35 WATT MONO BLOC
The SE35 mono blocs were built for a very discerning customer who
high levels of undistorted sound with two large modern floor
I-66, rated for about 90 dB/W/M.
He was amazed at the precision and detail these amps offered.
Unlike most 35 watt SE amps which one very large and expensive
such as an 833, or at least 2 paralleled 845 triodes, these use
EH6CA7 beam tetrodes or EL34 power pentodes.
The chassis is brass with aluminum top plate, the power supplies
within a mild steel box behind the tubes. The top-plate and the
plate is well drilled for ventilation to allow rising cool air
around the output tubes.
No printed circuit boards are used, so there is no obstruction to
air flow so
necessary to keep all tube amps cool.
All the transformers all use GOSS E&I lams, with very
to prevent high temperature rise in the power transformer and to
blameless bass performance
from the output transformer.
Without any NFB, the OPT response at full power to the rated load
to over 70kHz. The response with NFB is shown in the graphs below.
The weight is approximately 25Kg per chassis.
The power supply has all solid state rectifiers for the B+ with a
simple CLC filter
for the B+ anode voltage. There is shunt regulated fixed screen
filtered DC heater supply to
the two input tubes. Individual cathode bias is used
for each output tube so unmatched tubes
may be used.
The driver tubes for the 4 output tubes is an EL84/6BQ5 strapped
input is 12AU7 with both sections paralleled. The right front tube
follower mode to act as a buffer after the input signal to drive a
switchable frequency low pass input input filter to allow variable
frequencies for bi-amping. This buffer stage is not needed where
is not required, but it was found to be sonically transparent.
The owner wanted to try using an 8585 for bass and then have the
and treble powered by the SE 35, and filtering out the bass signal
from the midrange/treble amp effectively
raises the power ceiling of the SE 35.
Because bass frequencies below 250Hz take up most of the voltage
of any amplifier, the filtering out of bass frequencies
intermodulation distortions. The input cathode follower also
a "passive" preamp,
which in this case is a high grade resistance attenuator made
by Vishay, all without losses because input resistance of the
is so high
with very little shunt capacitance.
The schematic shows some unusual techniques to reduce the normally
THD measurements with any SE amps to levels more normally seen
PP amps, especially within the
first 10 Watts.
THD is less than 0.1% at any load between 3 and 12 ohms for up to
Most other SE amps cannot achieve this. I am using low amounts
in the output stage in what is called the "Acoustical connection"
low amount of global negative feedback from the speaker secondary
triode cathode. The total amount of local and global NFB is less
A more detailed description follows with schematics.
Power amp schematic.
Sheet 2, Power supply.
Sheet 3, Protection.
Sheet 4, Harmonic distortion 1.
Sheet 5, Harmonic distortion plus response 2.
Sheet 6, Harmonic distortion comparison to SEUL 13E1.
SHEET 1 - 2004, POWER AMP
This schematic was valid for 2004.
is fed into the high input
resistance of V1 12AX7 cathode
with its output driving a switchable cut off low pass
first order RC
F settings for -3db at 7Hz, 50Hz,
140Hz, 510Hz, and 1020Hz.
R6, R7 form a divider to set the sensitivity of the amp to
match a bass
so that equal voltages are sent to bass and treble
for use with a
The tube line up in the power amp is V2 = 2AU7, V3 = 12BH7.
12BH7 did work well, but 6BQ5/EL84 works/sounds better.......
- 2011, POWER AMP
The power amp schematic was revised this year because
interest by DIYers
around the World. Most did not want to have any input
buffer and low
filter so the tidier schematic here does not include it.
V2 is a paralleled 12AU7 with a transistor MJE350 which
acts as dc
with extremely high ac impedance, ie, a CCS, or constant
This maximizes the linearity available from V2; important
will *add* to
the 2H of the output stage. V3 is EL84 in triode which
slightly more gain, but approximately the same THD, but
The EL84 has about 12mA of idle anode current, ie, 3 times
the Ia of
12BH7, and a choke added and R15 and R16 have been
adjusted to suit.
The EL84 was found to give better sonic definition and
V4 to V7 are 6CA7 but may be EL34 without any changes, or
one could use
3 or 4 x 6550/KT88/KT90/KT120, or four 6L6 or 5881 for
same power outputs, providing the
bias currents are adjusted by changing
the bias RC networks to each cathode circuit of each
output tube so
the total anode and screen power dissipation of the tubes
used is no
than 90 Watts.
The output transformer is a large core of GOSS E&I
lams, tongue =
stack = 62mm. There is a well adjusted air gap. And there
The nominal anode load = 1k2, and nominal secondary is for
5 ohms which
is the load for maximum power. So in effect, each of the 4
output tubes sees
a load = 4k8.
Approximately 12.5% of the primary anode to cathode
voltage is carried
in the cathode winding
and used as local negative feed back to the output tubes.
This reduces OP tube THD from about 8% with no FB to 2% at
The OPT CFB with 5 ohms loading is 8dB of local NFB, and
= 7dB, so total NFB with 5 ohms = 15 dB.
Considerable cancellation of 2H generated by the EL84
driver stage and
output stage occurs because the 2H voltage generated by
each stage has
opposite phase for much of the expected load range.
This is not an easy issue to fully understand.
pentode or beam tetrode tube output stages
with a range of loads below and above the "centre value
RL", ie, in
case, 5 ohms, or RLa = 4k8 per tube where maximum possible
is delivered. For loads below centre value, 2H produced
has the same
phase as a triode. Usually the lowest load usable with an
SE amp is 1/2
the centre load, ie, 2.5 ohms in this case. THD may be 8%
max at 2.5
and as RL rises to 5 ohms, 2H falls to zero, and then
begins to rise as
rises so that it will be perhaps 8% at 15 ohms, or 3 times
But the 2H generated for
values has opposite phase to loads
under the centre
value RL where
2H is zero.
The effect of the local OPT CFB much reduces the 2H
produced on either
side of the centre value RL. In fact, there is more
reduction on high RL
values because OP tube gain is highest and hence more FB
when RL is low, and OP tube gain is low.
While such strange behavior occurs with 2H production
relative to load
the 3H is maximal when RL is low, and declines gradually
between the RL values of 2.5 ohms to 15 ohms. This 3H is
SE pentode and beam tubes, but is not any higher than what
occurs in a
class A1 PP amp. The 3H is also much reduced by the CFB
Now with all SE amps using a triode OP tube and triode
the 2H voltage produced by each stage cancels with all
Let me say a typical OP triode produces 5% 2H at near
The triode driver stage may produce about 2% 2H when the
OP tube clips.
The 2% 2H is amplified by the OP tube to produce an
2H signal so that overall 2H will be be 5% - 2% = 3% 2H.
This is not a
phenomena and occurs in all SET amps. There are some low
harmonics products called "second order products"
produced, but they
remain low enough to be ignored in this discussion.
If CFB is used in a pentode or beam tube OP stage, the
occurs where the OP tube 2H is the same phase relative to
tone, ie, at loads below the centre value. But the CFB
reduces the OP
2H to say 2%, and the driver tube produces 2% 2H, and at
this point the
overall 2H level becomes close to zero. It never ever
slight relative 2H phase differences between driver and OP
But the amount of natural 2H cancellation is highly useful
Where OP tube loads are above the centre value, the 2H in
the OP stage
is not cancelled by the driver stage because the OP 2H has
is opposite to where RL is less than centre value.
But the CFB is more effective to reduce whatever THD
exists at high RL,
and although the 2H of driver and OP tubes adds to
increase overall 2H,
there is no more than in other typical SE amps.
Graph 1. 2H Cancellation.
graphs of RL vs THD are difficult to understand because
bothers to ever draw such graphs.
The thickest lines are a set of 4 curves for CFB operation
0.25 Watts and 16 Watts.
There are thinnest lines of 4 curves for the same OP tubes
used in SEUL,
at power levels between 0.25 Watts and 16 Watts.
At the 0.25Watt level the SEUL amp produces 0.15% THD into
which reduces smoothly to 0.04% at 16 ohms for the same
At the 0.25Watt level the CFB amp produces 0.1% THD into 2
which falls rapidly to 0.014% at 4.5 ohms, and then rises
to equal the
SEUL level at 7.6 ohms, and rises further to 0.09% at 16
Now for both SEUL and CFB amps, the THD for 0.25Watts is
0.15% for any load between 2 and 16 ohms. This is a
any good SE amp. Most of the HD will be 2H in general.
But the CFB amp has the advantage that 2H reduces
between 3 and 7 ohms, and when a nominal 4 ohm speaker is
be an average of about 1/4 of the THD generated by the
The null of THD seen in CFB curves is all due to 2H being
with the remaining HD being mainly 3H. Other odd number H
are very low
the 0.25Watt and 1Watt levels and these do not rise
10Watt levels is reached. The CFB amp gives less THD even
8 ohm speaker.
Most of the power in the audio band is between 100Hz and
often there is low speaker impedance and high current, and
it is in
the CFB amp copes better than the SEUL amp.
The other benefit of the use of CFB is that most of the
in the OP stage, and any signal fed back to an earlier
input stage has
content, so less IMD products are produced by
non-linearities in the
or driver stages.
The curves above were recorded from the 2004 amp version
The amount of 2H in any chosen driver tube such as 12AU7,
or EL84 will vary, and this changes the position of the
nulls on the
As the driver tube becomes more linear, the null position
occurs at a
number of load ohms.
The lower THD and IMD with CFB does not seem to fully
improved fidelity heard from the SE35. The use of the EL84
contributes much to the sonic fidelity because it produces
such a small
amount of THD/IMD compared to other triodes with lower Ia.
found both EL84 and EL34 in triode to be extremely fine
I've found the use of EL84 as PP driver stages in my PP
amps to be
superior to other smaller twin triodes. It was for this
reason that I
from 12BH7 to EL84.
In 2009, I made a headphone
EL84 in triode with OPT
and driven with 6CG7. The amp was integrated, and could be
used to be
a very fine line level preamp.
When one examines the HD spectra in the 4.5 ohm THD, there
is some 2H,
there mostly 3H. There is also 3H, 4H, 5H, etc in
These are mostly hidden from view when viewing the THD on
at low levels used for most listening below 2 Watts. When
SE amps the 2H distortion is usually overwhelmingly
more than 15dB above the levels of any other harmonics, so
is 0.1%, 3H will be 0.02%, with 4H, 5H being buried in the
of the amp.
Distortion cancelling is frowned upon by some because they
of a driver tube are themselves distorted by the output
there is an
complex mixture of THD and IMD harmonics produced compared
to just trying to have a fairly linear
driver and normal output tube with fairly high
My approach was build the driver stage so it is naturally
at least no
less linear than anyone else might achieve with resistance
dc load. One
deliberately set up the driver tube to generate higher 2H
of the OP stage, but this is not the best approach. The 2H
achieve is really a by-product that is favorable, but not
2H distortion voltage cancellation occurs in every SET amp
little is ever said about it, but in SET amps where the
output tube is
or 845 and which have no CFB,
there is a substantial amount of 2H cancellation.
With a 300B there may be 180Vrms at its anode with THD =
6%, and 60Vrms
produced by the driver tube at 3% THD, so you get a
resulting 3% THD
If anyone wanted to use 4 x 6550 or 4 x EL34 all in triode
PO one could get would be 32 Watts for 6550, and 25 watts
for EL34, and
would be similar levels of THD to the SEUL levels shown in
In triode mode, there is no need for the CFB which becomes
with the low triode gain, about equal to 4dB of FB.
The THD graphs were measured in 2004 for various values of
THD is lowest with loads between 4 and 6 ohms, and at 1
watt into 4ohms
5 ohms THD < 0.032%, This is about 20dB lower than in
most other SE
where it may be 0.32%.
THD with 3 ohms is higher, but would be *much* higher than
is shown if
there was no 2H cancellation between the driver and output
The difference of THD between SE35CFB and SEUL 25W 13EI
amps can be
The SEUL amp with one x 13EI tube is a very good sounding
But like all SE amps, there is some THD, and it has a
shown above and with
about 15dB of applied global NFB. The SEUL
graph would be also typical for many pure DH triode SE
those using triode strapped pentodes or beam tetrodes
which might use
say 10dB global NFB.
The SE35CFB also sounds well, perhaps better, but its
5 ohms is so much better.
At low power levels where the amps are used, the SECFB
times less THD, and THD which is about the same as a good
amp with the same amount
of total NFB.
Frequency Response SE35
at full power,
5 ohms, and -6dB, -15dB.
The top graphs show the response of the amp with 5 ohms at
notice that some bandwidth limiting occurs at extreme LF
and HF, but
points are 14Hz and 32kHz. The responses are with full
amounts of NFB.
The top graph has the input filter switch set for zero LF
More bandwidth becomes available away from near clipping
as shown in
-6dB line, or 9 Watts.
The bottom graph at -15dB low levels show the frequency
of purely capacitive loads between 4uF and 0.33uF, along
the right side
the -15dB line, and there is virtually no peaking below
the amp can drive any
ESL load, and that without any resistance, any pure C load
cause HF oscillations. The line without peaking under C
peaks is for a
5 ohms. The peaking with pure C loads is considerably
reduced if there
a parallel resistance with capacitance load.
The response of the low pass filter is also shown for
The above PSU schematic gives close to what is needed for
and 2011 amplifier schematics.
The PSU does not use tube rectifiers because they
do NOT improve the
The CLC B+ filter with C1, L1, C3, C4 will give excellent
at the OPT connection = 3.6mVrms, 100Hz even without
This depends on L1 = 1H minimum and it should have Rw <
so that heat in L is less than 3Watts for a core size of T
= 25mm, H =
The choke may be larger with more L but Rw should be kept
< 40 ohms.
The C2 and R1 combine with L1 to make a 100Hz damped
network which increases 100Hz attenuation about +12dB so
that with only
1H, Vripple < 1.5mVrms.
L1 with C3, C4 have resonance at 5.2Hz, low enough to not
much emphasis of LF mains noise, caused by so many other
switching electrical gear on and off in your street or
Since the original 2004 PSU schematic was drawn, dc has
been applied to
the 12AX7 if used and 12AU7 input tube heaters to ensure a
floor. There is no need to use DC to the driver stage but
to do this if they like to trim values of series resistors
Active protection and
delayed B+ turn on for the SE35.
I repeat text on the schematic :-
DELAY. After turn on there is a slow rise in
voltage at C2 fed by current
through R2 so that after 25 seconds current will
flow through the 8.2V zener
turn the darlington pair of Q1 and Q2
to quickly close a relay in
series with the HT of the power transformer.
PROTECTION. The four cathode dc voltages are reduced by
through 1N4007 to the base of emitter follower Q3.
If one or more of the Ek cathode bias voltages rises due
current, a sample fraction of Ek is applied through diodes
to the base
which is an emitter follower buffer. The emitter voltage
at Q3 will
and if that exceeds the
threshold for forward current flow in 3 series red LED,
d6, d7, d8, then the SCR gate voltage will rise enough to
latch it on.
Once turned on, the SCR stays turned on regardless of the
Ek. And when
on, the SCR anode has low resistance to 0V thus draining
so that Q1 and Q2 are turned off and relay coil is denied
open, thus HT winding is opened and B+ reduces to near 0V.
The d5 red
is turned on to indicate there is no B+ present.
The EK voltage required to trip the SCR is +27Vdc. Vac at
away by networks 47k, 15k and 220uF so that AC signals do
not trip the
If the amp is turned off at the mains switch, the +18Vdc
rail in the
rapidly reduce to 0V because it is loaded with the low
input tubes. So then the SCR quickly becomes "unlatched"
and the amp
may be turned on again to "reset" it. If the SCR is soon
the 25second delay then something is wrong with an output
If any output tube misbehaves, there will not be any smoke
or any expensive repairs to output transformers.
The reason for active protection rather than reliance on
is because if
only one tube were to become saturated with say 300mA of
the mains fuse
will not blow.
So the active protection works well before an errant tube
The circuit will work when the Ik current in a
rises from the normal
62mA to 100mA.
Since 2006, the protection circuit fitted to SE35 has
earned its keep.
cannot resist the urge to buy expensive NOS EL34 which
they think sound
will better. While this may be the case, some NOS tubes
that have spent
perhaps 40 years sitting on a storage shelf may have
defects in the glass and after being used for a week, a
month, or 6
the defect slowly allows air into the tube and the tube
tries to conduct excessive Ia before it finally self
avoid the pyro-
technical display and collateral amp damage, the amp is
turned off well
faults become excessive. Protection also guards against
tube sockets going open, owners turning up volume with a
or coupling cap failure.