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

[stokes]

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.

I agree,it makes sense,some of these threads can get quite long,better to just add another post and keep it going.

[bob p]

Well, the whole idea for this thread was to have a How To guide for reference. By not allowing the "guide" to be updated, a new reader is forced to wade through the entire thread (including all of the irrelevant stuff) to hunt for the relevant material, and to figure out on their own how to apply any updates that may be applicable. That's a pretty inefficient way to do things, but that's just the way the system here is designed. <sigh>

Regarding the possibility that people who are already reading the thread might be inconvenienced: to say that focuses the "convenience factor" on the people who are already reading the thread, and totally discounts the value of having everything in one place for future readers, which is what reference material is all about.

In the future, hopefully there will be more people reading the thread than the small number who participate in it now. That would make the "keep it convenient for me" argument less compelling. In the big picture, my two-cent opinion says its better to put all of the information in one place for future readers, who will hopefully outnumber the small number of people who are already familiar with where information is located in this thread. If current readers are worried about missing updates, I promise to let you know if I update the original post. That would provide a solution that would be good to everyone.

I've spent the majority of my online life writing instructional guides on the Linux boards. I've got the procedure down pat. All of the information is consolidated in the top post of the guide and updated occasionally (with update notices in the body of the posts) so that its both convenient for everyone AND easy to follow. The only impediment here is that not being able to modify the posts tends to undermine the ability to write How To documents on this board. As long as that's the case, the board is really optomized to be used for nothing other than running commentary. Too bad.

[stokes]

I see where you are coming from,Bob.I've seen on other boards where someone would take an oft asked topic like biasing,for example,and post it as "sticky" so it stays at the top for reference,so people dont keep asking the same questions.I see the value in that,but I kind of like the "running commentary" format,as you get a lot of different opinions and techniques from a wide range of people.Maybe its possible to do something like that here,I dont know,my computer skills are marginal at best.

[Enzo]

I don't disagree with your feelings Bob, but this isn't where the stuff belongs then. This is a conversation about the topic, a place to work out the details. In my mind the difference between talking shop afterhours at the bar and a department meeting at work.

On the old Ampage I liked the links to all the various instructuve stuff. Once you get a finalized version of the bias report here, maybe you and tboy can create a reference section where we can collect all this stuff.

[bob p]

getting back to the original topic, the Back-Bias method discussed on Aiken's site looks useful for another reason that I hadn't specifically mentioned:

In addition to providing bias when a bias tap isn't available, it also allows another method of controlling the B+ voltage. Even if you're not going to use the back-bias method as a bias tap, the method effectively allows you to "throw away" some of your B+ voltage if your voltage rail is too high. This can be a helpful way of "turning down" the B+ voltages when your PT and filter section isn't exactly right for your application and you need to trim the voltage down a little.

Experienced readers will notice that using the zener diode method of generating back-bias is essentially the method that's been discussed on the board over and over again as a method of lowering B+ voltage. I don't remember ever seeing a good written explanation of the method. Aiken's page explains the method very well, though the focus of his article is on bias supply rather than B+ reduction. The good news is that the method (and his explanations) are good for either goal. Thanks, Randall.

[Rob Mercure]

Traditional methods

Well, you could consider this a bit tongue in cheek but since I've been considering it for a project you could always use a "C" battery. Afterall, this is the original bias method and with suitable circuit impedance the battery should last "forever." There are some cute little cells I bought a few years ago that are about the same diameter and 2/3 the length of a AAA cell that put out 20V - two of these in series puts you close to the bias range we frequently.

For myself I've been tinkering with a "Herzog" type effects box that uses 3V4 - and perhaps some other - subminiature directly heated pentodes. After some consideration I've decided to use batteries for the C- supply to simplify the bias circuitry and to allow some adjustment. As the amp will be using AAA cells for the A supply and three of the 20 V cells described above for the B supply adding a couple of coin cells for a C supply simply "goes with the flow."

Rob

[fiesta55]

Like so?

Could someone help a newbie out?I am building a Plexi from the Hoffman layout and using a full wave bridge rectifier in place of the 2-1N508 diodes.I have a P-TM-P50 power transformer with no bias tap.The hoffman board has pretty much the same components in the bias circuit as the drawing from Stokes.I have also read the Aiken page on this subject but am not good with schematics.I'm just not getting it with the extra components shown in his drawings.Could some explain to me(very simple ) how to implement this in my circuit.
Any help would be appreciated.I'm in a bit over my head and this is all I need to finish the amp.
I'm learning a lot from you guys.

[Bruce / Mission Amps]

Do you need "paint by numbers"?

There are a number of ways you can do this with a non center tapped PT but for a start, buy a little four legged, 600v FWB rectifier block that can handle one amp and hook it up to the two high voltage PT leads the same way as your high power rectifier is.
But, for a bias supply, ground the positive end and use the negative end to supply the B- voltage to your first bias supply filter cap... which is also connected with it's positive end grounded.
Then just build a resistor voltage divider circuit that can loose a bunch of the B- voltage but with a pot in it so you can set the voltage.

[fiesta55]

Thanks for the reply Bruce and yes I do need"paint by the numbers".This is my first build and I've done a good job until now.I'm trying to use this transformer that I got cheap.I am using a FWB rectifier block.
The bias circuit on the hoffman board consists of 2-10@100 with a 15k resistor and a 50k pot,a 1n4007 diode and a 220k bias range resistor.

[Bruce / Mission Amps]

Here's a rough sketch of one way to do it with a PT requiring a FWB rectifier...

Note that one FWB makes postive B+ and the other FWB makes the negative bias voltage.

[fiesta55]

Thank you very much Bruce.I'm guessing that the arrow marked bias voltage will go to the 220K bias range resistor on the Hoffman board?

It seems kind of redundant .Unless I'm missing something here that is pretty much the same bias circuit that already exists on the Hoffman Plexi50.

http://www.el34world.com/Hoffman/images/Plexi50.gif

[Bruce / Mission Amps]

Yes, very similar but the Hoffman one is based from a PT with a center tap so you only need a half wave rectifier.
Your's doesn't, so to make it easy, use a small FWB block.
These can be very small and you only need to make sure it can run at 800-1000v to be safe... loose your bias supply and the power tubes will go nuclear.

[soundmasterg]

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.

And then there are also the situations where your power supply IS a voltage doubler, and then it is a little bit harder to do as you suggest above. In this situation, it is probably better to just add another transformer that is dedicated for the bias supply. Since there isn't much current or voltage required, it will be small.

[bob p]

sure, one could always buy a transformer, but the whole idea behind the alternate methods we've been discussing is to use what's available so that you don't have to.

[Tom Phillips]

Here's a rough sketch of one way to do it with a PT requiring a FWB rectifier...

Note that one FWB makes postive B+ and the other FWB makes the negative bias voltage.

Bruce - was this just a quick idea that you havn't tried yet? At first glance it may seem OK but...
Feel free to tell me if I'm missing something here but I don't see how two FWB rectifiers can be hooked to one secondary winding as shown with positive ground on one and negative ground on the other. If you draw in the individual diodes so you can see the actual composit circuit I believe you will find that each half of the secondary will be connected to ground with a pair of back-to-back diodes. For all practical purposes, that's a dead short.

My approach to adding a bias supply to a FWB set up would be to use the capacitively coupled approach as seen in many Ampeg amps.

Cheers,
Tom