I
use this method to bias my amps. I also use the 70% of maximum
dissipation rule I learned from Randy Aikens site. Below is an exerpt, read the whole thing here.
In fact, go to Randy's
site and read all the tech stuff there. You'll be a lot smarter
when yer done. - Regis
From Randy Aikens site:
A general rule of thumb is that class AB amplifiers are usually
operated at no more than 70% of the maximum plate dissipation of the
tube (to account for the higher dissipation that occurs under signal
conditions), while true class A amplifiers generally run right at the
maximum plate dissipation (the dissipation at full power is lower than
the dissipation at idle in a true class A amplifier).
For example, the aforementioned EL34 tube has a plate dissipation of
25W, so at 400V class AB operation, it should be biased no higher than
(0.7 * 25/400) = 44mA. At 500V class AB operation, it should be
biased no higher than (0.7 * 25/500) = 35mA.
This doesn't mean you should automatically bias all tubes to 70% of max
dissipation!
They can be biased at any lower current if desired, and many people
prefer a point of around 50% to 60% of the max plate dissipation, which
contributes to longer tube life.
In true class A operation at 250V, it should be biased no higher
than (25W/250V) = 100mA. Note that a class A amplifier does not
necessarily have to be run at the maximum ratings. You could design a
true class A amplifier at lower plate voltages and higher currents, but
there is a limit to how high the plate voltage can be without exceeding
dissipation ratings, or having to go to class AB. There is also a
limit at how high the plate or cathode current can be for a particular
tube. A class B amplifier should be biased right at cutoff, or perhaps
a few mA standing current, to minimize crossover distortion. Class B
amplifiers usually have extremely high plate voltages in order to
maximize the output power, so they must be biased right at cutoff to
prevent over-dissipation at full power. If in doubt about the actual
operating conditions of the circuit, call the manufacturer or refer
servicing to a qualified amp technician
.
From Lord Valve: How
to bias your amplifier
DISCLAIMER: LORD VALVE (W. WHITTAKER
DBA NBS ELECTRONICS) WILL TAKE NO RESPONSIBILITY FOR ANY INJURIES OR
DAMAGE SUFFERED BY ANYONE AS A RESULT OF THE MISINTERPRETATION OR
MISAPPLICATION OF THE INFORMATION CONTAINED IN THIS ARTICLE. PROPER
HIGH-VOLTAGE SAFETY PROCEDURES AND PRECAUTIONS MUST BE PRACTICED AT ALL
TIMES WHEN SERVICING VACUUM TUBE AMPLIFIERS!! IF YOU ARE NOT
*COMPLETELY* FAMILIAR WITH THESE PRECAUTIONS, SEEK GUIDANCE FROM
SOMEONE WHO IS!
The cathode resistor method
This is the method that is best for hobby techs and do-it-yourselfers.
While not as accurate as the output transformer shunt method (detailed
below, after the cathode resistor procedure) it is far and away the
safest of the two methods, and can be successfully done with medium-
and even low-quality test equipment. It is performed by reading the
cathode current through each power tube; the accuracy is lower because
the cathode current is composed of the plate current *plus* the screen
current. Plate current can be identical on two tubes (tubes are matched
by plate current readings) while one tube is drawing more screen
current; with this method, the readings will appear to indicate a
mis-match when such is not actually the case. Since the cathode current
will always be higher than the actual plate current, the readings
obtained with this method will tend to make you set the tubes a little
colder than your calculations will indicate that they are. This
promotes slightly more conservative operation, which is beneficial to
tube life. Note that these instructions assume that your amplifier is
biased by applying a negative voltage to the control grids;
cathode-biased amplifiers cannot be adjusted other than by changing the
value of the cathode resistor(s) so this method does not apply to them.
BE AWARE THAT THE ACCURACY OF THE RESULTS YOU OBTAIN FROM *ANY* BIASING
METHOD WILL BE DIRECTLY AFFECTED BY THE QUALITY OF YOUR TEST EQUIPMENT,
AND YOUR SKILL IN USING IT. If any part of the following instructions
doesn't make sense to you, seek help from someone with more experience.
WHAT YOU WILL DO:
A) Replace the ground wire on each power tube socket with a 1-ohm
resistor.
B) Read the voltage drop across this resistor (in millivolts) with your
DMM.
C) Read the plate voltage.
D) Use the above readings to calculate the static dissipation wattage.
E) Adjust the bias to obtain the best tone, while keeping the tubes
within specifications.
A SUGGESTION: You may want to
practice taking these readings and making these adjustments with your
old tubes still in the amp, or with a spare (used) set. That way, you
won't fry your new tubes if you make a mistake. On some sockets, the
pins are numbered on the bottom (terminal) side; it is sometimes
difficult to tell which pins the numbers go with. The best way to tell
which pin you are looking at is to count clockwise from the notch on
the locator "keyhole" in the center of the socket, with the first
terminal clockwise from the notch being pin ONE. This assumes that you
are looking at the sockets from the BOTTOM, or UNDERSIDE. Most guitar
amplifiers use output tubes which have the same (or very similar)
basing. ("Basing" refers to the order in whichthe internal elements of
the tube are connected to the pins on the bottom of the tube.) The 6L6,
6V6, 6550, EL34, 5881, KT66, KT88, KT90, KT100, etc. are all easily
biased with this method.
You'll need a 1-ohm resistor for each power tube in the amp. All of the
tubes listed above have their cathodes on pin EIGHT, which will be
grounded to the chassis. On some amps, such as Marshalls, pin ONE will
be tied to pin EIGHT, and both will be grounded. On older Fenders, pin
ONE is usually used as a tie-point for the 1.5K grid-stopper resistor,
and the negative bias voltage will be on this pin. DO NOT GROUND PIN
ONE ON A FENDER AMPLIFIER, or you'll get a big surprise. (Expensive,
too. ;-)
REMOVE the ground wire from pin EIGHT on each tube, and REPLACE it with
a 1-ohm resistor. On older Fenders, the ground wire is a piece of
copper braid; unsolder it from the socket pin but *don't* cut it off
where it attaches to the chassis. Solder the resistor to pin EIGHT, and
attach the free end of the resistor to the ground wire you unsoldered
from pin EIGHT. Repeat this for all of the power tube sockets. I prefer
to use 2-watt resistors, but half-watters will work just fine. The
accuracy of your measurements will be directly related to the tolerance
of these resistors; precision 1% (or better) types are suggested.
Turn your amp on, but leave it on STANDBY. Set your DMM to the highest
DCV scale, ground the black probe to the chassis, and take a reading
from pin FIVE of any power tube socket. You should see a negative
voltage in the -35 to -50 volt range, if the amp has EL34s, or in the
-45 to -60 volt range if the amp uses 5881s, 6L6s, or KT66s. KT88s,
6550s, KT90s, and KT100s can have bias ranges that go as high as -100
volts. Amps which use 6V6s will usually have bias supplies which
produce voltages that are similar to EL34 amps...but not always. First,
locate the bias trimmer. (Possibly a little square blue thingy with a
screwdriver-adjust slot in the center, or a round black thing that
stands on three legs, or, for an old Fender, a full-sized pot with a
screwdriver-adjust slot on both sides; newer PCB-type Fenders use
three-leg horizontal trimpots, if they have a bias-adjust pot at all.)
Next, adjust the bias control until you have MAX NEGATIVE voltage on
pin FIVE. (In other words, rotate the bias trimmer until you obtain the
highest negative voltage that the bias supply is capable of
delivering.) Install your tubes (the amp is still on STANDBY, remember)
and wait a few minutes for them to warm up. Take the amp off STANDBY
and make sure your DMM is still set to the highest DCV scale; take a
reading between the chassis (ground) and pin THREE on any power tube
socket. Remember, the BLACK probe always goes on the CHASSIS. Write
this voltage down; you'll need it later.
Now, set your DMM to the lowest DCV scale (usually 200 mV) and take a
reading across the 1-ohm resistor(s). (This reading can be interpreted
directly in milliamperes, because one millivolt across one ohm equals
one milliamp. Ohm's law says so, and you ain't gonna argue with *that*,
are ya? ;-) It'll be pretty low, because you have the bias trimmer set
to max neg voltage.
Adjust the bias trimmer ("pot") until you get a reading across the
1-ohm resistor(s) somewhere in the 30-40 mV range, for everything but
6V6s. For 6V6s, you'll want to start out at around 20 mA and work
upward from there. Note that the polarity of this reading is
unimportant; only the numerical value means anything. (If you put the
black probe on the side of the resistor that is grounded to the
chassis, you will get a POSITIVE reading.)
MULTIPLY the voltage you read on pin THREE earlier by the reading you
just obtained from the 1-ohm resistor. (Example: 450 Volts times 35
milliamps, or .035 Amperes.) This will give you the STATIC DISSIPATION
WATTAGE at which the tube is operating. (It'll be wrong, but more on
that later.) The above example gives a static dissipation of 15.75
WATTS, which is well within specs for an EL34 (fairly cold, in fact) or
a 5881/6L6. See TABLE "A" (at the end of this article) for suggested
MAX static dissipation wattages for most of the common octal-based
tubes discussed here. To sum up what this calculation is, PLATE VOLTAGE
times CATHODE CURRENT equals STATIC DISSIPATION (IDLING) WATTAGE. It is
important not to exceed the manufacturer's specification for this
parameter, because tube life will be shortened. At extreme settings,
tube life will be measured in MINUTES...be advised.
Take another reading from pin THREE (remember to set your meter on the
HIGHEST DCV scale before you do!) and write it down. This new reading
should be LOWER than the first reading you took, because the tubes are
drawing more current now and the plate voltage will sag somewhat.
Multiply this new reading by the value you measured across the 1-ohm
resistor(s); this will give you the idling (static) wattage. The cooler
you run the tubes, the longer they'll last. If you dig the way the amp
sounds when the tubes are idling at only 12 watts, fine...don't worry
about it. 6V6s, though, will be running fairly *hot* at 12 watts.
Remember, each time you adjust the bias control, you'll have to take a
new reading from BOTH the 1-ohm resistor AND the plate (pin THREE) and
multiply them to see what the tube is dissipating. You can play your
guitar through the amp each time you adjust the bias, and see how you
like it. You can even adjust the bias by ear, and then take readings as
outlined above to see if the tubes are being operated within their
ratings. If you find that you only like the tone when the tubes are
operating near their limits, you may decide to trade some tube lifetime
for the tone you seek.If you like the tone with the tubes running cold,
you'll obtainsignificant extra tube life that way. It's *your* call.
If you see a few milliamps difference between the readings on the 1-ohm
resistors, don't sweat it; this could be due to poor matching (not a
factor if you bought 'em from *me* :), differences in screen current
between the tubes, or differing leg impedances in the output tranny's
primary. (All of those things are fairly common in guitar amps.) Note
that for an amplifier which uses four (or more) power tubes, balance
between the two sides is more important than having identical readings
from socket to socket. You should add the readings for each pair; if
the left pair is close to the right pair, things are fine. If the left
pair reads 32 and 34 milliamps (total = 66) and the right pair reads 35
and 31 milliamps (total = 66) then you've got a nicely balanced output
stage, even though some of the tubes are running slightly hotter or
colder thanothers. Having the currents balanced on the two legs of the
trannyhelps eliminate 120 Hz power-supply ripple from the output.
Notethat you can swap the tubes around to obtain the best current
balance,since you can take individual readings on each socket. If you
see a large difference between them (say, 8-12 milliamps) this means
you need to find out why this difference exists. One thing you can do
is SWAP the tubes into the opposite sockets and take new readings. If
the bogus readings are consistent on the SOCKETS, then you'll need to
look at the amp to find out the cause. If the readings MOVE with the
TUBES, you can be fairly sure you have a poorly-matched pair/quad.
Once you have everything adjusted to your taste and you're sure the
tubes are being operated within specifications, leave the amp fully
powered up for three or four hours. Eyeball the tubes every fifteen
minutes or so, to make sure the plates aren't turning red. You are
doing this to let the tubes "settle" into their new operating
con-ditions; at the end of the settling period, take a final set of
readings to make sure everything is still OK. If any readings have
drifted significantly, readjust the bias accordingly. Note that the
incoming line voltage directly affects all of the voltages in the amp;
you may want to read the line voltage occasionally to see if this is
happening. Line voltage will drop a bit around supper time (lots of
juice being used for cooking) and also after sunset. If the line was
120VAC when you completed your biasing procedure and it's 117VAC when
you take your final readings after the settling period, expect to see a
corresponding small drop in your measurements.
REMEMBER...THERE ARE VOLTAGES PRESENT
INSIDE EVEN THE SMALLEST TUBE AMPLIFIER WHICH WILL KILL YOUR ASS JUST
AS DEAD AS A HAND GRENADE WILL!! If you're not familiar with
high-voltage safety, seek guidance from someone who is. BTW, an oven
mitt or a pot-holder (real men like me use welding gloves) will come in
handy for handling hot power tubes if you need to switch sockets; you
don't want to let the tubes cool off too much while you swap them
before taking new readings.
The output transformer shunt method
This is the way most pro techs measure plate current. A *good* quality
DMM is required for this measurement. (When it comes to good DMMs, you
have three choices...Fluke, Fluke, and Fluke.) This section assumes
that you know a bit more about your amp, and how to use your testgear. If any of it is unclear, DON'T TRY THIS.
WHAT YOU WILL DO
A) Read the current flowing through each leg of the output
transformer's primary.
B) Read the plate voltage.
C) Use the above readings to calculate the static dissipation wattage.
D) Adjust the bias to obtain the best tone, while keeping the tubes
within specifications.
For this particular reading, you'll need to change your test leads to
the CURRENT input jacks, and select the 200 mA DC range. The two probes
are applied to the center tap and either of the ends of the output
transformer's primary. (On a Fender, for instance, the center-tap is
RED, and the two plate wires are BLUE and BROWN. On a Marshall, the
center tap is BROWN, and the plate leads are RED and WHITE.)
On some amplifiers, the easiest way is to put one probe on pin THREE of
either socket (or of either of the two sockets on each side) and the
other on the center-tap, which will be located at some distance from
the socket. Some amps (like the Marshall JCM 900 series, for instance)
have all the wires soldered to terminals on the bottom of the output
transformer, conveniently sticking up right where you can reach them.
The current that would normally flow through half of the transformer's
primary winding is "shunted" through the meter, and thus measured. A
small amount still flows through the part of the winding you are
shunting, but the transformer's resistance is much higher than your
meter's internal resistance. Nearly all of the current flows through
the meter.
BE ADVISED...for all practical purposes, a meter set to measure CURRENT
is equivalent to a STRAIGHT WIRE. This means that as soon as you touch
either probe to the high voltage circuitry, THE OTHER PROBE NOW CARRIES
THE SAME VOLTAGE. If you drop the probe and it lands on your arm or
leg, you could be electrocuted. If it lands on the chassis (or anything
else that is at earth or circuit ground potential) a huge spark will be
generated, along with a noise like a small firecracker. (Please don't
ask how I know this. ;-) The probe tip will be partially melted, and at
the very least, the meter's internal fuses will blow. At worst, the
meter will be history. Shorting the HV to ground isn't especially good
for the amp either, and may blow the amp's fuse or damage the
circuitry. You can easily kill a rectifier tube this way.
BE ESPECIALLY CAREFUL NOT TO LET A
PROBE SLIP OFF A TERMINAL AND HIT THE CHASSIS WHILE YOU ARE TAKING A
READING! BE *EXTRA* CAREFUL TO MAKE SURE YOUR FINGERS DON'T SLIDE DOWN
THE PROBE AND COME INTO CONTACT WITH THE METAL TIP!!
Once you've obtained the current readings from both sides of the output
transformer's primary, you'll need to take a plate voltage reading so
you can calculate the static dissipation wattage (as outlined above in
the CATHODE RESISTOR method) and decide whether you need to increase or
decrease the plate current. Note that if you are using the OPT shunt
method with an amplifier which uses more than one tube per side on the
transformer, you will need to divide the current reading on each side
by the number of tubes used. Example: you read 88 mA on one side of a
Twin Reverb's output tranny; that's 44 mA per tube, since there are two
on each side. (4 total.)
REMEMBER TO REMOVE THE TEST LEADS FROM THE CURRENT MEASURING JACKS, AND
TO SET THE METER TO THE HIGHEST DC VOLTAGE RANGE BEFORE YOU TRY TO READ
THE PLATE VOLTAGE!! If you attempt to read the plate voltage with your
meter still set up for a current reading, the results will be
spectacular (as outlined above.) Since you may need to take several
plate CURRENT and several plate VOLTAGE readings before you are
finished setting the bias, you will need to be extremely vigilant about
changing the meter settings (and the test leads) each time you take the
different readings. Most pro techs use TWO METERS for this procedure,
leaving one set up for current and one for voltage. (I use a handheld
meter for the voltage reading, and a bench meter for the current.)
Once you have the necessary readings, the procedure is the same as for
the CATHODE RESISTOR method: read, multiply, listen, adjust, read,
multiply, listen, adjust, read, multiply, etc. Don't neglect the
"settling" period, either. BE CAREFUL!! Types of (fixed) bias circuits
Many amps which use "fixed" (negative grid) bias have provisions for
adjusting the negative grid voltage upward or downward. Making the
grids LESS negative will cause MORE current to flow through the tubes.
Some amplifiers don't have a bias-adjusting control (pot) but instead
use a fixed resistor to set the voltage. If you encounter one with a
fixed resistor, the best thing to do is convert it to an adjustable
type. Most of the time, the fixed resistor will be in parallel with the
bias capacitor; the lower this resistor's value is, the lower the bias
voltage will be. If you can locate and identify this resistor, you can
replace it with a simple network consisting of a (lower value) resistor
in series with a potentiometer. What you'll be shooting for is a range
of adjustment that goes from LESS voltage to MORE voltage than is set
by the (existing) fixed resistor. Take the value of the fixed resistor
and divide by two; pick the closest standard value to your result, and
put it in series with a pot which is as close to the original
resistor's value as you can find. Example: the existing resistor is
33K; use a 15K resistor in series with a 25K pot to replace it. The
original resistor was 33K; you now have the ability to adjust the value
from 15K to 40K. This should provide you with sufficient adjustment
range to set any plate current you wish.
Some amps have a "balance" type bias adjustment, which allows you to
vary the negative grid voltage between the two halves of the output
stage; this makes a "matched" set of tubes less crucial to good
performance, although it can't compensate for tubes that are wildly
different. If you encounter this circuit, the easiest way to adjust it
is to simply "tune" the control for minimum 120Hz hum on the output.
This type can be modded to the *best* type, which is not only variable
from side-to-side, but adjustable up-and-down, too. Usually, this
circuit will have the "balance" pot's wiper connected to a resistor
which is grounded at the other end. You can replace this resistor
exactly as outlined above (half the value, add a pot, etc.) and have
the best of both worlds.
If the simple mods outlined above (and the reasons for making them)
don't seem perfectly clear to you, DON'T TRY THEM. A schematic (and the
expertise with which to interpret it) will go a long way towards
helping you do them correctly. You can have the mods performed by a
tech, and then do your own biasing from then on, if you wish.
Table A
Suggested MAX static dissipation wattages for common guitar amplifier
tubes. You can exceed these (although I wouldn't do it with a V6) at
the cost of some tube lifetime. The colder you run 'em, the longer they
will last. Remember, as long as you don't run the tubes hot enough to
damage them, there are *no* rules about how much current to set them
for. If you like the way your amp sounds when your 6L6s are only
pulling 14 watts, bully for you... you probably won't need to retube it
for 10 years. I know that many sources for biasing information just
specify plate (or cathode) current settings; telling you to bias your
6L6s at "35 milliamps" is nonsense. Unless you take the plate voltage
into consideration, a current specification is meaningless. For
instance, 40 mA at 250 volts is 10 watts; the same 40 mA at 500 volts
is 20 watts... TWICE as much. In both cases, the current is the same.
Amps vary; two identical amps can have plate voltages which differ by
as much as 20%. Just because you have a schematic that specifies the
plate voltage in your amp as being at 450VDC, don't expect to see that
voltage when you take a measurement. TAKE the reading, don't assume the
voltage will be as specified. Trust your meter. Most of these suggested
MAX wattages have been arrived at through my
own experience.
6V6
12 watts MAX
6L6GC (and variants, like the 7581A)
22 watts
MAX
5881
(American)
18 watts MAX
5881
(Russian)
22 watts MAX
EL34
24 watts MAX
6550
27 watts MAX
KT66
24 watts MAX
KT88, KT90, KT100 can be treated as 6550s, although all three of these
tubes are supposed to be able to take more current. The ultimate test
is to view the tubes' plates IN THE DARK, after they have been powered
up for 15-20 minutes. If you see any red spots, back the current off a
bit. One exception to this is the
NOS 6V6; some of these will show a slight red "stripe" down the center
of the plates even when they're set fairly cold. I've seen them run for
years in this condition. *Large* red blotches, or even the entire
plates turning red, is what you want to watch out for.