SEUL 22 watt mono bloc with 13E1.

Edited in June 2012.......

The two mono blocs shown were built in 1997 to prove nobody needed to spend an
absurd amount of money to achieve the best sound from a Single Ended design.
Some audiophiles said these amps have a midrange you would die for, but also
said the bass and treble was remarkable, and better than many might expect
from an SE amp. The 20 Watt full power response extends from 20 Hz to 65 kHz.

They produce 22 watts into 8 ohms, and 27 watts into 4 ohms.
There is enough power for most listeners with normal modern speakers of typical
sensitivity of 90dB/Watt at one meter.

The amps used the rugged 13E1 beam power tetrode which probably stopped being
made in the late 1960s. This tube never been fashionable among the hi-fi cognoscenti
and magazine paparazzi of tube-land mainly because it is a beam tetrode, and not a
triode. But the hi-fi press largely consists of people who have almost zero technical skills,
understanding, or experience, and who have irrational prejudices or who regularly give
good reviews of some ridiculously expensive junk submitted to them for appraisal.
This situation is good, because the stocks of the 13E1 tubes will last longer and prices
will remain lower if people are snobbish enough to ignore what is a very nice tube.
I would not want to see hoarders and sharks buy up stocks of such tubes because they
become both popular and rare, and end up under someone's bed gathering dust for 40 years!

I first bought a pair of these tubes in 1996 without knowing what I would do with them. I explored
their capabilities for SE triode and Ultralinear operation by connecting the screen to various
taps on an SE OPT.
In pure tetrode, regardless of Eg2, value the tube is not very linear and generates a large array
of even and odd harmonics even at a few Watts.  But this is to be expected with many beam
tetrodes and pentodes which may make 14% THD just before clipping. So I did not find it very
suitable for pure SE tetrode with a fixed screen voltage. But when the screen is connected to
the anode to make the tube work like a triode the linearity is no worse than many other large
genuine triodes.
It has a very low triode Ra of only 300ohms, and it will generate a very sounding nice 16 watts
into 1.5kohms. I found that the screen could be connected to a tap on the OPT primary at 66%
of the primary turns and there was negligible increase in the complexity of THD spectra compared
to triode, where the THD consisted almost entirely of 2H. 33% UL taps gave spectra too similar to
So I settled on the 66% UL tap for the 13E1, and got up to about 27 Watts instead of only 16 Watts
with triode. But because Ra with 66% UL is 600 ohms, some global NFB was essential to keep
the output resistance of the amp low for a good damping factor.

Unlike most transmitting tubes such as the 845 and 211, the 13E1 does not require a +1,000Vdc
supply. 13E1 were designed to work with anode B+ between +200V and +800V with a screen
supply low as +150V. These low voltages allow the use of an indirectly heated cathode as in
6550 or KT88 etc. In fact 13E1 has two cathodes, not just one, to allow a massive peak current
ability of about 800 mA. Some people have told me they have used the tube for an OTL amp!
Although a pair can make 200 Watts in class AB tetrode, I wanted to make an SE amp so
efficiency limits the pure class A output to about a maximum of 40% of the safe working maximum
of plate input power of 72 Watts. The tube data suggests plate power max, or max Pda is 90 Watts,
but this is a design figure for combined dc and ac Pda in class AB push-pull amps where the ac
duty cycle is never continuous as it is with pure class A. Hence I got a maximum power output
of around 28 watts for SEUL operation. If 13E1 is set up with Pda = 90 Watts at idle the anode
will glow dull red and the sound will be quite dreadful and the tubes will not last long.

Its now 2011, and the 13E1 I bought in 1996 lasted well until 2006 with thousands of hours.
Although there is some slight reverse grid current they are serviceable and produced the
same power and distortion before the owner replaced them as a precaution.
The new pair of 13E1 look like working for many years. The 13E1 has a similar power
output to a 211 or 845 transmitting tube, and the same excellent sonic characteristics,
IMHO, if set up just right.

The selected drive tube in 1997 was ECC32 and input was 12SL7, both used with their two
internal triodes wired in parallel. In 2006, the driver tube is a 6V6 in triode and 6SL7 at input.
The 6V6 driver tube gives slightly less thd, but allows the use of much lower bias resistors for
the output tubes to prevent slight DC bias drift after 8 years of constant use.

The amp was constructed so that if no 13E1 are available in future. 4 x EL34 or 3 x 6550
or KT88 or KT90 or KT120 could be used instead in SEUL or triode mode. The tubes are
mounted on a sub-chassis which could be unscrewed and replaced by a new chassis for
different tubes.
The power supply has several taps for different HT. However, even in 2012 stocks of NOS
13E1 are not too hard to source, and the owner of these amps had a spare pair for the

Chassis is brass plus aluminum and the enclosure right around the power supply is painted
mild steel. Size is 470 long x 240 wide x 220 high and weight is about 25 Kgs. A perforated
steel cover was supplied, and which screws down over the tubes, but is not shown.

These amps were used with pairs of my speakers at a March 2001 meeting of the
Audiophile Society of N.S.W., ( ASON ),  giving excellent sound for a large room with 30

Schematic of mono bloc power amp with the power supply included :-
schematic of
            13E1 SEUl amp, 2005

The schematic is fairly basic but a few items need careful attention should anyone wish
to build a sample of this amplifier. Where possible star earthing and good wiring practice
should be used with point to point on tag strips. The stability of any tube amplifier
including this one will depend on the quality of the output transformer. I usually wind all
my own OPTs because custom winders cannot be relied upon to wind the OPT how I
like them to be wound, if ever anyone to can be found to wind OPT. But the OPT
should have
primary inductance more than 15H, less than 5mH of leakage inductance,
less than 7% winding losses, and less than 1,000 pF shunt capacitance at the anode
connection. Primary load is 1.7k centre value.
This basic specification is not so easy to find off a shelf anywhere. Even with this spec,
the circuit needs to have some tweaking of the open loop gain and phase shift character
so that when global NFB is applied it is impossible for the amp to ever oscillate regardless
of whether there is a load connected or not, or whether the load is any value if L or C.
R & C values of the following components must be selected and checked for correct
maximum stability margins :-
R6&C5, low F phase shift reduction; values shown should suit most OPTs.
R33&C27, high F phase shift and gain reduction; they need to be carefully trimmed.
R20&C14, high F phase advance network for voltage NFB; must be trimmed.
R19&C12, high F zobel damping network to stop parasitic oscillations at RF.
R21&C15 high F zobel damping network to provide a load at above 100kHz.

All the values mentioned are fairly critical and cannot be assumed to the same as my
schematic because your OPT will have different primary L, leakage L, and shunt C
compared to what I wound myself.

The schematic shows two global NFB loops, one is negative voltage FB, the other
is negative current FB. There are 4 terminals on the rear of each mono bloc amplifier.
Two are connected to the "COM", or common active output terminal.

Each of the other two are connected to either the VFB or CFB terminals shown.

With CFB, the Rout of the amp is about 1ohm, and current FB helps stabilize the
amp at HF, and may give better sound into awkward ESL loads, or be better to drive
fairly constant impedance midrange speakers in a bi-wired situation.
The voltage NFB is still operative.

When COM to VFB is used for the speakers there is only voltage FB and since virtually
no speaker current flows in R22, 0.1 ohms, no current FB is sent back to the input at V1
cathode. But at above audio F there IS still some current FB since some HF currents will
flow in R21&C15 and these tend to make square waves into capacitor loads have less

Feedback application can be a complex issue; I make no apologies for confusing ppl
if I appear to over simplify what is involved. People are welcome to do their own analysis.
Most people select the COM to VFB speaker connection as being the one that sounds best.

NFB isn't evil unless it is abused by ignorant designers or wannabe DIY amp experts.

In my later versions of SE amps with 13E1, I gave up the idea of variable FB options and
settled for simple voltage FB only.

Here is another image of one amp with the psu cover removed, showing C-core output
transformer, large choke, caps, and large power supply transformer at the top right rear.


Distortion Graphs.....

The above graphs look quite strange compared to typical curves published in 1960.
The 1960 curves usually had a linear scale for both THD and
output voltage which
tended make the level of THD look lower than
it really was especially at low power
levels of a few Vrms output.
I have used a LOGARITHMIC scale for the THD quantities
so that the
amount of THD at low levels is VERY easily able to be read off.

Linear graphs reduced the reluctance of buyers to buy an amplifier which had high THD.
Most buyers thought that all distortion is bad, and it is, but depending on harmonic spectral
content, the perceived "badness" varies.
Common average listening power levels might be 1 Watt, so from the graph we see SEUL
with NFB gives THD = 0.12%, which is mostly 2H, indicated on Curve B.

The Curve C shows a PP triode amp using say 2 x 6550 set up for similar power and NFB.
At the same 1 Watt, THD = 0.02%, but it is mostly 3H. The measured distortion should be
"weighted" as H number rises because the 0.12% of 3H sounds worse than 0.12% of 2H,
and 0.12% of 5H is worse than 0.12% of 4H, and so on. To weight THD, apply the formula,
Weighting Factor = H number squared / 4, so then for increasing H we get :-
2H, WF = x1.0
3H, x 2.25
4H, x 4.0
5H, x 6.25
6H, x 9.0
7H, x 12.3
8H, x 16.0
9H, x 20.3

So the SEUL amp which has 0.12% of 2H needs no WF applied.
The PP amp with 0.02% 3H is equivalent to 2H = 0.02% x 3 x 3 / 4 = 0.045%

Clearly the PP amp measures better, but the THD is NOT the actual problem, and is merely
a way of indicating a more difficult to measure intermodulation distortion, IMD. It so happens
that listeners with above average hearing can detect distortion in a sound system if THD
is 2H and rises above 0.5% if the system has full bandwidth. That was the view mentioned
in text books of 1955. What told listeners that distortion was present may have been the IMD.
To many people, IMD occurring in an amp which produces THD of mainly 2H is much less bad
sounding than that caused by THD of mainly 3H.

In the case of the SEUL amp, if music is played with or without NFB applied, and AVERAGE
levels are kept at say 0.5W for either NFB condition, few people would notice any difference,
and if there was any difference it could be due to the change in damping factor. The use of NFB
reduces amplifier output resistance to less than 1 ohm, thus raising damping factor.
DF = Speaker impedance / Amplifier output resistance.
The NFB usually always makes bass sound tighter and less boomy or wooly, and most
people hear a general improvement especially at high levels. I am considering the case where
the amp has adequate power to give deafening levels of sound before distortion becomes
intolerable. If one were to use a single EL84 with maximum power of 5 Watts, then at 1Watt
the THD will be much higher than that of a 13E1 at one Watt.
With speakers rated for 90dB/W/M, 1 Watt average level for 2 speakers of a stereo system
produces an SPL of about 91dB. 
Most people think an average SPL level of 85dB is quite loud enough,
with peaks rising perhaps to 100dB. To achieve the 85dB average level, power from each amp
is approximately 0.33 watts,
which is 1.62 Vrms into 8 ohms, and where THD < 0.06%.

Such levels of THD and resulting IMD in tube amps are negligible, and cause zero distress with
all listeners I have known.

Music has a myriad of harmonics with relate mathematically and several different frequencies
played together sound pleasing in melody and chords. THD production does not change
the sound by itself very much.
But where IMD is produced, the sound can be changed, and
always made worse. Suppose
there are two signals produced with say 4 Vrms of 80Hz and 1Vrms of
1kHz, and there is harmonic distortion, then the 1kHz level will be amplitude modulated. This is
the equivalent of having both tones present but with additional tones of
920Hz and 1,080Hz.
These added IMD tones, or notes, are not nicely related harmonically to other notes in the music.
Exactly what IMD levels are is able
to be calculated, but there is little space to devote to that now.
But with many musical tones present, there is a huge number of "intermodulation products" and
if they could be played in speakers
without the wanted undistorted signal present, the sound is like a rustling
noise rising and falling in time with the music.
Many people find the general clarity of music using a good SE amp is often better than a PP amp with
less THD, and less IMD, so there can be a contradictory co-relation between measurement and subjective
assessment. But I always try to minimize
THD and thus minimize IMD, but without making the amplifier
too complex or over-engineered.

People who favor SE amps usually say the SE amp will better reproduce the natural recorded midrange
warmth without any loss of detail and
precision in the dynamics. But I have heard too many good sounding
push pull amps to agree which is generally better. I am at a loss to explain all the opinions I hear.


The above graph has a log scales for both THD% and Output Voltage, V).
There are two other scales below the graph for Output Power and dB SPL.
For example, where VO = 2.24Vrms, Power from each amp = 1.0 Watt,
and with 2 speakers each producing 90dB SPL at a Watt at 3 Metres in an
average room, ( more SPL than in an anechoic chamber ) then 2 speakers
should measure 93dB SPL.
The graphs show THD from the SE35 amps with local Cathode Feedback,
CFB, is about 1/5 of that of the SEUL with 13E1.
The SE35 and SEUL have been set up with approximately the same total
amount of NFB.
In years between 2003 and 2007 I had two customers to whom I had sold
samples of both amplifiers and they both listen to each others amps during
friendly house visits and they had a difficult time deciding which was the
better sounding amplifier. The owner of the SE35 sold it to fund yet
another amplifier purchase because he seemed addicted to regular amplifier
One cannot expect anyone to keep going to see the same movie, drink wine
from the same vineyard, or decorate the house with paintings from the same
artist, or remain married to the same wife forever. But I was flattered his
ownership of the SE35 was the longest time he'd ever kept a good piece
of audio gear. The fellow with the SEUL snapped it up when it came up
for for sale. He eventually decided the SE35 was a better amp than his
SEUL25. Some considerable modifications were done to the SEUL to
convert it to the SE32 at monobloc-se32-13ei-cfb.html
The performance of both SE35 and SE32 are now very similar, maybe
because both have very similar overall design with two triodes used for
input and driver tubes and the output tube is a multigrid power tube using
local cathode feedback.

SE35 THD figures are described in much more detail in the page devoted
to the SE35CFB amplifiers.
The THD in the SE35 is least when RL = 5 ohms, with THD being higher
when RL is a value either below or above 5 ohms. THD cancelling
between the driver stage and CFB output stage is most effective at 5 ohms.
The result of the work with SE35 with CFB led me to conclude that
the use of a tertiary OPT winding for local CFB for the 13E1 is used
may well prove to make a better sounding amp and one which would
also measure better than a plain UL connected tube. My customer who
owns both SE35 and SEUL thinks they do sound similarly stunning.
And to those who still doubt NFB does any good in SE amps, let me say
that each and every triode already has a local NFB loop within itself.
It occurs because of the electrostatic effect of the anode voltage upon
the flow of electrons to the anode. The control grid voltage changes the
electrostatic field between itself and the cathode, thus controlling electron
flow. So for a positive going voltage at the grid, there is an increase in Ia.
But as Ia increases, the load voltage across the load between B+ and
anode increases, thus making the anode voltage move negatively.
The tube is an inverting amplifier. Now the anode voltage swing is
usually much larger than the grid voltage swing. This anode voltage
change also affects the electron flow from the cathode. So as the anode
voltage falls, it tends to lessen the flow of electrons attracted to itself.
In other words, the anode voltage change opposes what the grid is trying
to do. The two electrostatic fields sum together to give a resultant field
that works on electrons in the Ia flow. This is a form of local shunt
feedback, equivalent to having a shunt resistance network of two
resistors between anode and the signal input, with the join of the
resistors going to the grid. I use such local resistance FB networks in my

The internal electrostatic FB in the triode is almost entirely prevented
by the addition of the screen grid as used in pentodes and beam tetrodes.
The screen usually has a fixed voltage relative to the cathode to enable
pentodes and tetrodes to have very high useful gain and much more
output power than a triode with a similar Pda rating. The price for this
extra "free" gain and power is much higher Ra giving a very poor
damping factor and a whole lot more harmonics, mainly odd numbered,
and changing in relative mix and phase with load changes.
The screen affects the Ia flow in the same manner as it would if it was
an anode.
So when the screen is connected to the anode, the multigrid tube then
behaves similarly to the genuine 3 electrode triode, because the tubes
internal NFB has been allowed to operate.
If external loop feedback is used for a pure pentode or tetrode tube,
then the complex harmonic spectral content is largely unchanged,
simply reduced in level by the NFB. 20dB of global NFB is needed
for pure pentode amps to get their Rout low enough and to curtail
distortion. IMHO, it is better to try to prevent formation of the odd
numbered H within the OP stage with the UL connection then less
global NFB need be applied.

The pentodes and tetrodes do allow partial application of the anode
signal to the screen, hence there is the Ultralinear connection. In fact,
UL is not "ultra" anything, but does result in the multigrid being able to
be operated in class A1 instead of A2 and to give virtually the same
power as pure pentode or tetrode, but with THD spectra like triode
with very low odd numbered harmonics.
When local CFB is used with a fixed screen voltage, the connection is
called the Acoustical, as it was used in early Quad-II amplifiers. The use
of only 10% of the total anode to cathode signal applied to the cathode
gives 10% series voltage NFB to the control grid to cathode interface.
With screen voltage fixed, there is some FB applied between cathode
and screen, ie, the Acoustical is really like having a 10% UL connection
but with 10% of applied voltage FB to the grid - cathode interface.

The result with KT66 in Quad-II gives the KT66 tetrodes the same
effective Ra as for triode connection, 1k6, much lower than the pure tetrode
Ra = 33k0, and yet power output is double what the A1 triode operation
can be.

Its now 2011 and listening tests have confirmed that the local CFB in the
OP stage worked very well with 13E1.

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