How To get Fixed Bias when you Don't Have a Bias Tap

[bob p]

Sometimes when working on an amp, you'll find that you need a source of negative voltage for Fixed Bias, and that your PT is either missing a bias tap or your PT's bias tap doesn't have enough negative voltage. This thread is intended to help you through these sorts of problems.

WHEN YOU DO HAVE A BIAS TAP, but the voltage is too low for your needs, you can use a voltage doubler or tripler to step up the bias voltage to a more suitable range. This approach could be useful, for example, if you have tubes that require a high magnitude of negative voltage, like KT88, but your Hammond OT only has a 50V bias winding.

WHEN YOU DON'T HAVE A BIAS TAP, the traditional approach to this sort of problem is to tap into the Hi Voltage secondary winding, and to use either a resistive voltage divider or a capacitive voltage divider to create a suitable bias source.

Here are a few examples of circuits that utilize the Hi Voltage secondary winding to develop a fixed bias supply (courtesy of the Fender Amp Field Guide):

6G3 Brown Deluxe

6G2 Brown Princeton

If anyone else has links to useful diagrams or schematic snippets, please feel free to post them here!

[famp69wv@yahoo.com]

Ahola's A1A

Bob

I don't have a schematic handy but it seems to me that Steve Ahola (and a host of others) were evolving a trainwreck express knockoff some years ago & at least version A1A derived its bias from the high voltage winding as well (they were using the Heyboer 'wreck' PT).

[Rob Mercure]

One of the reasons that the capacitive voltage divider has been popular with Ampeg - the original company - and other over the years is that the voltage and current through the cap aren't in phase so there is little power dissipation in the capacitor. Dropping the 250-500 B supply by 80-90% can dissipate lotsa power/heat with resistors but for some reason some designers/techs seem to distrust the cap mentod. Works for me.

Rob

[Satamax]

Hi Bob!

Well, 6G2 and 3 seem wrong to me, there's no cap between AC and diode???? THought that's full wave not bridge recto.

Here's the usual ampeg type

http://www.schematicheaven.com/ampegamps/v4bpowramp.pdf

[Rob Mercure]

Hey Max,

I think that Bob was trying to illustrate the resistive divider - as best I know Fender never used a cap type - that's why I added my comments.

Rob

[stokes]

A much simpler example (no tremolo circuit involved) is to take a 220k resistor from pin 4 of the rectifier,a reverse oriented diode in series to the left leg of a 50k pot,this leg gets a 10uf/100v reversed cap (+ to ground) the wiper gets another 10uf with the + to ground and a 10k resistor from the right leg of the pot to ground,your neg.voltage is taken from the wiper to the grid resistor(s).Looks much simpler when drawn out schematically than the text explanation.Components can be tweaked if you need to change the range of volts the 50k pot delivers.I have actually replaced the 10k resistor with a straight wire to bias a 7591A in a single ended output.I would prefer to use this type of supply in place of a bias tap that wasn't up to par for the tubes like the 50v winding described earlier.

[bob p]

back to the subject of CAPACITIVE DIVIDER BIAS SUPPLY, no, I wasn't pointing to the Fender schems to illustrate capacitive divider bias. i don't know of many good schematic examples of that type of circuit. the only one i'm familiar with is David Halfer's schematic for the Dynaco modification to the classic Williamson HiFi circuit. (For those of you who may not know the HiFi lore, the 1940s Williamson amp was one of the first true HiFi amp designs. It has been largely supplanted, to Williamson's frustration, by the ultralinear circuit of Hafler and Keroes. Hafler went onto form Dynaco, and the rest is HiFi history).

In the Dynaco Modified Willimason amp design, Hafler proposed that owners of Williamson circuits should rework the amplifer to eliminate parasitic instabilities that occurred at the HF and LF extremes. He also proposed the radical concept of reworking the Williamson-type amplifier for fixed bias, and he used a capacitive divider for the bias supply:

Dynaco Modified Williamson Amplifier

Unfortunately, the schematic doesn't pay much attention to the bias circuit, which looks like it was added as an afterthought by drawing it to fit in a margin.

If anyone has a better example, or some notes on the theory of the capacitive divider, please share!

[bob p]

A much simpler example (no tremolo circuit involved) is to take a 220k resistor from pin 4 of the rectifier,a reverse oriented diode in series to the left leg of a 50k pot,this leg gets a 10uf/100v reversed cap (+ to ground) the wiper gets another 10uf with the + to ground and a 10k resistor from the right leg of the pot to ground,your neg.voltage is taken from the wiper to the grid resistor(s).Looks much simpler when drawn out schematically than the text explanation.

Like so?

[Tom Phillips]

All hail the schematic diagram. A picture worth a kiloword.
Too bad that there isn't a super simple way of drawing a schematic in the posting window.

[bob p]

yeah, its a bit of a PITA isn't it? i had to draw it on a piece of paper and scan it. :-/

[galibier_numero_un]

Hi Tom,

Too bad that there isn't a super simple way of drawing a schematic in the posting window.

There are likely many simple drawing packages out there - ones where you don't have to learn a SPICE program.

I use a PCB board supplier named ExpressPCB for my turntable motor controllers http://www.expresspcb.com/. While I'm a hard wired guy for my tube stuff , a PCB is an ideal solution these circuits.

You can download their software which is actually two packages - one for drawing schematics and one for laying out the PCBs. If you happen to be manufacturing PCB's, drawing the schematic in their software allows you to link it to the PCB design and then to verify that traces are actually going where you think they are.

A while ago, someone sent me a group of tube related schematic symbols for their schematic package, and I'd be happy to forward them to anyone currently using the package. You can also draw your own custom symbols (what this fellow shared with me), which for us is necessary, since most of the symbols are biased toward silicon devices.

It'll probably take you about a half hour of messing around to get the schematic package working.

One of its print functions allows you to save to either .jpg or .bmp format (I forget which). If the file size is too dense for posting purposes, you can always use some graphics shareware program like Irfanview to reduce its size.

Cheers,
Thom

[stokes]

Drawing,scanning,attaching-too much computer stuff for me.But that is it,much simpler when you look at the schem.Nothing groundbreaking,but an easy circuit to start with when adding a fixed bias to something that doesnt have one to begin with,or as you pointed out if the bias tap is too small.

[Enzo]

Some older Marshalls tapped off the HV winding, for example 1986 (1987, 1989), 2205/4210, 9005

[bob p]

@rob brought up another method that I had overlooked:

BACK-BIASING. Essentially, you raise the center tap of the PT HiV secondary above chassis ground by interposing a resistor to create negative bias voltage. then traditional filtration methods are used to smooth out the bias supply.

There's a good explanation of the method at Aiken's site on the Back Biasing page.

Note: I would have added this information to the original post in this How To, but it appears that there is a time limit that has been imposed on modifying posts. It appears that all of my previous posts in this thread are "old" enough that I am no longer able to edit them.

[Enzo]

I for one would prefer you NOT go back and edit previous posts for content. SOmeone finding the thread for the first time would benefit, but anyone who already read it is not likely to go back and read it over and over to see if anything was added. But that is just my own opinion.