A dumb question...how do you set Bias without a bias pot?

[capnjim]

Ok..so I have collected and worked on tube guitar amps for 20 odd years and have always just put whatever tubes I have lying around in.
I know what bias is and how to check it, yet I have never actually adjusted it.
I read constantly about how you should re-set your bias after a tube change. How do you do this without a bias pot? Do you need to make a resistor? If there is, for example, a 100K bias resistor and you want to change the bias, do you need to "make" a 98K or 103K?
Can you actually change the bias without a pot?
This has been on my mind for a while and I finally got my first tube bass amp and am wondering if I should install a bias pot.
Thanks

[okcrum]

You change parts, usually resistors. Resistors come in standard values, depending on their tolerance, and you use the closest standard value.

[BassmanPaul]

I'm sorry OP, but if you ask this question you shouldn't be messing with bias at all. All too many guys are messing with stuff that can kill them. Incidentally bias is just as important in a guitar amp as it is in a bass amp. It's just that bass players seem to have gotten obsessed with it.

[77PBass]

A great book for understanding tube guitar amps is "The Guitar Amp Handbook" by Dave Hunter.

It walks you through famous circuits starting with a Fender Princeton. Great reading...I am reading it for the 4th time now as I am gearing up for one of the project amps at the end of the book.

[77PBass]

BassmanPaul is correct....unless you know what you are doing, messing around inside a tube amp is dangerous. There are lethal voltages involved and residual voltage in the capacitors even when the amp has been unplugged.

[capnjim]

Thanks...I have been soldering, and changing caps etc on my amps for 20 years. I also worked in a university physics lab and have a good electrical knowledge. I understand how bias works, I just have never bothered on my old guitar amps.
I know resistors come in many varieties...but not really close together.
Like my 100k example, there are no 105k or 95k.
Maybe I'm not really making sense...I dunno...

[demon666]

Originally Posted by BassmanPaul

I'm sorry OP, but if you ask this question you shouldn't be messing with bias at all. All too many guys are messing with stuff that can kill them. Incidentally bias is just as important in a guitar amp as it is in a bass amp. It's just that bass players seem to have gotten obsessed with it.

+1

[capnjim]

Well..I don't think anyone has actually died from working on a tube amp. I did spend a couple years as a technician repairing electrical components in a physics lab. We used to build our own power supplies. I just wanted to know how you can make fine adjustments without a bias pot....
Geez..you sure are a nervous bunch!

[billfitzmaurice]

Originally Posted by capnjim

I read constantly about how you should re-set your bias after a tube change. How do you do this without a bias pot?

Install one.

[BassmanPaul]

Originally Posted by capnjim

Well..I don't think anyone has actually died from working on a tube amp. I did spend a couple years as a technician repairing electrical components in a physics lab. We used to build our own power supplies. I just wanted to know how you can make fine adjustments without a bias pot....
Geez..you sure are a nervous bunch!

You are very wrong and also right:

You're wrong - People HAVE died messing with tube amps - that's a fact not conjecture!!!

You're right - I AM a nervous type! I've seen a man die from abusing his equipment. I don't like the thought of that happening again.

If you do indeed know bias circuitry then it should be child's play for you to redesign the bias circuit to include a twenty turn pot to vary the bias above and below (never to zero) the required voltage.

[okcrum]

Originally Posted by capnjim

Thanks...I have been soldering, and changing caps etc on my amps for 20 years. I also worked in a university physics lab and have a good electrical knowledge. I understand how bias works, I just have never bothered on my old guitar amps.
I know resistors come in many varieties...but not really close together.
Like my 100k example, there are no 105k or 95k.
Maybe I'm not really making sense...I dunno...

What's got people worried here is that, after 20 years, you don't seem to know that the resistors you need exist, and you seem to think no one's ever died from working on tube gear.

[Cannabass]

i don't know WHAT the hell you're all talking about but it IS killing me to read all this bickering instead of question answering so..

it seems to me... isn't the solution...

just install a pot?

[BassmanPaul]

Originally Posted by Cannabass

i don't know WHAT the hell you're all talking about but it IS killing me to read all this bickering instead of question answering so..

it seems to me... isn't the solution...

just install a pot?

Not bickering in any way, just trying to save the life of someone who, after twenty years of "working" on tube amps, doesn't seem to know tube amp repair and maintenance class 101 stuff.

Would you prefer us to tell him what he should already know and to hell with the consequences???

[okcrum]

Originally Posted by Cannabass

i don't know WHAT the hell you're all talking about but it IS killing me to read all this bickering instead of question answering so..

it seems to me... isn't the solution...

just install a pot?

I've answered the OP's question, twice. So have others. Click the link in my last reply.

[capnjim]

Geez what a bunch of old ladies...I guess no one has the intelligence or common sense to give me a real answer....boy a link to a resistance chart, thats brilliant!!!! I bet AES sells a lot of 103K resistors...
Forget I asked.

[Passinwind]

Originally Posted by capnjim

Ok..so I have collected and worked on tube guitar amps for 20 odd years and have always just put whatever tubes I have lying around in.
I know what bias is and how to check it, yet I have never actually adjusted it.
I read constantly about how you should re-set your bias after a tube change. How do you do this without a bias pot? Do you need to make a resistor? If there is, for example, a 100K bias resistor and you want to change the bias, do you need to "make" a 98K or 103K?
Can you actually change the bias without a pot?

Yes, but good luck finding the correct value without using a pot first.

I generally use a decade box, then if I arrive at an oddball value I just make a series or parallel network of resistors to give me what I need.

[okcrum]

Originally Posted by capnjim

Geez what a bunch of old ladies...I guess no one has the intelligence or common sense to give me a real answer....boy a link to a resistance chart, thats brilliant!!!! I bet AES sells a lot of 103K resistors...
Forget I asked.

OK, I'll be blunt: buy 1% resistors. They're not expensive and they come in every value you need. Try Mouser, not AES.

Given that you couldn't figure that out from the chart, I wonder...

[capnjim]

Thanks

[beans-on-toast]

As others have stated, you substitute the value closest resistor or install a variable bias circuit.

Here is a schematic for a bias board that you can piggyback onto your amp.

Here is a kit that you can buy.