SEUL 22 watt mono bloc with 13E1.
The two mono blocs shown were built in 1997 to prove one did not
need
to spend
an absurd amount of money to achieve the best sound from a Single Ended
design.
Some audiophiles have said these amps have a midrange you would die
for.
But they also have an excellent 20 watt full power response from 20 Hz
to 65
kHz.
They produce 22 watts into 8 ohms, and 27 watts into 4 ohms, which
gives them enough power for most listeners with normal modern
speakers of typical sensitivity of 90dB/watt at one meter.
These amps use the rugged 13E1 beam power tetrode. Although it is about
30 years since this tube was made
the stocks are only slowly dwindling because 13E1 has never been a
fashionable tube among the
paparazzi of tube land mainly because it is a beam tetrode, and not a
triode, and the hi-fi press largely
consists of people who have almost zero technical skills,
understanding, or experience.
This is good, because the stocks of the tubes will last longer and
prices will remain lower.
I would not want to see hoarders and sharks buy up stocks of such tubes
because they
become both popular and rare!
I bought a pair of these tubes in 1996 and explored their
possibilities
for SE triode and Ultralinear connection
by taking the screen to various taps on an SE OPT.
In pure tetrode the tube is not very linear and generates a large array
of harmonics even at a few watts.
It is not suited to SE use in pure tetrode, ie, 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,
and it has an 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 turns and there was negligible increase in the complexity
of THD spectra compared to triode, where the THD consisted
almost entirely of 2H. 33% taps were too similar to tetrode.
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
+1,000V supply because they were designed to run on
a supply of only +440V,
which allows the cathode to be indirectly heated; in fact 13E1 has two
cathodes, not just one
to allow a massive peak current ability of about 800 mA. Although a
pair can make 200watts 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 is 90
watts, but this is for class AB push-pull amps where the duty cycle is
never continuous as it is with pure class A.
Hence I got a maximum power output of around 28 watts. If 13E1 is set
up with Pda = 90 watts at idle
then the anodes will glow red hot and the sound will be quite dreadful
and the tubes will not last long.
The output tubes have a similar power output to a 211 or 845
transmitting
tube, and
the same excellent sonic characteristics, IMHO.
The selected drive tube in 1997 was ECC32 and input was 12SL7,
both with each
half wired in parallel.
In 2006, the drive tube is a 6V6 wired in triode and 6SL7 input
triodes.
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
the last 8 years years of constant use.
Last year when i serviced these amps for their happy owner the power
output
was the same as in 1997, so no loss in emission has occurred in the
last
9 years.
The amp has been constructed so that when the 13E1 wears out in
years
to come,
four EL34 tubes could be used instead in single ended UL 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, stocks of NOS 13E1 are not hard to source, and the owner of
these
amps has a spare pair for the next 10 years of use.
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 is normally supplied, and which screws down
over the tubes,
but is not shown.
These amps were used with pairs of Turner Audio speakers at a
March
2001 meeting of
the Audiophile Society of NSW, ( ASON ), and they provided excellent
sound for a large room with 30 listeners.
Schematic of mono bloc power amp with the power supply included :-
The schematic is fairly basic but a few items need careful
attention should anyone wish to build a sample of this amplifier.
Where possible start 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 wind all my own OPTs because custom winders cannot be relied upon to
wind the OPT
how i like them to be wound. But the OPT should have more than
15H of primary inductance,
less than 5mH of leakage inductance, less than 7% winding losses, and
less than 1,000 pF shunt capacitance
at the anode connection. That basic specification, my friends, isn't so
easy to arrange.
Even with a spec like that 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.
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 be as I have suggested
because your OPT will have different stray leakage L and capacitances
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 overshoot .
Feedback application can be a complex issue; I make no apologies for
confusing ppl if i appear to oversimplify.
People are welcome to 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 would
be DIY amp experts.
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.
Now for some nitty gritty facts about distortions........
But allow me to explain. Most THD graphs which were drawn in 1960 to show the THD were
drawn with linear scales for THD and output voltages to flatter the amplifier and to calm the doubts in buyers minds based on the pre-conceived notion that all distortions are bad. 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.
Distortion is all bad. But we cannot ignore that distortion harmonics are always generated in all equipment
used for audio and we should be brave enough to draw graphs which will show how much total harmonic
distortion, ie, THD, is present at the low levels at which most amplifiers are used.
Most ppl think an average SPL level of 84dB is quite loud enough, with peaks rising perhaps
20db above this. To achieve such levels with a pair of modern speakers at 3 metres away in an average room
needs an average power level in each amplifier of approximately 0.25 watts, which is 1.41 Vrms into 8 ohms.
The Curve A graph shows THD rapidly rising from what is 0.0% THD at 0.0 watts.
Curve A shows that THD = about 0.3% at 0.25 watts when no global NFB is used.
Curve C shows THD falls to 0.05% when 16db of NFB is applied when 8 ohm speakers are used.
As long as THD remains below 0.1% at normal levels the THD will not affect the sound quality.
Curve B shows the amp loaded with a 5 ohm speaker instead of the ideal 8 ohms speaker.
In this case the voltage gain of the output stage is about 3db less than with 8 ohms, so the applied
amount of NFB is also 3dB less since no change has been made to the value of the R20 through which the
feed back signal flows to the V1 cathode.
Thus the THD with the lower load value is higher because of two reasons, one being that less NFB is
effectively applied, and the other reason being that the load line that the tube sees for 5 ohms at the output
is a steeper load, and one in which more THD products are generated.
Its even a worse situation when 4 ohm loads are used with dips to 3 ohms. But I have demonstrated these amps
to audiophiles using such low impedance speakers that I have built myself, and obtained rave receptions;
they liked the sound from the system very much.
The THD with 16 ohm speakers would be a lot lower than what is measured with 8 ohms, since the
higher load generates less THD in the output tube, and there is is greater tube gain hence more effectively
applied NFB.
When in doubt, ALWAYS use speakers that have a higher impedance than the load matching
of the amplifier; ie, 8 ohms is better connected to a 4 ohm outlet.
The THD itself at low levels is virtually all second harmonics of all the music frequencies present
and this amount of 2H will not cause sonic degradations. Since all musical instrument sound and voices
contain a very rich and complex mix of harmonics, not all of which are harmonious to the fundamental
or each other, a small % of harmonic distortion will not make a great difference to the percentages of
harmonics of the fundemental already present, and the difference between an old Italian violin and a Susuki
trainer will still be quite obvious to trained ears.
What may cause sonic problems would be the intermodulation products formed as a
result of the transfer linearity character which we describe in terms of the THD figures.
IMD products are the sum and difference F between differerent tones present. If you have 100 frequencies present simultaneously in a piece of music, they all interact to form sum and difference frequencies with each other
and such a myriad of artifacts manifest themselves as being like background hiss and noises at a level rising and falling with the music levels. The IMD products are usually so low in level that they are inaudible, unless the
amplifier is used at a high level and there is a high amount of natural THD or there is no global NFB.
However, SE tube amps tend to produce a benign potpourie of IMD artifacts
even though their levels may measure poorly when compared to solid state amps with much better
all round measurements.
I would add that my Curve A in the above graph sheet is quite typical of most
SET amplifiers which boast thatv they do not have any global NFB.
But Curve C shows a max THD = 0.5% at 20 watts. This is a perfectly acceptable result.
For comparison purposes, I have plotted Curve D and Curve E being the typical THD curves for a triode
( or UL ) class AB1 push pull amp also with the same levels of NFB used, and with two load values.
Notice that PP amps have much lower THD. However it is of little significance since the THD
is mostly 3H, and the resulting mix of IMD products are alleged to sound worse than those produced by
a similarly powered SEUL/SET amp with NFB.
I am of the opinion that the class A PP triode or UL amp will sound excellent. People who like SE sound usually say the SE amp will better re-produce the natural recorded midrange warmth without any loss of detail and precision in the dynamics. I am at a loss to explain all the opinions i hear.
Another graph to stir up the emotions......
This graph has a log scale for THD% and for the output voltage so that THD at lower
levels is clearly discernible. As you can see the THD produced by the SE35 watt amps
with Cathode Feedback, CFB, in their output stage 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.
I have two customers to whom I have sold samples of both amplifiers and they both listen to each others amps
during friendly house visits and niether can tell me which is the better sounding amplifier.
I might add that the 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 above comparison of THD that i measured has led me to conclude that the use of a tertiary
cathode winding for CFB on the OPT when 13E1 is used may well prove to make a very healthy
sounding amp and one which would also measure better than a plain UL connected tube.
In the next pair of amps with 13E1, I will try out using a cathode winding for local output CFB from
the OPT and try lesser % tapps for the screen connection or just take the screens to a fixed B+ like I have in the SE35.
And to those who still remain unconvinced that NFB does any good in SE amps,
let me say that each and every triode already has a local NFB loop within itself;
It is the electrostatic effect of the anode voltage movement on the flow of electrons to the anode.
And I will tell the same people that if one builds a decent and "fast enough" OPT, ie, one with
low leakage L and shunt C then global NFB works OK if the amount is kept to a moderate level
and to at least ensure that output resistance is low enough for a good damping factor.