Biasing Cathode Biased Amps

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March 2nd, 2008, 07:44 PM #1 (permalink)

Guitarslinger1Banned

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How to bias a cathode biased amp with just a voltmeter

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 readingby the current reading to figure power dissipation.

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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 measuredon 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 powerdissipation (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 valuecathode resistor.

6. After installing the new cathode resistor, do ALL of the steps again to see what you nowhave. 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 by2" in step 5, as you will already have the power dissipation of the single 6V6

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bradpdxTele-Afflicted

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Not quite, IMHO.

This is trickier than is sounds because of the feedback relationship between the cathoderesistor and the current draw, because the cathode resistor sets the grid voltage in tandemwith 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, getcloser 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 linesfight each other, in a sense.

This was classically solved by picking a bias line on a tube chart and doing the calculation




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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 ofwear, 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 specifiedpower 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 othertubes (e.g. Chinese) might not be so happy.

The best way to reduce excess power dissipation is to reduce the plate voltage. On BF/SFChamps, 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 humrejection from the B+ supply.

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

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Quote:

Brad,

Originally Posted by bradpdx

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 gridvoltage 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 youvary 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 mustdecrease. 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 thecalculation 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/SFand 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 - thespecified 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) andadding a series resistor BEFORE the B+ hits the output transformer. Typically valuesof 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 ofthe 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.




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Oh my.

My post was designed to aid those who are not experienced with biasing their reasonablydesigned cathode biased amps, to do so with just a voltmeter by changing the value of thecathode 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 thedesign and other factors.

Yes changing the cathode resistor, can only manipulate the power dissipation so much due tothe 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 seriesresistor/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 needsimple bias adjustment...which is what my post was about in the first place.


emu!Friend of Leo's

Join Date: Aug 2007Location: Missing

Posts: 4,756

Quote:

Great explaination!

Just to add...

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 thechassis...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. Theplate voltage just needs to be measured once, and written on the inside of the cabinet or

Originally Posted by specialty guitarsThis 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 biasedcorrectly. 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 platevoltage 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 amountof 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 bythe 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 youneed 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 itis 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 single6V6




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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 cathoderesistor (mounted somewhere out of the way, of course). Thus changing the cathode resistorbecomes as simple as turning a knob.



Tele-Holic

Join Date: May 2005Posts: 971

Quote:

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 tofigure 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 isdialed 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.

Originally Posted by emu!

Great explaination!

Just to add...

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 onthe 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 thepower 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 yourcathode resistor (mounted somewhere out of the way, of course). Thus changing the

cathode resistor becomes as simple as turning a knob.

March 3rd, 2008, 11:50 AM #6 (permalink)


Join Date: Jul 2006

Location: Portland, ORAge: 52

Posts: 1,573

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Quote:

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: itwon'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 onlymeaningful thing to vary is the plate voltage.

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

__________________

Originally Posted by specialty guitarsBrad,

Oh my.

My post was designed to aid those who are not experienced with biasing theirreasonably designed cathode biased amps, to do so with just a voltmeter by

changing the value of the cathode resistor. No more, no less.

Your method of reducing excess power dissipation by adding an additional seriesresistor/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 firstplace.




6/9

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March 3rd, 2008, 12:47 PM #7 (permalink)


Tele-Holic

Join Date: May 2005Posts: 971

Quote:

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 properpower 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 powertube 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 Chinesetubes of yesteryear that can't operate within the typical voltage ranges of vintage guitaramps. Fortunately, those days are past, and along with good availability of American andother 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 cathodebiased amps.

If I can set up cathode biased 6V6 amps for correct power tube dissipation with amps thathave 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 benchand try it...

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

Mike at KCA Tubes touches on cathode biasing by changing the cathode resistor value in thetext 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 thatthe 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 newEL84s. 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 mustbe installed. Please heed this warning as we will not be responsible for tubes that expire

quickly...

http://www.kcanostubes.com/categorie...EL84-7189A.htm


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 thatare 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 leavingit alone.


Guitarslinger1BannedTele-Holic

Quote:

Sorry, but every book I read and website that I find disagree with you. I could post links and


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




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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% maxdissipation may not be necessary as it is with fixed bias amps, but that one may want toexperiment with the given amp to determine what's best.

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

What about cathode-biased amplifiers?

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 differentmanufacturers 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 withcathode-biased amplifiers.

Checking the bias current in a cathode biased amplifier is easy, just measure the voltageacross 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 currentreading 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 isdrawing 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 canadd 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, todetermine the voltage drop, and thus the cathode current. You can also use individualcathode 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 fourtubes 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 itcan 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 theamp 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 toexperimentally 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. Ifthe amp is actually class AB, you might still be able to get away with biasing at maxdissipation because of the large bias shift at full power that pushes the amp into the class ABregion, 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 toavoid exceeding the dissipation at any point in the tube's operation.



Join Date: Jul 2006

Location: Portland, ORAge: 52

Posts: 1,573

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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 powerdissipation 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 justdoesn'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. Thevalues 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.__________________

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Guitarslinger1BannedTele-Holic

Join Date: May 2005

Posts: 971

Quote:

Exactly! And that's what my original post was about.

nuff said

Last edited by Guitarslinger1; March 5th, 2008 at11:48 AM..


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

March 4th, 2008, 10:34 AM #11 (permalink)

emu!Friend of Leo's

Join Date: Aug 2007Location: Missing

Posts: 4,756

Quote:

Using the voltage drop across a 1 ohm resistor allows a direct 1:1 correlation to V & I. Ittakes 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 lessconfusing.

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.

Originally Posted by specialty guitars

Also, it's redundant as you are using Ohm's Law to figure the current when youdivide the voltage drop on the cathode resistor by the resistor's value.

March 4th, 2008, 12:07 PM #12 (permalink)

1293Friend of Leo's

Join Date: May 2006

Posts: 2,280

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

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Biasing Cathode Biased Amps