Auto Bias?

Ok time for another "newb" question.

As I am getting ready to start my build, I was deciding on an aesthetic look (Wifey wants the front tube sockets that are only used for Bias measure gone due to "ugly" factor ) and I was planning on putting a few banana terminals on the top to make checking/setting the bias voltage easier. But that got me thinking, I know myself and I doubt I can remember to check the bias every 30 days or so, but I can design a simple ckt to do it for me.

My thought process is use a voltage regulator (Like an LM317) to create a stable 1.2v output (Lowest that VR will go) regardless of input voltage and a few resistors to drop the voltage to a stable .500 output, add in a comparator network to let met know if the input voltage drops too low to go to the required voltage or goes too high (AKA bad rectifier tube) and a potentiometer to "tweak" the voltage coming out of the regulator and I now have a very stable auto bias ckt that I can easily tweak for whatever tubes I choose to run and would make it much more tolerant to swings in input voltage.....

I will build it as instructed for now, but was thinking this ckt could be an after upgrade

What do you all think? Is this a horrible idea?

TIA!!

FWIW, you can use any box you want. I'm not a fan of the original boxes either.

My ST120

My only immediate concern would be noise from the VR. If that can be contained, it could be a very good idea.

why would there be noise from the VR? Wouldn't the variations coming from the transformer be noisier?

mcgyver74 wrote:why would there be noise from the VR?   Wouldn't the variations coming from the transformer be noisier?  

The LM317 is fairly clean - and with care can be very clean indeed. But lacking that care...

http://www.tnt-audio.com/clinica/regulators_noise2_e.html

All I am suggesting is that a careful approach is good. And as you are going pure first, you have all the time you need to develop your solution.

I will also state for the record, that if you do this and you are successful, there will be great interest in this forum and elsewhere!

Sounds like i have a project!!! Once this baby is built I will get to designing

mcgyver74 wrote:Ok time for another "newb" question.

As I am getting ready to start my build, I was deciding on an aesthetic look (Wifey wants the front tube sockets that are only used for Bias measure gone due to "ugly" factor ) and I was planning on putting a few banana terminals on the top to make checking/setting the bias voltage easier.  But that got me thinking,  I know myself and I doubt I can remember to check the bias every 30 days or so,  but I can design a simple ckt to do it for me.

My thought process is use a voltage regulator (Like an LM317) to create a stable 1.2v output (Lowest that VR will go) regardless of input voltage and a few resistors to drop the voltage to a stable .500 output,  add in a comparator network to let met know if the input voltage drops too low to go to the required voltage or goes too high (AKA bad rectifier tube) and a potentiometer to "tweak" the voltage coming out of the regulator and I now have a very stable auto bias ckt that I can easily tweak for whatever tubes I choose to run and would make it much more tolerant to swings in input voltage.....

I will build it as instructed for now, but was thinking this ckt could be an after upgrade

What do you all think? Is this a horrible idea?

TIA!!

I like auto bias...however i wanted to be more of a part in my listening experience. Call me old fashion. Why re-invent the wheel? You may wind up with problems causing you aniexty as well as money. Bob focused on a strong flawless design, made in the US and cost effecint for both the manufacturer and the cusumer.
I bias my m125's more than once a month. Actually twice each time I turn them on. First after fifteen minutes and a second time 45 minutes into play. It's especially important if the voltage varies in your area during certain times of the day, as they do for me.
Of course this is my HO.

[quote="bluemeanies"]

mcgyver74 wrote:Ok time for another "newb" question.

I bias my m125's more than once a month. Actually twice each time I turn them on. First after fifteen minutes and a second time 45 minutes into play. It's especially important if the voltage varies in your area during certain times of the day, as they do for me.
Of course this is my HO.

Do you run with or without a cage?

Peter W. wrote:

bluemeanies wrote:

mcgyver74 wrote:Ok time for another "newb" question.

I bias my m125's more than once a month. Actually twice each time I turn them on. First after fifteen minutes and a second time 45 minutes into play. It's especially important if the voltage varies in your area during certain times of the day, as they do for me.
Of course this is my HO.

Do you run with or without a cage?

Without...I am using two Sovtek KT88's in each m125

Thanks guys I will build as written to start, but I know myself. I'll always forget to check the bias so I just KNOW I'll bust a tube that way LOL

In reality, checking the bias once a month is good enough. There are others that are more critical than others. I have found that on my ST70, once a month is more than enough.

Without...I am using two Sovtek KT88's in each m125 [/quote]

Running without a cage makes biasing easier than running with. The OP has small children chasing about, so cageless is not an option.

I dremeled off about an inch of the plastic handle on this skinny screwdriver and can now (using a small flashlight) adjust the bias on my VTA ST 70 with the cage on. Super easy...

I know ARC and I think McIntosh have been auto-biasing their tube gear for years. My worry with auto-biasing is that you get no warning if a tube is about to go ka-blooey, whereas a daily check noting that a tube is suddenly wandering between .3 and .7 might mean things are about to go pear-shaped. with consequent damage upstream and down.

Dave_in_Va wrote:I dremeled off about an inch of the plastic handle on this skinny screwdriver and can now (using a small flashlight) adjust the bias on my VTA ST 70 with the cage on. Super easy...

...or you could drill holes in your cage right above the pots so you could use a regular sized screwdriver

Or you could buy a $35,000 pair of Vandersteen 5As: they come with a nifty little subwoofer-adjusting wooden screwdriver that fits the bill just ducky. When I sold my Vandies I kept the wooden driver. It's yours for, oh, $5k.

Dave_in_Va wrote:I dremeled off about an inch of the plastic handle on this skinny screwdriver and can now (using a small flashlight) adjust the bias on my VTA ST 70 with the cage on. Super easy...

This is a good idea if you have a cage or even if you do not.
I purchased online, a ceramic screwdriver making sure not to have any questions if I slip while biasing.

deepee99 wrote:Or you could buy a $35,000 pair of Vandersteen 5As: they come with a nifty little subwoofer-adjusting wooden screwdriver that fits the bill just ducky. When I sold my Vandies I kept the wooden driver. It's yours for, oh, $5k.

Are your amps included ?

For you, Brother Bill, absolutely. But insured shipping on that screwdriver could get spendy.

This WAF is the reason I'm single.

Years ago my girlfriend asked me if I'd sell my motorcycle. I asked her why since she like riding with me. She said she wanted me to sell the bike so I could buy her a ring. I said I don't need to sell the bike, that I had enough for a ring for her. She insisted that I should sell the bike anyway. Eventually she moved out. A while later I missed her and wanted her back. I said I would be willing to sell the bike. She strung me along and a year or so went by. Out of the blue she called me and said she wanted us to get back together. I was traveling for work and she told me to call when I got back into town. Two days later I called her. A few times. The next day I called again. And I called the day after that. I tried calling a few more times over the next week. A few weeks later, since I hadn't heard from her I bought a new Harley. I email a photo of the bike and told her it was her new engagement ring. She called to tell me she never wanted to see me again. She insisted I didn't call her after getting home. I told her I did. She still told me she didn't want me to bother her again. I still have the motorcycle.

I bet she would have wanted me to sell my tube amp and speakers also.

There are about 7,000,000,000 individuals on the planet.
Of which about 3,500,000,000 are the wrong sex.
Of which about 3,350,000,000 are geographically challenged by more than 3,000 miles.
Of which about 90,000,000 are too old, too young, too this or too that.
Of which about 35,000,000 are involved, diseased, damaged or otherwise unsuitable
Of which about 4,500,000 are challenged for any number of other reasons.
Of which about 300,000 are simply incompatible for everything from the wrong eye-color to the wrong body type.

Leaving you with about 200,000 valid options with a reasonable distance. Whether you like it or not.

By the way, the above line of reasoning was my mother's way of saying: There is a lid for every pot.

It is April 1, after all.

If only we could bias a woman with a small screwdriver . . .

deepee99 wrote:If only we could bias a woman with a small screwdriver . . .

Even a large one made with orange juice and vodka would work for me.

Dogstar wrote:

deepee99 wrote:If only we could bias a woman with a small screwdriver . . .

Even a large one made with orange juice and vodka would work for me.

Ha! You boys are forgetting that a woman wields a larger, more effective tool than any screwdriver:
The V-----l Wrench. Good luck wid dat.

deepee99 wrote:If only we could bias a woman with a small screwdriver . . .

That is a BEAUT!