This page explains cathode bias and fixed bias for all output tubes
and application of Dynamic Bias Stabilization for PP amps with cathode
bias to give much better class AB operation.
Biasing of tube amps is a critical design issue and is not much understood
by anyone without technical training.
Before I explain Dynamic Bias Stabilization, DBS, and its role in my 300Watt
monobloc amps, readers should be aware of terminology used in relation to
the use of KT88 and 6550 or similar output tubes.
The mention of a whole lot of differently named Vdc voltages such as Ea, Eg2, B+
Eg1 are confusing to many but I must write with the many abbreviated terms
to be concise for those prepared to learn the language of tube electronics so
they may more easily build an amp and understand what they are doing.
Practice makes perfect, after I began 1993 I found the book with consistent
terminology was the Radiotron Designer's Handbook, 4th Ed, 1955, so I have
tried to keep terms used in 1955 here as this website.
"Bias" is a term used to describe the relation between the DC voltage and
DC current operation of an output tube in relation to the applied Eg1.
Bias is usually stated for the idle condition with no signal present.
The amp operation is optimal when the bias condition at idle is maintained
throughout the range of signal operation from idle to beyond clipping level.
Unless stated otherwise, signal voltages are 400Hz to 1kHz sine waves at
constant amplitude and THD < 1%.
Briefly, "the amp bias" means the relationship between Eg1 and Iadc. But bias can
also be "screen bias" which nominates the Eg2 which is usually between slightly less
than Ea or much less than Ea, and which ideally should be well regulated.
In Triode of Ultralinear amps the screen bias is very close to Ea because the screen
is connected to anode for triode, or to a tap along OPT primary winding which has
very low winding resistance. But for pure beam tetrode or cathode-feedback (CFB)
operation the Eg2 is a fixed Vdc below Ea.
Setting the bias, or adjusting bias or "auto biasing" or "cathode biasing" describes
the method used to stabilize DC working conditions of a tube at the idle condition
without any signal present. "Biasing" may not remain constant during signal
operations especially at elevated levels and in class AB, and usually much
unwanted signal distortion occurs when biasing changes during such operation.
There are two common methods for biasing,
1. cathode bias, aka auto bias.
2. fixed grid bias.
Fig 2, Bias methods....
Fig 2 shows Cathode Bias aka Auto Bias on left, and Fixed Bias on right.
The KT88 are in triode in both schematics and have identical idle conditions of
Ea = +379V, Eg2 = 379V, Ia+Ig2 = 50mAdc, Eg1 = -41Vdc.
The biasing level is chosen so an output tube produces a safe and constant
level of heat from the anode and screen which allows optimal class A or AB
operation with minimal crossover distortion. With screen connected to anode
we may include screen Idc as part of Iadc.
Anode heating is called Pda, power dissipated at anode, in Watts.
For simplicity, Pda here means anode and screen heating.
So, Pda = Ea x (Ia + Ig2) = Ea x Ikdc, where Ikdc is the cathode dc current.
For Fig 2, KT88 Pda at idle = 390V x 0.05A = 19.5Watts. This is a comfortable
level of heat well below the maximum Pda rating = 42Watts.
For class AB, Idle Pda should be < 50% max Pda and for pure class A can be
up to 67% of max Pda. No octal tubes should ever be idled at the maximum Pda.
In the Ea and Ia idle condition for Fig2, KT88 triode has Ra = 1k6, µ = 8,
gm = 5mA/V. This is very different from KT88 data figures measured with much
higher Ia and gm with lower Ea but if you carefully examine the KT88 triode
Ra curves you will see what I mean.
Cathode bias - Advantages:-
Cathode bias aka auto-bias offers excellent and simple idle current and grid bias
voltage regulation with use of Rk & Ck parallel network between each cathode
and 0V. There is no need for an owner to make any periodic adjustments.
Owners do not need to remember to do bias checks and there is no need to to
use a screw driver and voltmeter after removing amp covers to make voltage
measurements while exposed to dangerous voltages.
Cathode bias is most ideal for all pure class A amplifiers such as all SE amps.
If there are multiple output tubes there must be separate Rk & Ck for each tube
and not one Rk & Ck network for commoned cathodes. The separate Rk & Ck
ensure all tubes have equal Ia and Ek. With separate Rk & Ck a single tube of
say 4 output tubes can be replaced without needing to worry about it being closely
matched to others, ie, with exactly the same electronic parameters.
So if the replacement is a different brand and year of production, it will work fine.
If output tube parameters of gm, µ, Ra change over years, cathode bias tends to
keep Ia and Ek the same and maintain the same tube operation and extend their life.
Pda of output tubes is kept fairly constant despite large mains voltage changes
between say 220Vac and 250Vac, because at DC the effective Ra of tubes is
much increased with presence of Rk.
Cathode bias - Disadvantages:-
All tube amplifiers whether SE or PP will be used with speakers which have
higher or lower impedance than recommended by the amp manufacturer.
Owners routinely ignore user manuals. The SE class A amps with cathode
bias usually or fixed bias can tolerate a range of RL between half and double
the ideal load for maximum Po and which should be labelled on output.
The SE amp may have only one labelled outlet as 6r0 for 25Watts and if 3r0
or 12r0 are used the Po may be 12.5Watts in each case. The SE amp cannot
produce more Po if RL is lowered, and the practical load limit is 12.5Watts
for all loads between 3r0 and 12r0, but it will be all class A and usually sound
well if that is enough power without any clipping. SE amps usually have output
tubes idling at 67% of max Pda and neither fixed or cathode bias can improve
bias stability when RL is low, nor allow higher Po when load is low.
In PP amps the idle Pda is often less than 50% of Pda maximum and the maximum
class A Po might be 12.5Watts with 12r0. When 6ro is used, Po is 25Watts class AB
and with 3r0 is is perhaps 40Watts class AB. With cathode bias, the Ek remains stable
for all loads above 12r0, but for any load below 12r0 the Ek will to more positive and
cathode biasing is the least suitable, with high 40Watts into 3r0 "not worth having"
so in fact the PP amp with cathode bias offers little more than the SE.
Many old PP amps from 1950s did use cathode biasing such as Quad-II and Leak.
The makers realized that owners seldom used more than 1/2 a Watt average
to each of two speakers which had sensitivity of 93dB/W/M, so class AB was fine
because even with 4r0 speakers connected to amps labelled for 16r0, the outcome
was listenable. But many ppl now want louder levels of music with high bass content
and with speakers rated for 87db/W/M, so 4 times the Po is needed, so instead of
2 x EL84 or 6V6 per channel, you now need 2 x KT88 or 6550.
Instead of having a pair of KT66 or 6L6 in triode for 12Watts pure class A,
people now will prefer a 30Watt capability for all loads between 3r0 and 9r0
from 2 x KT88 or 6550.
Fixed bias works better for where low RL loads are inevitable.
Fixed bias - Advantages:-
In general fixed bias benefits operation of PP amps which are to be used with a
wide range of load values usually all below the RLa-a which gives pure class A Po.
In Fig 2, the fixed bias example requires 17r0 at "8r0" outlet of OPT to get the
same 12.5Watts of pure class A Po as for cathode bias. When the RL is reduced
4r0 about 20Watts is available with fixed bias and there is no bias drift of Ek.
Nearly all guitar amps are PP with fixed bias but have only one adjustable -Eg1
which is applied to say 2 or 4 x 6L6GC in the amp output stage for 50Watts or
100Watts class A Po. The reason for fixed bias is always the better high Po
performance and crossover distortion caused by cathode bias drift when
all output tubes are driven way beyond class AB clipping and into class C operation.
Fixed bias - Disadvantages:-
Owners need to check the bias regularly and keep re-adjusting as years go by,
which they forget to do, or they adjust everything wrongly because it involves
technical skill, knowledge of exactly what they are doing, use of Vdc voltmeter,
and a screwdriver.
Once you have 4 or more output tubes per channel the number of adjust
pots and time needed to get bias correctly adjusted becomes a challenge
for non-technical owners. Bias adjustments are interactive, and when you plug
in 4 new tubes and turn on for first time the bias currents of each may vary hugely.
So you must first turn all bias pots low, and the pot position for low bias current
may not be known. So an owner needs to measure current quickly and turn bias
down quickly before any tube overheats during the bias setting procedure.
With bias on each tube turned low, each is then raised slowly.
One first to be raised to 50mA will then go lower as other tubes are raised
because the anode B+ is being pulled low under increasing total Ia.
So one may have to repeat 8 adjustments several times before all bias
is set at the maker's recommendations and all have equal Ia.
Tubes will change with age, and after say 5,000 hours of use they may
develop faults with air molecules leaking past the glass to pin junctions.
Often as a tube ages, the silver gettering turns brownish-grey, and grid draws
small grid current at idle, so hence the need for grid bias resistors for
fixed bias amps to be to 100k or less. Expensive NOS tubes which have sat
on a shelf unused since 1960 will have a short life because even without use
some air leaks into the tube, or soon will when tube has had a month in an
amp after doing nothing for so long. I have known owners to regret not
buying an extra couple of spare tubes to cover early losses. In all aging tubes
gettering becomes unable to absorb all the gas. The tube seldom slowly
reduces electron emission from cathode, but tends to emit more and increase
Ia for a given EG1 bias voltage. With fixed bias, there is no cathode Rk
to regulate tube current and tubes can overheat and enter thermal run-away
operation with spectacular and expensively smoky results.
My previous attitudes and solutions for biasing:-
SE amps. In all the SE amps I have built I have used cathode biasing with separate
Rk & Ck networks for each output tube. Sometimes I have used
a combination of cathode bias plus a fixed Eg1 because the Rk value could be
lower value than if the full Eg1 needed was all across Rk to get Ek = Eg1.
PP amps. In many PP amps I have used fixed bias, and in 100W ULAB amp
with 6 x EL34 I have used 6 adjust pots. In later versions I have used 3 pots
for fixed bias balancing with LED indication which tells an owner if any one
output tube is not working.
All amps have active protection against bias failure.
In 2003, I invented the idea of Dynamic Bias Stabilization for biasing all PP
amps so that:-
1. Cathode bias with separate Rk & Ck could be used to eliminate all adjustments,
2. Operation with very low RL and at high Po up to and beyond clipping could
occur without any significant positive Ek drift.
Fig 3 schematic was drawn in 2014 and is virtually the same as I originally
posted up in 2003, but it shows ALL the working voltages correctly and
for use with Sovtek KT88 which are very similar to Sovtek or EH 6550.
Use of 6CA7 ( EL34 ) were also tried in the same circuit and Ek at idle = +35Vdc,
so Ikdc = 40mAdc approximately, and R14 and R15 could be increased to 8r2.
10r will a little too high.
The data for some common NPN
TO220 package power bjts is :-
The cathode Iac and Idc currents must flow through R14&R15.
At idle with Ikdc = 50mAdc, the Vdc +0.33Vdc and in class A operation the Iac
flow in R14&R15 is +/- 50mA approximately, so the V change is from +0.33V
to +0.66Vpk to 0.0V approximately. In class AB the peak positive Iac at cathode
is up to +350mA and Vac at R14&R15 could be +2.4Vpk. But well before this
Vpk is reached Q1 & Q2 are turned on via R18&R19, and Q1 & Q2 conduct
peak current through R20&R21 33r so the peak cathode current is bypassed through
Q1&Q2 and C5&C6 do not charge up very much. C5&C6 do act to prevent high
distortion voltages which would appear between cathode and 0V.
The actual Vac at cathodes is the same as appears at R14&R15, and is a square
wave of +/- 0.7pk with top having a slightly rounded top. This cathode waveform
has extremely low contribution to THD, and the total operation at clipping
with very low RL is equal to fixed bias operation.
The 33r value is not critical and can be between 27r and 47r so that maximum peak
current when shunting 1,000uF with Q1&Q2 is limited to less than the bjt maximum
Ic rating, often about 5Amps. The 33r and bjts will become warm during continuous
sine wave clipping. I found they did not need heat sinking even with a 2r0 output load.
If Ik peak = 400mA, average cathode Ik = 126mA.
I have included a basic protection board ( bottom left ) which turns off the large mains
PT if excessive Idc should ever flow at idle in V1 or V2. The same board
could be used with many output tubes.
But in September 2014, I found this schematic better than the 2003 schematic....
Fig 4 schematic shows R14 and R15 without any Idc flow which allows the R value
to be chosen for better Ek regulation during class AB operation without any
worry about the effects of idle Ikdc.
I found R14&R15 needed to be higher ohms than for Fig 2, because the Idc current
flow does not flow in R14&R15, and at idle the Vdc across each is 0.0Vdc.
For Q1&Q2 to turn on the base voltage must be above +0.55Vpk.
The minimum Vac pk at R14&R15 for beginning of Q1&Q2 conduction must be
as a result of peak Ik = 2 x Ikdc, 100mA pk in this case. I found 15r quite suitable
and Ek rise was less than +10% with very low RLa-a at clipping. The operation
becomes equal to fixed bias operation.
With normal low average Po being all in pure class A, the Q1 and Q2 never do
anything and there is no effect on signal path.
But where a sudden music transient occurs, all high peak currents are instantly
bypassed by Q1&Q2 and Ek remains constant. During high Po in class AB,
it is difficult to measure any additional THD above what is measured with fixed bias.
The DBS action is reduced at HF by the effect of C7&C8 20nF which create pole
so that DBS is reduced at F above 10kHz where little musical energy exists.
Some of you may try to add DBS to an amp where idle Ia is higher or lower than
what I have here. The values of R14&R15 and R18&R19 can be reduced and in my
300W amps equivalents to R14&R15 are 2.35r, and I have R18&R19 at 200r, with
diodes limiting applied Vpk to bases of gutsy 2SD424 bjts.
With just 2 output tubes you could use 22r for R14&R15, and then have an additional
R from bjt bases to 0V to determine beginning of bjt base current turn on.
It is possible that if bjts turn on with Ik peaks less than 2 x Idle Idc, then Ek will
decrease after a few first Watts, then slowly increase with class AB, or stay lower
which means that tubes are pushed into heavier class A operation and this extends
the amount of class A Po available before class AB threshold is reached, but I favor
keeping Ek constant and allowing a maximum of +10% at sine wave clipping.
Some may like this DBS idea, and they are not entirely baffled. The bjts act as
silent slaves to allow the tubes to work better.
Without DBS as I have it, operation with a sine wave is at clipping with low RL
is entirely depressing to behold!
The protection circuit.
Fig 4 offers best protection against any one of more output tubes conducting
too much Idc at idle. The best way to prevent overheated tubes from causing
damage to themselves of other amplifier parts is to turn off the amplifier.
In Fig 4 the idle Pda = Ea x Iadc = 382V x 0.05A = 19.1Watts. Ek is +41Vdc.
Suppose idle Idc increased from 50mA to 70mA, then Ek = +57Vdc.
Ea will then be 423V - 57V = 366V, so Pda = 366V x 0.07A = 25.6Watts.
If Ek at one KT88 were to rise to +57Vdc at idle it indicates something is very
wrong with the KT88, because the increase is more than normal Ek +25%.
If there is a difference of say 20mA of Ikdc between both KT88, it means there
is a net 20mAdc flow across the whole of the OPT primary which would cause
the ungapped OPT core to become saturated by Idc, therefore causing serious
THD & IMD and high peak signal currents.
The amp should be shut down before anything gets too hot, and KT88 replaced.
If a known good KT88 is fitted, and problem continues, the grid coupling cap
may be faulty.
The protection circuit turns off the amp if Ek1 or Ek2 or both reach +57Vdc
The other reason Ek must not be allowed to rise too high is that the V rating
for Ck 1,000uF caps is usually 63Vdc, and if Ek stays above +63Vdc for
minutes, the cap may leak current, short circuit, or overheat and explode.
Ek1 and Ek2 are applied to two R&C filter networks R3&R4&C1, and
R6&R7&C2. The Vdc at tops of C1 and C2 have low Vac content and slow
down rise signal from Ek. Both Vdc at C1&C2 are applied to diodes to R5
and base of Qx which is a cathode follower which is high base Z in, and low
Z emitter output which feeds current through 5.6V zener diode to R2 and R1 and
scr1 gate. The Vdc at Qx base is determined by whichever Ek is highest.
One could have cathodes and 100 diodes, and the Qx base is still only driven
by the highest Vdc before the diodes.
Scr1 is sensitive gate C106D scr needing +0.67Vdc at 0.03mAdc to latch on.
For this to happen, Ek must be +57Vdc. During normal operation, Qx has no
collector to emitter current because no current flows in 5.6V zener diode.
When scr1 latches on, it turns on the coil of Relay 1 in PSU. The relay opens
the Neutral line to mains input to large PT1 and the whole amp is turned off.
The green "on" LED turns off, red "fault" LED turns on. The +12Vdc rail
for relay 1 is supplied via a 7VA auxiliary mains transformer with 12Vac
secondary. It is turned on with PT1 but its Neutral line is before Relay 1.
Scr1 stops conducting only when the amp is turned off at mains switch
and reset by turning back on after 2 seconds.
In my 300Watt monobloc amps I have applied the principles of DBS.
The amps have CFB windings which make DBS application slightly more
complicated. I found I only needed 2 x 2SD424 to look after cathode
current management of 12 x 6550.
In my 300W amps I also have a regulated Eg2 at a lower Vdc than Ea.
If Eg2 were ever to rise to same value of Ea, the protection circuit would turn
off the amp. However this is very unlikely because the Eg2 SS regulator has a
current limiting collector R and zener diodes tend to fail by reduction of their
Now let us consider a 300Watt+ monobloc amp.
With 12 x KT88/6550 output tubes, fixed bias would mean having 12 adjust pots
and with 2 channels there would be 24 pots, and all are a little interactive, and
bias setting would be an onerous procedure.
The use of cathode bias plus fixed bias worked just fine if I used :-
1. Anode B+ = +512Vdc. Anode idle current = 40mAdc,
2. Screen B+ = Regulated +375Vdc to +387Vdc, depending on real zener Vdc.
I found "75V" zener diodes idled at 78Vdc with only 4mAdc. Ig2 at idle for
12 x 6550 = 48mAdc and maximum at clipping with low RL = 150mA approx.
3. Cathode resistors = 500r, using 3 x 1k5 x 5W in parallel per output tube.
4. Cathode bypass caps = 1,000uF x 63V per output tube.
5. Fixed grid bias = -17.6Vdc applied to all 6550 grids, and derived from rectified
12.6Vac heater windings.
If I used my method of Dynamic Bias Stabilization in conjunction with the 12 x R&C
cathode bias networks, the rise of Ek at each of 12 cathodes was from +23.7Vdc
to +27Vdc at clipping when using 1/2 the recommended RL at Po = 390Watts.
As far as I know, my idea of dynamic bias stabilization or "clamping" of Ek
in class AB amplifiers using cathode biasing has never been officially
invented before 2004 when I tried to describe a basic version of it to a
news group, rec.audio.tubes. I don't think
saw my invention
could understand how it worked, even after posting
the basic schematic
to the news group
One lad tried to tell me my idea could not work at all, so I
test it, and of course, he would do no such thing, so typical of men on
the Internet. You are at....
300W amp dynamic bias
input/driver and output stages.
300W amp power supply.
300W amp active protection.
300W amp power vs load
300W amp images, tubes with blue
glow, and more views of amps.
Back to Power amplifiers.
Back to Index Page.