tech friend and said I would bring over your diagram so he can see what I need. E man if you have some better values for this circuit I would appreciate it or if they will work ok with a trim pot thats cool too.

Eman did you come up with any different values I am going to go to my Tech friends place later this afternoon.

Sorry, I temporarily lost track of this thread. Didn't mean to bail out on ya!

I can get some values in the AM, but Bill's method is tried and true.

OK IF Bills method is good with the values he suggested I will go with that and build that circuit as is with the trimmer pot and the different cap value. Can anyone tell me what the Bias Voltage should be for the 5f4 Super?

OK IF Bills method is good with the values he suggested I will go with that and build that circuit as is with the trimmer pot and the different cap value. Can anyone tell me what the Bias Voltage should be for the 5f4 Super?

Schematic says -40 V, but you really should adjust it so your particular tubes are biased correctly.

I just wanted it for a starting point thanks once I get every thing put together I will figure out the biasing for the tubes. Pretty much got every thing here to finish it my tech friend said that he would try to dig up the stuff I need for the bias circuit tonite and have it for me tomorrow. Supposed to get up to 18 inches of snow Thurs night through Sat morning. Blah the only good thing is knowing this time of year it does not last. Be a good time to do some work on the amp as long as we are under a blizzard warning. (NW WI)

No, I meant that Bill's breadboard, and mess with the values method is tried and true. The circuit (PJ) to be 'copied' won't even be close to what you need.

Will get back later!

All righty, Here's what I came up with -

You can build the bias supply exactly the same as the PJ that Bill posted, except - Change C12 from 0.047uf to 0.068uF. .

To make it adjustable, add a 5K pot to the TOP side of R29 (the 15K resistor). This ought to give you a range of -38V to -52V.

As usual, these numbers are approximations based on an imaginary circuit. Cap tolerances, input voltage variations, etc. weren't taken to account.

Thanks Eman for your help.

Oh, and don't be tempted to increase the size of the electrolytic caps for "better filtering." They'll take too long to fill up. The capacitive coupling from the high voltage tap just allows a trickle. And of course you want the bias voltage to be there before the tube is warm.

The two stages of 10uF caps with the 33K between them will give you a very clean bias supply.

Well gonna have to wing it with the parts I have on hand and see what I come up with. I could not come up with a .068 cap but I do have a .047 630 v cap and I have the diode. Closest trimmer pot is 15 K and I have the 333 resistor and I had to put two resistors in series to get 54 K and the closest I can come to a 15 K resistor is a 33 K and a 22 K wired in parallel for 13 +K I have the 10 UF Caps in 35 V too so some of the values are not exact but I will put it together and see what happens.

Bill's right, the C/R/C filter makes for an ultra clean bias voltage. It's really, an AC voltage divider as it divides the ripple by the ratio of the second cap's Xc and the 33K ohm resistor. If the cap's Xc at 60hZ is 265 ohms and the resistor is 33K ohms, the ripple reduction is at least 99%. And as Bill said, too much cap in the bias supply can drag the charge time out past what may be "safe". This modded PJ bias supply takes ~3 seconds to get to full voltage.

BTW, if you don't use the exact values listed, it probably won't do what you need.

Well I will try it and see what I get and then I will modify if need be. Parts are hard to come by around here unless you order on the internet. Radio shack carries little and my tech has parts but he has them squirreled away so he aways has to hunt and look for them. As far as the resistors go they can be off when measured. The .047 cap is all I have at the moment as he could not find a .068 but said he would continue to look if need be. The trimmer at 15 K was all I could get. He could not find any though he said he has a box somewhere with some and Radio Shack sells them in a grab bag assorted type deal.