In 1997, I built the 22W SEUL amps using a single 13EI in single
ended
ultralinear mode.
The details are well covered in my web page on the SEUL
22W.
The SEUL amp pleased anyone lucky enough to hear music piped through
it.
Since 1997, I have increased my experience of using local negative
feedback in amplifier output stages.
A customer of mine who'd bought a pair of SEUL amps had borrowed
another
customer's SE35 amps
and he thought the SE35 to be slightly more accurate and detailed.
It isn't uncommon for audiophiles to
lend their amps to others occasionally. I wondered if any better
sonic and technical performance
could be had, and I had suspected it to be possible ever since 1997
but had not explored the practicalities.
After the SEUL with 13Ei, the next SE amp I built was the SE35 using
a quad of parallel 6CA7/EL34 and
fully detailed at my page at SE35W
monoblocs.
My customer with SEUL has always found that other projects I have built
for him resulted in a worthwhile
and pleasing outcome so he went ahead with the change from the
ultralinear
operation with feedback to the
13Ei screen from a tap on the output transformer primary to having
the primary divided into two windings
with 66% of turns for the anode and 33% of turns for the cathode for
applied cathode feedback.
He had also purchased a pair of my Sublime speakers, also described
in my website page on loudspeakers.
The original SEUL amp was in fact capable of about 22W into 8 ohms
and about 25W into 4 ohms.
But with 4 ohms there was more than twice the THD than with 8 ohms
and because the Sublimes had an
impedance of about 5 ohms average, it was thought a change to the
output
transformer ratio would
give a much better match of load to the 13Ei and thus reduce the
distortion
and give a higher maximum
output power of 32 watts because of increased anode efficiency
with a much lower screen dissipation.
The sound of the new amp circuit is very clear and natural, never
clinical
or blandly cool, and conveys the
recorded warmth of a real live performance to give high emotional
engagement
with music that is the
hallmark of a good tubed system. Bass is tight and gives the music
its foundation, treble is sweet,
with midrange that is glorious without being euphonic.
Rather than wade through the changes to the SEUL22W schematic in a
laborious discussion, I will simply
provide the SE32 schematic I used and explain how it works, with some
provisos and notes about limitations etc.
People are then free to compare the SEUL22W to the SE32 schematic,
and are free to adopt the principles
of the operation. Tube amp design is somewhat flexible.
Fig1.
Fig1 shows the audio circuit with input V1 6SL7, driver V2 EL34 in
triode,
and output V3 13ei.
V1 input stage, 6SL7
C1&R1 form a high pass filter with pole at 7.2Hz to keep out dc
or really LF signals.
V1 Input stage signal is applied to the 6SL7 grid. There is a very
mild amount of 9dB of
global negative feedback 9dB applied to the cathode via FB resistance
divider, R5 and R11,R12.
The voltage difference between the grid input signal and cathode
feedback
signal is amplified 47 times
by the 6SL7 and applied to the network beginning with C5. The network
after C5 has a shelved response
at LF and HF to reduce the 6SL7 gain and phase shift at frequencies
where otherwise oscillations might occur
below 10Hz or above 60kHz because of the use of the global NFB.
The 6SL7 is among the world's most linear triodes and happily produces
the 15Vrms required by the driver stage.
V2 Driver stage, EL34.
The EL34 is triode connected and has a gain of about 8.7. I had hoped
to use a choke plus resistance to feed the EL34
with anode dc so that this gave a high impedance dc feed to the tube
but there was no room to put any filter chokes.
So I created a 700V supply for the EL34, and used a simple 25k anode
R13.
The following grid biasing R17 is bootstrapped to the cathode FB
winding
and appears as a load on
the EL34 of 203k, and the total anode load for EL34 becomes 203k in
parallel with 25k to give 18.2k which allows
a maximum anode signal of 177Vrms at about 2.5% of mainly 2H. But only
130Vrms is needed to drive the
13Ei grid to clipping levels and at this level the EL34 produces only
about 1.8% 2H distortion and
could not be made more linear easily. This 2H has a phase relationship
with fundamental frequencies so
that there is substantial cancellation of the 2H produced in the output
stage, and most
most effectively where loads are less than rated nominal, when output
stage distortion becomes highest.
All SE amps where you have a single ended triode driving a single ended
output tube do have some
distortion cancellation naturally occurring between the two stages
but it is not a useful amount because the
output stage produces a much higher amount of distortion than the
driver
stage and at all levels.
In this amp and the SE35, the use of CFB in the output stage reduces
the output stage's distortion
to similar percentages to that of the driver stage and at all levels
so the cancellation is very effective
in reducing distortion but without having to use global NFB which
reduce the distortions.
In the SEUL, global NFB is about 16dB, so all distortions get reduced
by a factor of 1/6.3,
correction but in this CFB amp the GNFB is only 9dB and distortion
reduction factor is 1/2.8, less than half
what must be used with the SEUL. Distortion measured much lower with
CFB, as well as the amp output resistance.
Less GNFB could be used, and the CFB amp would still equal the THD
and Rout performance of the SEUL.
V3 Output stage has the 13ei set up as a beam tetrode with a screen
Eg2 = +175Vdc,
Ea = +475V, and Ia = 155mA, for a Pda = 73.6W. The screen heat
dissipation,
Pdg2, is very low
because with such a low screen voltage with which the 13Ei is designed
to operate the screen current at idle is also low.
There is an OPT cathode winding devoted to giving 33% of the total
Va-k signal as local cathode
voltage feedback in series with the grid input signal. So why was this
33% of primary turns chosen for local CFB
when 12% to 15% would be plenty?
When I wound the OPT for these amps in 1997, I used the following recipe :-
Core = double C-cores with strip width = 55mm, and build up = 36mm,
low grade GOSS
which was all I could obtain in 1997. Max µ = 4,500 without a
gap, but with a gap µe is about 350.
The air gap was set so 200mAdc would magnetize the core to about
0.6Tesla.
The Primary is 1,800 turns in 3 P sections of 600 turns each with
the
center section
subdivided to give two 200 turn windings and two 100 turn windings
to allow a variation
of screen connection points for UL and for future arrangements.
The Secondary has 4 sections interleaved symmetrically between the
3 P sections.
Each S section is a single layer of 57 turns each, with the last on
section divided into 3 sub sections
of 19t each, and the arrangement allows 4 parallel 57t secs, 3 parallel
76t secs, or 2 parallel 114t secs,
so if the anode RL was 2.8k, secondaries give a match to 2.8 ohms,
5.0 ohms, or 11.24 ohms respectively.
The 2.8k to 5 ohm match was selected for the above
schematic,
1,800 P turns to 76 S turns.
It was decided that all of the center P section of 600 turns would
be used for a CFB winding.
This is at near 0V Vdc potential. I could have used 1/2 the center
P section for 16.5% CFB
and this would have resulted in only 50Vrms cathode FB and an easier
drive voltage of about
80Vrms at the grid. But then I would have had a high Vdc potential
between two adjacent P layers of turns
without the same P-S insulation thickness, and to avoid the risk of
dc arcing, I used the whole center section of P turns.
In any case, the amp is used at low levels for hi-fi where average
signals are 1/10 of the peak signals,
and well away from high distortion levels.
The best screen arrangement took a day to work out. At first I just
had the screen going to a fixed
voltage of +150Vdc above the cathode, as the data on this tube says
Eg2 at +150V is OK even
though Ea might be 5 times this voltage. The 13Ei was designed at a
time when designers tried to
produce beam tetrodes which did not need a high screen voltage or
screen
current for mainly economic
and efficiency reasons, but also for better reliability with less
voltage
and current involved. It is mainly luck that the
13Ei works in triode mode or UL mode at all because in these modes
the screen is at the same potential as the anode and
the limits for the Ea are determined by the effect screen voltage has,
so Ea of +375Vdc is the maximum for
the 13E1 in triode or UL. With CFB, you could have Ea much higher,
perhaps +600V which would be useful
in a push pull amps and then a pair could produce an output power in
class AB1 of
over 100 watts with at least 35 watts in class A and as clean as using
a quad of KT88.
The operation of the 13ei was chosen for 2.8k load to give a match
for
maximum clipping power into 5 ohms.
Now for all beam tetrodes and pentodes, the load for maximum power
approximately = 0.9 x Ea/Ia .
Pda at the anode = Ea x Ia, so Ia = Pda / Ea, so RL = 0.9 x Ea
squared / Pda.
In this case the load was selected at 2,800 ohms so 2,800 = 0.9 x
Ea squared / 73.6, so Ea = 478.51Vdc.
With Pda = 73.6 maximum, Ia = Pda / Ea = 73.6 / 478.5 = 153 mA.
In practice, these Ea and Ia calculations proved to be correct.
At first I tried to have the screen supplied with a fixed Vdc
voltage
at 150Vdc above the cathode Vdc.
But I found that with 33% of primary turns at the cathode and
66% at the anode, the cathode voltage would swing upwards and
so close to the fixed screen voltage that the tube would go into cut
off and the distortion became high, and power limited to less
than SEUL. So I then connected the earthy end of the screen supply
to available tapping points on the cathode winding
which was wound with these taps to allow varied UL % taps. The best
outcome was when the screen was bypassed to
the CT of the CFB winding, or at 16.5% of the total primary turns.
This meant the minimum voltage between screen and
cathode was well above the threshold for Ia cut off caused by Eg2
becoming
too low. Then as a double measure I raised
the Eg2 supply slightly to +175Vdc above the cathode and no premature
"cut off distortion" cold occur at any loading.
The final result gives 32 watts and twice the power at clipping that
anyone gets when they try to use this tube in triode
and slightly more than with SEUL and much lower THD and output
resistance
at all levels than for either triode or UL.
So the screen connection method and Eg2 remains high enough at all
times to have its proper influence on the electron stream.
There are TWO local NFB circuits. Any distortion voltage between
anode
and cathode appears at both anode and cathode
but in a ratio of +2 : -1 respectively. So let us call the distortion
voltage appearing at the anode = -2d, then at the cathode it
is +1d because of the relative transformer winding phases, and it is
as if -d appears at g1 and Ek was at 0V.
The open loop gain between grid and anode is about -10, so a +10d
"correction
signal" must be produced at the anode.
But we would measure only -2d, so there must have been -12d produced
without the FB and the +10d correction
signal reduces it to -2D by summing action. But -d also effectively
appears at the screen g2, and the screen has a gain
into the anode load of maybe -3x, so +3d also appears at the anode
to give an additional correction signal of +3d,
and it also subtracts from what must have been the distortion without
any NFB of -15d.
So the distortion reduction with the 2.8k as shown is from -15d to
-2d, or by a factor of 2/15, = -17.5dB.
The amount of distortion reduction is much more than if you only had
a UL screen tapping.
I won't bore everyone silly with all the complex reasoning behind
why
the Ra of the 13Ei is
so much reduced from about 10.6k in pure beam tetrode with no FB
present
and with the same dc operating conditions.
But were the screen bypassed to the cathode instead of a tap along
the cathode winding,
then the tube works in pure beam mode but with only one loop of NFB
around the grid to anode
circuit, and Ra with FB is easily calculated as Ra' = Ra / ( 1 + [
µ x ß ] )
where µ = amplification factor = 220, and ß = faction fed
back = 0.33.
So Ra' would be 10,600 / ( 1 +[ 220 x 0.33] ) = 144 ohms.
But with the screen taken to a tap and fed with signal the way I have
it is as if the tube is
working with a 16.5% ultralinear tapping, and µ becomes a lot
lower for UL,
and the reduction on Ra is much less. But in either case, the 33% of
grid-cathode CFB reduces Ra from 10.6k
to about 400 ohms and near the 300 ohms you get with triode connection
so that with an OPT Z ratio of 2,800 : 5, or 560 : 1,
the anode resistance appears as about 400/560 = 0.71 ohms at the OPT
speaker secondary connection.
Winding resistance of the OPT adds about 0.2 ohms and so final output
resistance totals about 0.9 ohms.
The 9dB of global FB reduces this output resistance to 0.32 ohms giving
a damping factor of 10 even with a 3 ohm load.
The easier and simpler way to set up the 13ei tube is to have a
fixed
Eg2 at +175V above the Ek at +33V
developed with the cathode R&C biasing network, and limit the
applied
CFB to less than 20% of the total anode turns.
people winding suitable OPT can have a total of 20 layers of wire for
the anode and cathode primary windings,
and devote up to 4 layers to the cathode and 16 to the anode, and
arranged
so the cathode winding is split into
2 windings and placed among the other 16 layers for anode windings.
Speaker secondaries should also
be well interleaved with the primary in at least 4 or perhaps 5
sections
of 1 layers each. This will give you
an output stage with far less THD/IMD than any UL or triode stage,
and lower Ra near a triode,
and needing only about 75Vac maximum g1 drive voltage. The single 13Ei
stage will thus perform about
the the same way as four 300B in parallel, giving 32 watts output for
a total of 75W input for anode plus screen pda.
If 300B were used, Pad total might be 120W, and the tube cost would
be a lot higher than a single 13Ei at present 2008 prices.
Insulation between any adjacent anode winding layers at +500V should
be about 0.07mm,
and between earthy cathode layers and layers at +500V = about 0.4mm,
with
all insulation between secs and any other windings = 0.5mm.
I have used only 9dB of global NFB, and Rout at the speaker
terminals
became a low 0.32 ohms for the 5 ohm rated output.
At very loud listening, THD < 0.05% into any load above 2.5 ohms.
Noise is extremely low, even with a non-potted "open frame" OPT and
PT on the same chassis. But at least I have placed the PT
away from the OPT and thus prevented most stray magnetic
coupling.
The local CFB would be reducing whatever
small amount of stray magnetic coupling exists.
The measured THD of the completed SE32 was very much like the
results
I obtained
with the SE35, and well below the SEUL22 levels and was because of
the same reasons I cited for the SE35
regarding natural unforced 2H distortion cancelation between the driver
stage and output stage.
So there is little point to me publishing the THD graphs I obtained
for the SE32.
Distortion is low, and its all anyone really needs to know.
There is a less understood reason why local CFB works and sounds so
well.
And it seems true even though a similar total amount of global and
local applied
around 2 stages of triode gain and a single 13E1 acting in pure beam
tetrode mode
would measure slightly better. If there is local FB in a single gain
block with one tube in class A,
the distortion correction does not have to travel through other stages
on its way to correct
distortion and thus generate other low level IMD products along the
way which also have to be
then corrected. Its better to have the slightly higher measured
distortion
of a series of
stages each with their own loop of NFB if that level is low enough.
There is plenty of electrostatic shunt NFB in the input and driver
triodes of the amp I have described here
and the amp will work well without the global NFB if it is really not
wanted,
especially where the speaker load was perhaps 8 ohms or more when the
damping factor
would be fine without the global NFB.
In my case I am adding only 9dB of global NFB, a tiny amount compared
to the typical 60dB
around a class B solid state amp. The numerical difference is between
3 times to 1,000 times.
The amount of global FB I apply is around a substantially linear
circuit
to begin with,
so few IMD low level products are formed, but the extra global NFB
lowers the Rout
to get a very good damping factor which translates to good speaker
driver control even if the the speaker
Z dips to 2.5 ohms. Any DF increase would be inaudible imho.
If ever anyone were to try to use the 13Ei as a pure beam tetrode
without
any FB but with the
above load and dc operation, they may well be shocked at the
non-linearity,
with 2H, 3H and other
H reaching above 10% at the onset of clipping. Open loop gain would
be maybe 40 though,
so there is lots of gain that can be easily be reduced with external
and linear NFB mechanisms such as provided
by tight magnetic coupling in an OPT. The linear NFB path around the
tetrode here is a more linear
path than exists in all triodes which don't have a screen to interrupt
their NFB action internally.
Triodes are fine, but their NFB delivery path is one obeying a rate
of change equal to a
cube root of a constant squared, and triodes only really become very
linear when
there is minimal Ia change, but in a power tube we want a lot of Ia
change because there is real work to be
done at a speaker. So we can use a beam tube, and apply the local FB,
and drive it with a triode
which has minimal Ia change, and as long as the driver tube doesn't
go anywhere near clipping, the
total outcome will produce low distortion. I'd never use more than
33% CFB if I began from scratch
because at 50%, drive voltage needed leaps to over half the total Va-k
on the output tube, or about 180Vrms,
and then the driver tubes begin making more distortion % than the
output
stage and few benefits are gained.
Those wanting to use all 9 pin tubes instead of octals for the
driver
amp should consider the input and driver amp
I have used in my Deep Space 845 amps detailed elsewhere on this site.
It uses one 6CG7 plus three EL84 to make about the same drive voltage
needed for an 845.
Never be tempted to let 13Ei Pda exceed 73W. The anode will begin to
glow red at 80W,
and the sound becomes mud. The heater generates over 30 watts
of heat,
and radiates this heat at the anode which passes it on through the
glass. The old data says
Pda shouldn't exceed 90W, but that isn't a rating for continuous
operation.
I have the two 13Ei
which I originally fitted to this pair of amps back in 1997 and they
still measure as well for maximum
power as when new, but now do develop some positive idle voltage at
their grids which means
there are a few stray positive gas ions in the tube which cannot be
absorbed by the gettering.
They have given over 7,000 hours at least.
If the drive amp is linear enough, you may omit global NFB entirely,
and you'll
still have a very nice sounding amplifier. Stability must be checked
as always with local or global NFB,
and the R24 + C19 worked fine for my output stage, but maybe different
R&C values
would be required in something made by someone else, and with slightly
different
amounts of leakage inductance and capacitances in the output circuit
with OPT.
Fig2.
In Fig2 above, there is a total of 4,700 uF for the main 500V B+
supply
filtering.
There are no filter chokes, and they are not needed in this case
because
in fact if you increase C enough
then the R values for R&C filtering become low enough not to
dissipate
much heat.
Ripple voltage Vr at top of C9 = 1.6Vrms, 100Hz, and is reduced by
a two stage RC
filter with an attenuation factor of 0.0017, so Vr at the OPT
= 2.8mV, and quite low enough.
R12 and R16 are mounted on a heat sink to keep their temp low as they
dissipate 4.5W each.
They each consist of 5 x 820 x 10W all in parallel.
The +780Vdc at the top of C3 is developed by means of a 1/2 wave
voltage
doubler
working from the +500V doubler making anode supply current. The +780V
is made by
the doubler formed with C11, and two 1N5408, and feeds C3 through
R15, and peak
charge currents are low, and don't affect the switching of the anode
diodes for anode supply.
If anything in the EL34 shorts to 0V, the cheap R will burn open before
the circuit produces smoke from the PT.
A short in the main 515Vdc anode supply will blow the mains fuse.
Active protection circuitry is yet to be fitted and it is due to be
done this week.
it will have a simple RC filter using 4.7k from the cathode to a 470uF
cap to reduce the ac voltage but allow
the Vdc at the cathode to be divided down further by a resistance
network
and applied to a C106D sensitive gate SCR.
If the cathode vdc rises to 50Vdc, Idc in the tube would be 217mA,
and Ea would drop by about 25Vdc, making
Pda = about 93Watts, and the tube would show some red and be over
stressed,
but able to cope for a short time.
At Vdc at cathode = 50V, the SCR is arranged to turn on, causing a
relay to open in the HT winding on the PT
so that the whole anode supply is turned right off, and no damage is
sustained. With such a small Ia change involved
between correct operation and a fault condition, active protection
which has precision is imperative and fuses cannot be
depended upon. Owners are notorious for fitting the wrong value of
fuse after a fuse blows, and therefore causing
much more expensive damage. My protect circuits can be triggered if
there is a shorted speaker load connected,
or if bias failure or tube failure from any reason occur, and the amp
may be re-set by turning off, then back on.
repeating fault conditions mean the amp needs a visit to a capable
technician.
The amps will retain their red "on" LED, but this will be used only
for the fault condition and a blue
LED will be fitted to show the "on" condition, and that everything
is working OK.
The 6SL7 has a dc supply to its heater as shown to minimize its hum.
Those wanting a similar gain and Ra and wonderful sound and less hum
but from
a 9 pin tube should use a 12AY7.
Have I covered everything? Maybe, and if I think of something
additional,
I'll
add more to this page.
If you wanted me to build you a pair of these amps, I have plenty of
13Ei
which are no longer made, but the price would be around aud $8,000,
and at 2008 exchange rates.
There are few SE 30W mono SE class A amps offered on the Net for
less
than what I want, and those that are
offered may not have as much simple sophistication included, or be
genuinely hard wired point to point.
The SE VA350 made by KR Audio is available from the local Duratone
Hi-Fi shop in Canberra. It is an integrated
amp using 2 x KRT100 output triodes similar to 845 for 30W per channel
and with solid state
driver circuits and the price is aud $18,000.
These days I have moved away from using 1.2mm brass for any chassis
material.
I found it better to use all steel at least 1.6mm thick or have a steel
rectangular channel frame with a 2mm
aluminium top plate like I use in my 300W monoblocs.
The SE32 amps come with a sturdy steel grille over the tubes to allow
you to see them at night,
but act to prevent the inevitable "oops" when something falls onto
the amp and breaks a tube
or pushes one sideways in its socket.