Bias - fixed? cathode?

[rollingelbow]

So I am over at the Aiken amps site site trying to learn about bias and honestly, I haven't a clue as to how you tell what type of bias an amp is using? (tell tale signs) Furthermore, if I want to bias an amp using the cathode bias method, can i use this on a fixed bias amp? I'm really confused. I've got a plexi clone that I need to bias and i am trying to understand the difference so I know what methods should be used to successfully bias it..

Thanks

[Bulatovic]

I am a newbie here as well, but as far as i understand:

Cathode bias - biasing done by putting resistors (which in my opinion should be called fixed, as the only way of changing it is to change the resistor) - used on lower power amps 18watter, 36 watter and so on.

Fixed bias - biasing by setting the value of small bias potentiometers to the right value. Used on higher powered amps. If you have a plexi el34 driven you should have these small pots inside which u most often adjust by using a small screw-driver. So if you have and amp which uses 2 or more KT66, EL34 or so you will probably have fixed bias.

hope that this is helpful

Cheers

[Bulatovic]

this is what Aiken amp tech info says:

"There are two main types of biasing: fixed biasing and cathode biasing. Fixed biasing does not mean the bias is not adjustable, in fact, it usually means the opposite. Cathode biasing is usually fixed, and not adjustable, and fixed biasing is usually adjustable with a small trimmer potentiometer, or "trimpot". It is no wonder the subject is confusing to people!"

[Phil_S]

If you have a plexi el34 driven you should have these small pots inside which u most often adjust by using a small screw-driver.
Cheers

It is a really bad idea for a novice to stick a metal shaft screw driver in a trim pot. It is fine for someone who knows what he's doing. If you are intent on trying to get this done, I recommend two things. One is to get a "TV" tool, basically an all plastic (non-conductive) screw driver, and the other is to find Lord Valve's bias instructions -- easily found with a google search.

[johnny5]

keep reading and searching. you'll figure it out. there's tons of info out there.

Fixed Bias: a negative voltage is applied to the grids of the output tubes, let it be more negative than the cathode.

Cathode bias: small positive voltage created at the cathode by placing a resstor there (120 - 300 ohms, 5-20 watt), making the grid more negative than the cathode.

I also understand that the above probably doesn't make sense:)

looking at a layout: find out which pin of you output tubes is the cathode and follow it. if it goes directly to ground, it's fixed (most likely). if it goes to a big brick resistor, it's cathode biased.

[zaphod_phil]

It is a really bad idea for a novice to stick a metal shaft screw driver in a trim pot.

That maybe depends. The Darwinian theory suggests that this helps breed future generations of smarter, less electrically conductive novices.

[dartanion]

I'm not sure if this is the reason for the nomenclature, but bias shifts on cathode biased amps when played, thus it is only "fixed" at idle. Fixed bias puts a "fixed" negative DC voltage on the grids to set the bias.

Yeah, it's a bit confusing to say the least.

[Michael]

And if you use a zener or LEDs for Cathode bias, it's really fixed.

[rollingelbow]

Well i built the damn thing so at least i know where everything goes . Actually reading back through the site the only thing i really could figure was that yeah, larger watt amps tended to be fixed bias though if you can use a trim pot to change the amount of negative current to the plate then its fixed..cathode is as you mentioned, used with resistors of fixed values. So the question then is, what is our average Joes way of biasing their fixed bias tube amp? Safety is something you make yourself aware of and take the proper precautions not to blow yourself up, but beyond that, i am interested in the method or the cheap tools that can be used to do this effectively so i don't have to take it to a tech. Essentially, i want to learn how to do this myself.

[Michael]

Well i built the damn thing so at least i know where everything goes . Actually reading back through the site the only thing i really could figure was that yeah, larger watt amps tended to be fixed bias though if you can use a trim pot to change the amount of negative current to the plate then its fixed..cathode is as you mentioned, used with resistors of fixed values. So the question then is, what is our average Joes way of biasing their fixed bias tube amp? Safety is something you make yourself aware of and take the proper precautions not to blow yourself up, but beyond that, i am interested in the method or the cheap tools that can be used to do this effectively so i don't have to take it to a tech. Essentially, i want to learn how to do this myself.

Easy! If your fixed bias is adjustable with a pot, get a bias probe. They can be had new on Ebay and other locations for about $20. Pull a tube; plug probe into tube socket; plug tube into probe; take your readings. Easy as cake, safe, and cheap. Takes literally minutes (or less).

[Francis_Vaughan]

It occurs to me one way of thinking about the different bias configurations is this:

Axiom: You need to set the quiescent grid to a negative potential relative to the cathode.

If you tie the grid to zero volts - i.e. ground - you need to float the cathode up relative to zero - which is done by application of Ohm's law across a cathode resistor. Hence it is called Cathode Bias. Cathode bias has an additional feature - the local feedback inherent in the system due to the cathode resistor (cathode degeneration) makes the system stable and tolerant of variations in the tube characteristics. Thus you don't need any further adjustment. This points to an interesting feature - Cathode Bias isn't fixed - the grid to cathode voltage will settle to a voltage that is determined by the feedback loop - which is dependant upon the μ of the tube amongst other things.

However, if you don't want a cathode resistor for some reason (and dissipating lots of power in the resistor is a good one) then you need to set the quiescent grid voltage at some potential lower than zero volts. Done with a negative rail and a potentiometer this is a solid reference voltage relative to zero volts (and hence the cathode.) So the grid to cathode voltage is firmly set, and the design is called Fixed Bias. With no cathode resistor in a fixed bias system there is no feedback, and the voltage must be adjusted to exactly the right fixed value, and you need to re-adjust for each tube's vagaries. Thus, in a real sense the feedback loop is you with a screwdriver and meter.

[kd]

So the question then is, what is our average Joes way of biasing their fixed bias tube amp?

The easiest way is to put a 1 ohm 1% resisitor from the cathode to ground. Before your put the tubes in, set the pot so you get the maximum amount of negative voltage to the tube socket. Put the tubes in, set the meter to DC millivollts and read across the 1 ohm resistor. If you were to read say 40 milivolts that would be interpreted as .040 milliamps since you are using an accurate 1 ohm resistor. Then .040 x plate voltage = the wattage the tube is dissipating. Example: ..040 x 380 volts = 15.2 watts. As a rule of thumb, to figure most dissipation you should set a tube to, find the maximum dissipation for the tube and and multiply by 70%. For an el34, 30 watts is max, so 30 x .7 = 21 watts. Also remember, plate voltage will vary up and down as you change the pot for more or less current draw.


Ken

[rollingelbow]

Damn you guys are braniacs on this stuff.. impressed with the knowledge both practical and technical that you guys always have on here.

So KD, the lazy man's way (not wanting to take anything apart and resolder) would be to use a bias probe attached to their volt meter. I like the idea of a dual probe so i can get both reading s at once rather then waiting for tubes to cool. Sure it is a bit of an investment, but so are tubes when you blow them prematurely.

[kd]

If you want to build a bias probe cheap, go to Hoffmans website and there are instructions there for building one. He has all the parts and they don't cost much.

Ken

[Merlinb]

And if you use a zener or LEDs for Cathode bias, it's really fixed.

No, zener, diode and LED biasing is cathode biasing, because the bias voltage is "automatically" developed by the cathode current.

[zaphod_phil]

Yes, but you get a fairly well fixed bias voltage! Despite what the conventional terminology may be, this is a clear exception - so let's not split too many hairs....