THÖRESS SE300B User manual

THÖRESS
300B Single-Ended Triode . Mono Power Amplifier
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"SE300B Monoblock"
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I N S T U C T I O N M A N U A L…....... .
Thank you for purchasing the THÖRESS SE300B Mono Power Amplifiers!
Our SE300B Mono lock has een tailored around the highly regarded 300B
filamentary power triode in single-ended zero-feed ack operation mode for an
output of up to 10 watts into a 4, 8 or 16 ohm load. The amplifier is uilt with
meticulous hand construction using our proven point-to-point hard wiring
techniques. Much care has een taken in arranging each aspect of the internal
construction to ensure low noise performance, ease of service and the highest
relia ility for many years to come. Please read the following explanations and
instructions carefully to get the most out of your SE300B amplifiers.
CATHODE POINT BALANCE
The 300B power triode is a filamentary tu e (FT) with a weakly glowing 5V/1.2A
filament developed y Western Electric in the early times of vacuum technology. The
filament of a 300B tu e consists of a thin oxide coated thoriated tungsten wire
mounted with spring suspensions to mica supports pressed into the glass envelope.
The electrons which constitute the cathode-to-plate current are emitted y the
glowing wire itself (directly) and not y a cathode pipe insulated from the filament
as in case of the more widely used indirectly heated tu es. The anode current flows
through the filament along with the heater current!
FTs require a specific cathode point implementation outside the vacuum envelope in
order to separate the signal current from the subsidiary heater current!
In the inception of vacuum technology (when semiconductor rectifiers were not in
existence) FTs were AC heated with the aid of dedicated windings on the mains
transformer whereas a M.IDLE T.AP of the winding served as cathode point (ACMT
cathode point implementation). In our mono lock the 845 tu e is DC heated
whereas the cathode point is given y the junction of a two-resistor (RR) voltage
divider, ypassed with a alance potentiometer (BALPOT, mounted on the rear
panel) connected in parallel with the filament. We call this arrangement DCRR
cathode point implementation. The BALPOT allows to alance the cathode point of

the amplifier with respect to the filament of each individual 300B power tu e in
order to ensure equal current flow through oth ends of the tu e filament. Optimal
cathode point alance is attained when the hum induced in the wanted signal via
residual ripple of the heater voltage vanishes (reaches a minimum).
Thoroughly balance the cathode point of the L R amplifier with respect to the filament of
the individual 300B power tube with the BALPOT in the way described in the SETUP
section of this manual!
Check the L R amplifier for optimal cathode point balance from time to time and finely
tune the balance if necessary!
Re-balance the cathode points of the L R amplifier after changing the 300B power tubes!
For a given setting of anode voltage and current, the grid ias voltage Ug(DCRR) of a
given 300B tu e under DCRR cathode point implementation is a out 2.5V (that is
half the heater supply voltage, 2.5V=5V/2) lower than the ias voltage Ug(ACMT) of
the same tu e under ACMT cathode point conditions,
Ug(DCRR) <---> Ug(ACMT)-2.5V,
Nota ly, Ug(DCRR) is the actual voltage etween the 300B control grid terminal and
the DCRR cathode point under alanced conditions in the sense a ove (including the
effects caused y residual grid current of the tu e). It is important to take this ias
shift into account when making reference to the characteristics of the ideal (average)
300B tu e as given y the original Western Electric data sheets.
300B OPERATION CONDITIONS
Under ACMT cathode point implementation the ideal (average) 300B tu e draws an
anode current of Ia=0.065A when it is exposed to an anode voltage of Ua=425V and
a grid- ias voltage of Ug(ACMT)=(-)93V according to the Western Electric data sheets
(January 1950). This is an excellent operation point for single-ended class-A
operation which we have chosen for the 300B tu e in our amplifier. Corresponding
to a plate dissipation Pa=28W=425VXx0.065A well elow the permissi le design
maximum of 35W,
(Ug(ACMT), Ua, Ia, Pa) = ((-)93V, 425V, 65mA, 28W).
The corresponding quiescent operation point under DCRR cathode point
implementation is
(Ug(DCRR), Ua, Ia, Pa) = ((-)90.5V, 425V, 65mA, 28W)
Ug(DCRR)=(-)90.5V=(-)(93V-2.5V).
The grid bias voltage of the 300B tube is factory preset to (-)90.5V and is not meant to get

trimmed by the user in order to adjust the idle current!
This design choice makes it important to use 300B tu es with characteristics out of a
specific tolerance window with respect to the ideal 300B tu e. Allowing for a 10%
tolerance, only tu es with an anode current
59 mA=65mA-6mA (Pa=28.5W) … 71mA=65mA 6mA (Pa=30W)
(under the given DCRR voltage conditions (425V, (-)90.5V)) are suita le for service in
our amplifier. Accordingly, it is advisa le to use only hand picked matched pairs of
300B tu es tested for the a ove specs supplied y the manufacturer or a relia le
tu e supplier.
The use of 300B tubes with inadequate characteristics will lead to inferior sound quality
(too low idle current) or to a reduced lifespan of the 300B tube due to overheating (too
high idle current)!
A carefully selected good quality 300B tube of current production is likely a better choice
than a rare and expensive NOS part with questionable characteristics!
If there is dou t that a given 300B tu e fulfills the a ove requirements, the actual
idle current Ia can e determined indirectly ( y ohms law) y measuring the voltage-
drop Ud over the winding resistance Rw of the output transformer primary winding
on the living o ject (after the 300B cathode point has een accurately alanced with
respect to the filament), Ia=Ud/Rw.
Measurements within the SE300B amplifier circuitry should be carried out by an
experienced technician only!
POWER TUBE MICROPHONY
Mechanical vi rations of the inner life of a vacuum modulate the anode current and
such are to a small degree converted into noise artefacts which lur the wanted
signal. This effect, called tu e microphony, is particularly pronounced with FTs
ecause the freely suspended filament wire is a le to swing rather easily. Therefor
amplifiers employing FTs need more considerate placement than amplifiers featuring
more commonly used indirectly heated tu es in order to keep the wanted signal free
of microphony artifacts. Tu e microphony is especially critical when highly efficient
loudspeakers are involved, for o vious reasons.
Place the Amplifier on a rigid rack, shelf or platform carefully decoupled from the floor in
order to keep the signal free of noise artifacts due to tube microphony!
DRIVER TUBES
The SE300B mono amplifier employs a matched pair of CV2382 (EL822) power
pentodes operated in triode mode (g2 tied to the anode, g3 connected to the

cathode) at high idle current in the driver stage. The amplifier comes with a set of
tu es which have een carefully hand-picked to meet tight specifications. When
operated in triode mode, the ideal (average) CV2382 tu e draws an anode current of
Ia=40mA when it is exposed to an anode voltage of Ua=290V and a control grid ias
voltage of Ug1=(-)8.5V. Allowing for a 10% tolerance in anode current only tu es
with
Ia=36mA=40mA-4mA ... Ia=44mA=40mA 4mA
(under the given voltage conditions (Ua, Ug1)=(290V, (-)8.5V) are suita le for service
in our SE300B amplifier.
It is strongly advisable to use only carefully tested matched pairs of driver tubes as
supplied by the manufacturer!
The use of driver tubes with questionable characteristics may lead to inferior sound
quality and, in extreme cases, to damage in the driver circuit!
Never switch on the amplifier until all tubes have been installed!
Never pull out a driver tube while the amplifier is powered on!
POWER AMPLIFIER GAIN
The SE300B mono amplifier has een specifically designed to have a moderate
voltage gain (low input sensitivity). This design choice has two enefits. At first, the
idle noise of the line amplifier remains in-audi le even when the amplifier is used to
drive a highly efficient loudspeaker, for example our 2CD12 model (or an even more
efficient horn arrangement). Secondly, the volume control can e operated at a
higher angle of rotation so as to allow for conveniently fine volume adjustment.
Low voltage gain is a desirable feature of power amplifiers, particularly when the
amplifiers are meant to drive highly efficient loudspeakers!
When the SE300B mono amplifiers are used to drive loudspeakers with moderate to
low efficiency it will e necessary to set the volume control kno (on the line device)
to a somewhat higher than usual angle of rotation to achieve a saturated listening
loudness.
LOUDSPEAKER LOAD MATCHING
The SE300B Mono Amplifier allows for precise 4, 8 and 16 ohm load matching y
way of jumpers soldered to the secondary terminals of the output transformer (OPT)
according to the patterns indicated elow. Each of which pattern corresponds to a
specific (primary versus secondary) turns ratio of the OPT.

4 ohm loudspeaker
Connect (1 and 2), (5, 6, 3 and 4), (7 and 8).
8 ohm Loudspeaker
Connect (2, 3 and 5), (4, 6 and 7).
16 ohm loudspeaker
Connect (2 and 5), (3 and 6) and (4 and 7).
Output Transformer Coil
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1 o o 5
2 o o 6
3 o o 7
4 o o 8
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o = secondary terminal, 1 = loudspeaker RED, 8 = loudspeaker BLACK
A single-ended triode amplifier can adequately drive a loudspeaker when the OPT is
configured for a lower load impedance than the rated impedance of the
loudspeaker, especially when the loudspeaker is highly efficient. In this case the
(primary versus secondary) turns ratio of the OPT is higher than the nominal index
value. This reduces the voltage gain ut also the harmonic distortion and the output
resistance of the amplifier, at the expense of a somewhat lower maximal power
output.
It can be beneficial to use a 16 ohm loudspeaker with the 8 ohm or even the 4 ohm load
pattern of the OPT, especially when the loudspeaker is highly efficient!
It is possible to use a 8 ohm loudspeaker with the 4 ohm load pattern of the OPT,
especially when the loudspeaker is highly efficient!
On the other hand, a compromised amplifier performance is definitely to e
expected when the OPT is configured for a higher load impedance than the rated
loudspeaker impedance. Thus:
It is not advisable to use a 4 ohm rated loudspeaker on the 8 ohm or 16 ohm load pattern
of the OPT!
It is not advisable to use a 8 ohm rated loudspeaker on the 16 ohm load pattern of the
OPT!
SETUP
To set up a pair of SE300B Mono Amplifiers power off all devices of the setup and
proceed as follows.

Do not connect the amplifiers to the mains until steps 1 to 5 have een taken!
Do not connect the amplifiers to the preamplifier until steps 1 to 8 have een taken!
1. Make sure that the power switch on the power inlet module is in OFF position on
oth mono locks.
2. Bring the BALPOT into middle position on oth units.
3. Install the tu es carefully.
Never switch on the amplifier until all tubes have been installed!
Never pull out a tube of the socket while the amplifier is powered on!
Filamentary tubes (FT) such as the 300B are fragile devices and must be handled with
exceptional care!
Never move or even transport a 300B power tube as long as it is still hot!
Always deinstall all tubes and wrap them in their original protection case before shipping
or transporting the amplifier!
4. Bring the amplifiers into their final position. Here y take into account the notes
made in the section TUBE MICROPHONY. Make sure that there is sufficient clearance
around the tu es to allow for adequate ventilation.
5. Connect the L+R amplifier to the L+R loudspeaker. Ascertain that the load
matching installed on the OPT suits the loudspeaker impedance. Take notice of the
explanations presented in the section LOUDSPEAKER LOAD MATCHING of this
manual.
6. Connect the L+R amplifier to the mains .
7. Balance the cathode point of the L-channel monoblock with respect to the
filament of the installed 300B tube in the way described below.
Power on the L-amplifier with open input (no ca le connection) while the R-
amplifier remains switched off. Wait for a out one minute until the warm-up
process on the amplifier has come to an end. O serve the residual hum radiated y
the L-loudspeaker and adjust the BALPOT of the L-amplifier (see paragraph CATHODE
POINT BALANCE) until the hum noise vanishes (reaches a minimum). Here y it is
important to o serve the noise at a out 0.5m distance to the woofer(s) rather than
at the listening position, with regard to room modes (specific areas in the auditory
where the sound power distri ution peaks or dips thanks to standing waves
etween pairs of (sound reflective) room oundaries, particularly opposite walls).
Power off the L-amplifier after the cathode alancing procedure has een
accomplished.
8. Balance the cathode point of the R-channel monoblock with respect to the
filament of the installed 300B tube in a similar manner. Power off the R-amplifier
after the balancing procedure has been completed.
Check the amplifiers for correct cathode point balance from time to time and finely tune
the balance if necessary!
Always re-balance the L R amplifier after changing the 300B power tube!

9. Connect the L+R amplifier to the preamplifier.
10. Make sure that the volume control kno rests in zero position.
11. Power on the preamplifier and the program source(s) while the R+L power
amplifier is powered off. Wait until the warm-up process on these components has
come to an end.
12. Complete the setup y powering on the L+R power amplifier.
Always power on the program sources and the preamplifier first and then switch on the
power amplifiers, observing a delay of at least 30 seconds!
When powering off the system, always switch off the power amplifiers first, then switch
off the other components of the system, observing a delay of at least 30 seconds!
Keep the original crates and tube protection cases for later use. They have been
specifically designed for safe transport under rough conditions!
FUSE
The SE300B Amplifier draws a current of 0.325A (0.75A) from the 230Vac (115Vac)
mains corresponding to a power consumption of 75W. It is protected with a
1A slow blowing 5x25mm fuse
inside the power inlet module. Very occasionally, the fuse may low at the moment
the amplifier is powered on, (due to the current spikes drawn y the power
transformer in this instant) especially when power cords with very low resistance are
used with the amplifier. Should this pro lem arise more regularly it may e advisa le
to use a fuse with slightly higher current rating (2A/3A).
F E A T U E O V E V I E W
•All-Tu e Mono Power Amplifier utilizing the famous classic 300B filmentary
power triode in single-ended operation mode.
•10 watts of output power.
•Powerful single-stage driver utilizing two CV2382/EL822 pentodes operated in
triode mode at high idle current.
•Minimalist (pure class-A) zero-feed ack schematic.
•High grade foil-in-oil capacitors (made in Germany) in the power supply.
•C-core filter choke made in-house.
•Ultimate tape wound cut core (C-core) output transformer manufactured in-
house.
•Precise 4, 8 and 16 ohm loudspeaker load matching via jumper setting on the
output transformer.
•Low noise, low leakage mains transformer produced in-house for 230Vac,
(115Vac via jumper setting), 100Vac (Japan), 120Vac (USA, Canada), 220Vac

(South Korea, China, Thailand, Indonesia), 240Vac (UK) or 245 Vac (Australia).
•Full hand construction, point-to-point wiring throughout.
•Non-magnetic (aluminum) case, anodized printing on front and rear panel,
powder-coated chassis and lids.
•Dimensions: 150x595xH330 mm,
330=20/feet+210+100/power tu e over case,
595=575+20/speaker inding posts, weight: 14 Kg each Mono Block.
•Dimensions of the shipping crate: 950x400xH460 mm.
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THÖRESS...
A Tribute to Professional Audio Components
from the Golden Age of the Electronic Tube !
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