# How to bias a cathode biased amp with just a voltmeter

Discussion in 'Amp Central Station' started by Guitarslinger1, Mar 2, 2008.

1. ### Guitarslinger1Banned

May 26, 2005
This is taken from a post I posted in another thread. It may be of help to some of you.

As with fixed bias amps, cathode biased amps sound their very best when biased correctly. The following steps are a way to figure the bias without a bias probe, and only a voltmeter.

If you have a bias probe, it is much simpler, as the bias probe is already reading the plate voltage in relation to the cathode, so all you need to do is multiply the plate voltage reading by the current reading to figure power dissipation.

The voltmeter only method requires a few more simple math steps to get you there.

1. Measure the voltage drop on the cathode resistor to ground. Write the value down.

2. Divide this voltage by the value of the cathode resistor. This gives you the amount of current being drawn by both power tubes in milliamps. Write this value down.

3. Measure the voltage on the plates of the power tubes to ground. Write this down.

4. Now, subtract the voltage from the cathode resistor in step 1 from the voltage measured on the plates. Write this value down. Take this value, and multiply it by the current (milliamps) from step 2. This will give you the dissipated power (in watts) of both power tubes. Write this figure down.

5. Take the figure from step 4 and divide by 2. Write this figure down. This is the power dissipation (in watts) of each tube. For 6V6s, if it is over 12 watts, then you need to install a higher value cathode resistor. If it's 10.5 watts or less, you need to install a lower value cathode resistor.

6. After installing the new cathode resistor, do ALL of the steps again to see what you now have. You may have to repeat this process several times to get it dialed in, but it is worth it, and your ears will thank you.

Of course if you are biasing a single power tube amp like a 5F1 Champ, omit the "divide by 2" in step 5, as you will already have the power dissipation of the single 6V6

Jul 16, 2006
Portland, OR
Not quite, IMHO.

This is trickier than is sounds because of the feedback relationship between the cathode resistor and the current draw, because the cathode resistor sets the grid voltage in tandem with the current.

As the cathode resistor is reduced in value, the tube will draw more current in order to reach an appropriate grid bias voltage. But is certainly isn't 1:1 - in fact, if you vary a resistor over +/-50%, the actual power dissipation in the tube will change only slightly.

This is because in order to draw more current, the bias voltage must be reduced - that is, get closer to zero. That means that the drop across the resistor must decrease. Yes, the resistance is less (that helps), but the current is HIGHER (that hurts). The two trend lines fight each other, in a sense.

This was classically solved by picking a bias line on a tube chart and doing the calculation from visually observed chart values. The resulting calculated resistance varies surprisingly little over the curve, take a look for yourself at a typical 6V6/6L6/EL84 spec sheet.

Having observed many pure class-A amps in the field (mainly Champs (5F1, BF/SF and GA-5s) it is thus not surprising to see that even with a wide variety of tubes at different levels of wear, the typical power dissipation remains pretty much the same. In other words, there is no point in changing the cathode resistor.

The main thing I worry about with Champ circuits is that they run very hot - the specified power dissipation in a BF Champ is 13.4 watts for that 6V6, which does run them down. Fender could safely assume in the 50s/60s that US made 6V6s were quite capable, but other tubes (e.g. Chinese) might not be so happy.

The best way to reduce excess power dissipation is to reduce the plate voltage. On BF/SF Champs, I do this by adding a 4 section filter capacitor (instead of 3) and adding a series resistor BEFORE the B+ hits the output transformer. Typically values of 470ohms to 1Kohms (2 watts or so) will drop the B+ by 20 - 45 volts, which puts the tube in a happier place. It also eliminates that "Champ hum" which is a result of the class-A circuit's total lack of hum rejection from the B+ supply.

The result is a slightly "browner" sounding Champ that runs cooler and longer. Overall headroom is negligibly affected.

3. ### Guitarslinger1Banned

May 26, 2005

Oh my.

My post was designed to aid those who are not experienced with biasing their reasonably designed cathode biased amps, to do so with just a voltmeter by changing the value of the cathode resistor. No more, no less.

At no time did I say that changing the resistor value was a 1:1 ratio.

Obviously reducing the B+ is the best way to reduce excessive power dissipation, but that is far outside the scope of my post, and not necessary with many amplifiers, depending on the design and other factors.

Yes changing the cathode resistor, can only manipulate the power dissipation so much due to the relationships you cited, and it certainly can not overcome a really poor design. At no time did I claim that it could.

Your method of reducing excess power dissipation by adding an additional series resistor/filter cap, is a fantastic way to do so, as is changing rectifier type, but should not be necessary for most amps that are within reasonable design parameters, and that only need simple bias adjustment...which is what my post was about in the first place.

4. ### emu!Poster Extraordinaire

Great explaination!

You could make it even simpler by adding a 1 ohm resistor between your cathode resistor and ground...tie in some externally mounted voltage probe connections on the back of the chassis...use these probes to measure mV across the 1 ohm resistor. According to Ohm's law, the resulting mV will equal the mA being conducted by the power tubes cathode. The plate voltage just needs to be measured once, and written on the inside of the cabinet or somewhere convienient. After changing tubes, measure the probes to get mA...multiply it by plate voltage to get watts.

And...you could make it even simpler yet, by using a variable resistor as your cathode resistor (mounted somewhere out of the way, of course). Thus changing the cathode resistor becomes as simple as turning a knob.

5. ### Guitarslinger1Banned

May 26, 2005
Sure, you could do this but 1% 1 ohm 5 watt resistors (in the case of 2 x 6V6 amps) would be very expensive and difficult to find. Also, it's redundant as you are using Ohm's Law to figure the current when you divide the voltage drop on the cathode resistor by the resistor's value.

No doubt 5 watt rheostats would be handy but quite expensive and large. And as Brad eluded to in his post, variations among tubes shouldn't cause too much trouble, so once the amp is dialed in, hopefully this would not need to be done again except in the event of a tube change with tubes with operating characteristics that are too far outside of typical.

Jul 16, 2006
Portland, OR
Sorry, I wasn't trying to go over the top, but to explain the mechanism in lay terms.

I understand your point and I don't doubt the sincerity one bit. What I was trying to say is: it won't work in practical terms. All reasonably designed class-A amps that are out there (Champ, GA-5, etc.) are already about right with respect to cathode resistor, and the only meaningful thing to vary is the plate voltage.

There simply is no benefit to tweaking the cathode resistor, and I recommend leaving it alone.

7. ### Guitarslinger1Banned

May 26, 2005
Yes it does work in practical terms, and is the accepted method to bias cathode biased amps. I didn't make this stuff up.

I have biased many a cathode biased amp, with B+ voltages all over the map, within proper power dissipation range by adjusting the cathode resistor value. I am 100% certain that I could bias one of your BF or SF Champs to correct power dissipation by simply adjusting the cathode resistor value. I have on many occasions set up tweed Champs for correct power tube dissipation with whichever rectifier (5Y3, 5V4, GZ34) the customer thought he wanted, and then for a different rectifier when he changed his mind.

But, keep in mind, I'm not talking about lowering the B+ for the sake of some feeble Chinese tubes of yesteryear that can't operate within the typical voltage ranges of vintage guitar amps. Fortunately, those days are past, and along with good availability of American and other NOS tubes, the Russian, Chinese and Slovakian tubes made today can handle the voltages found in vintage guitar amps. I'm simply talking about properly biasing cathode biased amps.

If I can set up cathode biased 6V6 amps for correct power tube dissipation with amps that have B+ voltages ranging from 340V to 410V by changing the cathode resistor, so can anyone else.

IMHO, you may want to put down the slide-rule and the Smith chart and get on the bench and try it...

Saying there is no benefit to properly biasing a cathode biased amp is like saying there in no benefit to biasing a fixed bias amp. A properly biased amp, of any type, will sound better than one that's improperly biased.

Mike at KCA Tubes touches on cathode biasing by changing the cathode resistor value in the text and link below, and I'll dig out my old tube amp books, see what I can find, and post more later.

Most guitar amps that use these tubes are cathode biased (aka "self biasing") and generally don't require rebiasing when installing new EL84s. However, it is always best to check that the idle power of these tubes does not exceed the maximum rating (12watts) in your amp. Be sure to check or have your amp tech check the idle current/power when installing new EL84s. With some pairs/quartets (any brand, NOS or new production) the tubes will run too hot and have a very short life. If the tubes run too hot, a larger value cathode resistor must be installed. Please heed this warning as we will not be responsible for tubes that expire quickly...

http://www.kcanostubes.com/categories/32/1/6BQ5-EL84-7189A.htm

8. ### Guitarslinger1Banned

May 26, 2005
Sorry, but every book I read and website that I find disagree with you. I could post links and text all day, but I'll leave it with this info below from Aiken Amps.

Now one thing where I dropped the ball completely, and this Aiken info reminded me, is that with a class AB amp cathode biased amp, setting the bias at lower than 100% max dissipation may not be necessary as it is with fixed bias amps, but that one may want to experiment with the given amp to determine what's best.

From http://www.aikenamps.com/Biasing.html

Do cathode-biased amplifiers need to be biased? The short answer is yes. The cathode biasing method is self-regulating, to an extent, because increases in cathode current create a larger voltage drop across the cathode resistor, which in turn, creates a larger negative grid-to-cathode voltage, which counteracts the increase in current. The tube will reach a stable point of equilibrium and stay there. However, just as different tubes from different manufacturers will draw varying amounts of current in a fixed-bias amplifier, the same is true of a cathode-biased amplifier. For this reason, the bias should always be checked, even with cathode-biased amplifiers.

Checking the bias current in a cathode biased amplifier is easy, just measure the voltage across the cathode resistor and divide by the resistance value to obtain the cathode current. Note that if the output tubes share a common cathode resistor, you must divide the current reading by the number of tubes sharing the resistor. Note also that a common cathode resistor does not allow you to determine the individual currents of each tube, so if one tube is drawing more current than the other, you would not be able to determine which is causing the mismatch, and, in fact, you would not be able to tell there was a mismatch at all. You can add individual 1 ohm resistors from the cathode of each tube to the common bias resistor, but you must then measure across the 1 ohm resistors, not from the cathodes to ground, to determine the voltage drop, and thus the cathode current. You can also use individual cathode bias resistors on each tube. The value of the resistor will be double that of the common resistor if two tubes are used, or four times that of the common resistor if four tubes are used. Each resistor would also have to be bypassed with its own electrolytic bypass cap.

The difficulty with cathode-biased amplifiers is that the cathode resistor must be physically changed for another one of different value in order to change the bias current. Although it can be done, very few guitar amplifiers have adjustable cathode bias.

Cathode-biased class AB amps are usually exempt from the "70% rule", because their cathode voltage rises when a signal is applied, effectively reducing the bias, and shifting the amp further into class AB operation. This means you can bias them hotter than a normal fixed-bias class AB amp and the tubes will still survive. Having said that, you have to experimentally determine how hot you can bias them by finding out how far the bias shifts during signal flow.

If the cathode-biased amp is "true" class A, there will be no bias voltage shift seen on the cathode when signal is applied, so you can bias at max dissipation and not worry about it. If the amp is actually class AB, you might still be able to get away with biasing at max dissipation because of the large bias shift at full power that pushes the amp into the class AB region, but you should check the tube dissipation at all signal levels. Note that max dissipation may not occur at full power, rather at somewhere between idle and full power (usually around halfway), so you have to carefully determine the safest max idle current to avoid exceeding the dissipation at any point in the tube's operation.

Jul 16, 2006
Portland, OR
Gosh, I didn't mean to create a tempest in our teapot.

You are correct that one can set the bias via the cathode resistor, of course.

I only wanted to indicate that the correlation between the cathode resistor and the power dissipation is a bit weak - in other words, you may have to adjust it quite a bit to see an important difference. Of course it can be done, and the general value does matter. It just doesn't matter to within a value like +/- 20%, which is why Fender and others used this method. It was cheap and got the job done without fuss, even with sloppy resistors. The values picked by Fender were right out of the tube manuals and are pretty good, even though Fender elected to run the bejeezus out of the tubes with respect to plate voltages.

The reason I prefer the method of adding a power supply stage is that it is more consistent and has the added benefit of reducing hum (by a lot). That's all.

10. ### Guitarslinger1Banned

May 26, 2005
Exactly! And that's what my original post was about.

nuff said

Last edited: Mar 5, 2008
11. ### emu!Poster Extraordinaire

Using the voltage drop across a 1 ohm resistor allows a direct 1:1 correlation to V & I. It takes out the step of dividing voltage by resistance to get current. If you know V, you know I. To someone who doesn't know about Ohm's law (most guitar players, IMO), it's less confusing.

The only reason I posted was to tell about what I have seen in other peoples amps as far as cathode biasing is concerned. I don't make any claims as to whether one way is better or worse.

12. ### 1293Poster Extraordinaire

May 18, 2006
I have an even easier baising method using duct tape and a pen knife, but I'm afraid if I post it somebody will just tear it apart.

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