Not exactly a stompbox...

[ranchak]

I know this is not a stompbox, but I thought you guys might like to see a bias tool that I made. Special thanks to R.G. for his help.

[xshredx]

Very nice...
would you consider documenting it a little more, and making this into a project for everybody?  This would really be handy for lots of people...

Anyway, a bias meter is handy, and this one looks great!

[aron]

WOW, what a pro job!

[trevize]

great idea! please provide us some more informations!

[soulsonic]

You can find pretty much all the info at RG's site.

It looks a damn site better than those ^!@%#*$(!*#@!  Bias Rites I had to build all day. I really like how you did up the box... though the switching arrangement seems a little strange. I woulda just done it "1,2,3,4" with just a single rotary.

[ulysses]

looks nice

cheers
ulysses

[R.G.]

From the GEO front page, click on "Tube Guitar Amp Tech Pages" and then on "DIY Bias Probe".

Ranchak wired his tube holders with the plug, socket and 1 ohm resistor all in one piece per tube. The illustration at GEO is for a separate plug and socket because I've worked on some amps that don't have enough room over the tubes to insert the extra height of the plug/socket. To be fair, those are hifi amps, so maybe most guitar amps will work fine with a one-piece.

The specifics of the box depend on the meter you get.

A hacker's way of making this work is to notice that the cheap $3 meters from Harbor Freight have a 200mV range. You can just put one of those inside your box with the selector switch permanently set to "200mV" and the wires going to the current inputs. You could even pull the plastic case off the meter. But the digital panel meter is simpler if you can find one.

[DWBH]

What's bias after all? I never understood that. And how do you 'fix' your amp's bias? What's that on the lcd? What measure?

[ranchak]

Thanks for the input guys. I could have used one rotary switch, but I picked up two data transfer switches at the thrift store for $6. I think this design will work for tube heads, but not for combo amps. I have to remove the chassis from my Marshall combo, there is not enough room to fit the sockets and tubes inside the amp.

My design is a cross between R.G.s design and the design at Hoffman Amplifiers site. I like the idea of measuring the voltage from pin 8, this way you can use lower voltage wire and stay safe at the same time.

There are a few changes I would make. I got cheap and used shielded coax for the leads, I would use the leads from a DMM next time. This way you can use the red wire for the left tube and the black wire for the right tube. Secondly the wires are more flexible than the coax. I would also use pieces of shrink tube to keep the wires together so they don't lay and twist around. I probably would use one rotary switch instead of two. I would also use large diameter heat shrink tubing on the sockets. Again they would be color coded. I used Plasti Dip and it does a good job, but doesn't look as professional as I would like. None of these changes affect the usage or functionality, they are definately cosmetic. You can go one step further and skip the dedicated meter and install banan plugs on the end of your leads and plug directly into your DMM. I wanted the ability to see the bias voltage on each tube and each "side" of the amp. If you guys have any further questions I'll be glad to answer them.

[R.G.]

What's bias after all? I never understood that. And how do you 'fix' your amp's bias? What's that on the lcd? What measure?

From your question, it is clear that you never read the articles at GEO - http://www.geofex.com.

In particular, the Tube Amp FAQ tells you what bias is, what it means in the amp, and how it's set. The short version is that it's the DC current that flows through a tube when there is no signal. This is important in tube amp output stages because it both minimizes cross over distortion in the output stage and also balances your output transformer so it does not distort in saturation on one or the other side of its swing.

You set the bias by setting a precise amount of current flow through the tube. This is done by changing the reverse voltage on the grid of the tube with respect to the cathode. In Fenders this is usually done by unsoldering and replacing fixed resistors. In civilized amps it's done by adjusting a bias voltage pot. You read the tube current with a meter, and adjust the bias voltage until the current is correct.

What's on the LCD is the number of millivolts across a 1 ohm resistor. The resistor is put in series with the cathode of the tube it's plugged into. Since ohm's law says that the voltage across a resistor is equal to the current through the resistor times the resistance, the number of millivolts across a 1 ohm resistor is equal to the number of milliamperes which are flowing through it. So by reading millivolts across 1 ohm, we read milliamperes through the 1 ohm, and hence the current in the tube. The LCD reads the number of milliamperes in the tube directly.

[Paul Marossy]

Nice job! 

[Timebutt]

Thank you R.G. for the very clear explanation about biasing: suddenly I had this 'click' and everything fell into place
Just something I was wondering about: if biasing a transistor/tube, what value exactly are you aiming for? This can probably be found in the datasheet?

[snoof]

Nice job ranchak!!!!

correct.  as far as tubes go, the bias is dependent on the plate and/or cathode voltage.  you don't want to bias the tubes so that they are dissipating more wattage than they are supposed to.

[jaytee]

How about a gutshot?

[ranchak]

Just something I was wondering about: if biasing a transistor/tube, what value exactly are you aiming for? This can probably be found in the datasheet?

You can find most bias values here:

http://www.diyguitarist.com/GuitarAmps/PowerTubeBias.htm

[Timebutt]

Ok, so these values are valid for tubes only, but lets say an ordinary transistor (which will mostly operate under 9V in stompbox applications which probably leads to a smaller 'amount' of values?) what's the correct value I am aiming for in that situation and is it for the same reason as power dissipation as with tubes?

[ranchak]

I have been really busy at work, but when I get some time I will get a gut shot of this.

[R.G.]

Just something I was wondering about: if biasing a transistor/tube, what value exactly are you aiming for? This can probably be found in the datasheet?

It's not that simple.

The circuit dictates what bias level the active devices have to be set at, not the other way round. And other considerations determine what currents the circuit demands.

For instance:
Tube amp, Class A: Class A operation demands that the output tubes be set at 1/2 of the maximum current. This is to prevent them from ever turning completely off when they swing to minimum current.

Tube amp Class AB: Tubes are set less than Class A so they'll turn off some of the time, but more than zero to lessen crossover distortion. Exactly where that value is depends on the tubes and their internal linearity at the voltages and currents of the power amp.

Transistor preamp stage, high impedance: generally 100uA to 1ma, as this is the lowest noise range for most bipolars

Transistor line level driver: zero (class B) to maybe 100ma (class A)

12AX7 triode: 10uA to 1.5ma depending on power supply available, noise considerations and the incoming signal level.

You aim for the right current for the circuit, which varies. And you have to know the circuit and application to know the right current. Yes, it's circular, and you have to just go learn it. I have wished many times that it was easier.