SE 35 WATT MONO BLOC





The SE35 mono blocs were built for a very discerning customer who wanted to achieve high levels
of sound with two large modern floor standing speakers, the VAF 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 use a very large and expensive single triode, these use four parallel inexpensive
EH6CA7 beam power tetrodes, whose electronic characteristics are almost identical to EL34
which could also be used.

The chassis are brass with aluminum top plate, the power supplies are within a mild steel box behind the tubes.
the bottom plate is well drilled for ventilation of rising cool air around the output tubes.
The transformers all use GOSS E&I lams, with very generously sized cores to prevent high temperature rise
in the power transformer and to get truly blameless bass performance from the output transformer.
Without any NFB, the OPT response at full power to the rated load is from 20Hz to over 70kHz.
The response with NFB is shown in the graph 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 anode B+,
shunt regulated fixed screen bias, and RC filtered DC heater supply to the two input tubes.
Individual cathode bias is used for each output tube so unmatched tubes can be used.

The driver tubes for the 4 output tubes is an EL84/6BQ5 strapped as a triode, input is 12AU7 with both sections
paralleled, and the remaining tube at the front is a 12AX7 in cathode follower mode to act as a buffer
to drive a switchable low pass input input filter to allow variable cut off frequencies for bi-amping.

The owner wanted to try using an 8585 for bass and then have the midrange and treble powered by
the SE 35, and filtering out the bass signal voltages effectively raises the power ceiling of the 35 to that of a much higher
powered amp, since bass frequencies take up most of the voltage headroom of any amplifier.
The filtering out of bass frequencies also drastically reduces intermodulation distortions.
The input cathode follower also allows the use of a "passive" preamp, which in this case is a high grade resistance
attenuator made by Vishay.

The schematic shows some unusual techniques to reduce the normally very high THD measurements with any SE amps
to levels more normally seen with good PP amps, especially within the first 10 watts
where THD is under 0.1% at any load between 3 and 12 ohms, something almost no other SE amps can achieve,
although I am using only a mild total of  local cathode feedback in the output stage  and some small amount
of global negative feedback from the speaker secondary to the input triode cathode.

A more detailed description follows with schematics.

Sheet 1, 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, POWER AMP SCHEMATIC.
    
 
The power amp works as follows:-
Input is fed into the high input resistance of V1 12AX7 cathode follower with its output driving a switchable cut off
low pass first order RC filter with 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 power amp, so that equal voltages
are sent to bass and treble speakers designed for use with a normal single amplifier.

V2 is a paralleled 12AU7 with a transistor CCS DC supply, Q1, an MJE350.
This maximizes the linearity, important because any 2H will *add* to the 2H of the output stage.
V3 is a paralleled 12BH7, which I first used when the amps were built, but are now changed to a trioded EL84,
which gave slightly more gain, but approximately the same THD.

The EL84 has about 10mA of idle anode current, so R15 and R16 have been adjusted to suit.
The EL84 was found to give better sonic definition and dynamics.

V4 to V7 are 6CA7 but may be EL34 without any changes, or one could use THREE x 6550/KT88/KT90
or four 6L6 or 5881 for about the same power outputs, providing the bias currents are adjusted by changing
the bias RC networks to each cathode circuit of each output tube so that the total anode and screen power dissipation
of the tubes used is no more than 90 watts.

The output transformer is a large core of GOSS with a suitable air gap, lots of interleaving,
and has a load match for 1.2k to 5 ohms, so in effect each of the 4 output tubes sees 4.8k.
Approximately 12.5% of the primary voltage is in the cathode windings and is fed back to the output tubes
to reduce THD from about 8% with no FB to 2% at 35 watts with CFB.
The OPT CFB with 5 ohms loading is 8dB of local NFB, and the global NFB = 7dB,
so total NFB with 5 ohms = 15 dB.
The spectra  of the distortion of the CFB output stage  is  less complex than  pure beam tetrode, but like beam tetrodes,
the 2H is minimal at RL = 7ohms approx, with oppositely phased and rising 2H either side of
where the 2H null occurs at 7 ohms.
Without the CFB in the output stage the THD at full power in beam tetrode mode would be far too high
at about 9% at clipping, and not low enough at low levels, and will have many more odd order
harmonics.
Triode or SEUL have 2H which is always the same phase, and much lower levels of odd order harmonics.
The harmonics of triodes and UL amps are more tolerated by the ear. The local CFB reduces the beam tetrode
THD by about 10dB in two ways, one by way of the FB applied in series with the grid circuit and the other because the
cathode signal moves in relation to the fixed screen voltage so the CFB is also applied to the screen circuit.
CFB use does result in slightly more high order harmonics present, ie, harmonics above 2H, compared
to triode or UL, but they are at a lower level.

The triode driver is set up to have a normally linear as possible operation with a resistance load for DC supply,
and cap coupled biasing resistor load.
The driver triode has its own particular rate of 2H distortion increase which tends to cancel the 2H produced in the output
stage when the RL is below 7ohms. Above 7ohms the driver amp 2H becomes additive to the output stage 2H
but because tube gain in the output stage rises with higher RL, there is effectively more applied CFB so
the total 2H at the output remains low.
One problem with SE amps is their inability to properly cope with load values below the rated load value
so that where an SE amp has been designed for maximum power into 6 ohms, and some part of the speaker Z =3 ohms,
the THD will rapidly rise due to the lower tube gain, less NFB, and higher distortion due to the lower load.
In the case of the SE35 CFB the maximal 2H cancellations of distortion voltages occur at around
4.5ohms, just below the rated load value of 5 ohms where 35 watts is available.
During tests I found that THD was much reduced when 3 ohms was connected.
At this point we need to see the THD measurements taken for one amp....



The THD graphs have been prepared for various values of RL from 3ohms to 16 ohms.
A black dot on each load value indicates the 1 watt level for each load.
THD is least with loads between 4 and 6 ohms, and at 1 watt into 4ohms or 5 ohms THD < 0.032%,
about 20dB lower than in most other SE amps where it may be 0.32%.
THD with 3 ohms is a lot higher, but would be *much* higher than what is shown if there was no 2H cancellations
between the driver and output stages.

We should compare the THD from both SE35CFB and SEUL 13EI amps to see what differences
exist......



The SEUL amp with one x 13EI tube is a very very nice sounding amplifier. But like all SE amps, there is some THD, and it has a typical amount shown above and with about 15dB of applied global NFB.

The SE35CFB also sounds nice, perhaps better, but its performance with 5 ohms is so much better.
At low power levels where the amps are used, the SECFB amps have 6 times less THD,
and THD which is about the same as a good PP amp with the same amount of total NFB.

Now we must look at the response and thd graphs...



The top graphs show the response of the amp with 5 ohms at clipping, and notice that some bandwidth limiting
occurs at extreme LF and HF, but -3dB points are 14Hz and 32kHz.
More bandwidth becomes available away from near clipping as shown in the -6dB line, or 9 watts.
The frequency peaking effects of purely capacitive loads between  0.33uF and 4 uF are shown clearly along
the -6dB line, and there is virtually no peaking below 20kHz, showing that the amp
can drive any ESL load.
The response of the low pass filter is also shown for different switch positions.

The bottom graphs of RL vs THD are harder to understand because nobody else bothers to ever draw such graphs.
But basically, consider the line showing the 1 watt output power level.
This is the second unbroken curve line from the bottom.
One can clearly see that THD is least when RL = 4.3 ohms at a watt of power.
At 16 watts, the top unbroken line, THD is only 0.2% at 16 watts. By comparison, I have drawn in dashed lines
for the SEUL amp and  the only time the UL amp has less THD for a given load and power level
is where RL > 12 ohms approximately.

Clearly the CFB amp has a  lot less THD in the most critical load areas where the most
output power will be delivered, and on this basis SE CFB is very much worth doing.
The principle is that the output CFB from the OPT reduces THD in the output stage so that
its quantity and rate of increase is more nearly equal to the driver amp THD,
and since most of the THD involved is 2H, then good cancelation is possible.
When one examines the spectra in the 4.3 ohm THD, there is some 2H, but there is also 3H, 4H, 5H, etc
in diminishing quantities. This is mostly hidden from view when viewing THD on an oscilliscope while testing SE amps because the 2H distortion is usually overwhelmingly dominant, often being more than 15dB above the levels of any other harmonics.
Distortion cancelling is frowned upon by some because they say the distortions of a driver tube are
themselves distorted by the output stage thus there is an increasingly 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 THD.
My approach was build the driver stage so it *was* fairly linear, no less so than any of the better makers use.
And distortion voltage cancelation occurs in every SET amp made, and nothing much is ever said about it,
but in SET amps where the output tube is a 300B or 845 and have no CFB, there is a substantial
amount of 2H cancelation going on.
I just had to reduce the THD of the output stage with CFB so its dominant 2H could be more easily cancelled.
I believe the outcome of sonic purity and detail following this approach  validates my technique,
and the measurements tend to back this up.


Now let's see the power supply....


The power supply doesn't use tube rectifiers. I don't believe they improve the sound.
But I still have CLC for the B+ filter. And I have a C2 & R1 across the choke  of the CLC
so that the C2 is resonant at 100Hz with the choke. The R1 damps the resonant circuit
and prevents HF noise from being passed from C1 to C3 and C4.
The result is that 100Hz ripple noise is reduced about 12 dB compared to having a choke alone.

Since the schematic was drawn, dc has been applied to the 12AX7 and 12AU7 input tube heaters
to ensure a super low noise floor.

There is an active protection and delayed B+ turn on for the SE35.....


 At 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 diode and turn the darlington pair of Q1 and Q2
to quickly close a relay in series with the HT of the power transformer.

The four cathode dc voltages are reduced by dividers and each fed through 1N4004 to the base of emitter follower Q3.
If one or more of the cathodes has high enough a rise in cathode current the cathode voltage will rise,
and the emitter volotage at Q3 will also rise and if that exceeds the threshold turn characted of
D3, D4 and D5, then the SCR gate voltage will rise and it will latch on and turn on a red led indicating a fault condition.
The scr will also pull the voltage in C2 down towards 0V via R3 and Q1 & Q2 are turned off and the relay
in series with the HT will be opened, and the B+ supply turned off.
If any output tube misbehaves, there will not be any smoke in your lounge room,
or any expensive repairs to output transformers.
Ther reason for active protection rather than reliance on fuses alone is because if one tube were
to become saturated with say 300mA of idle current then the mains fuse will not blow.
So the active protection works well before an errant tube can become saturated.
The  circuit will work when the idle current in a single tube rises from the  normal 62mA to 100mA.

Back to index page