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THÖRESS
300B Single-Ended Triode . Mono Power Amplifier
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"SE300B Monoblock"
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I N S T R U C T I O N
M A N U A
L.......... .
Thank you for purchasing the
SE300B Mono Power Amplifiers!
THÖRESS
Our SE300B Monoblock has been tailored around the highly regarded 300B
filamentary power triode in single-ended zero-feedback operation mode for an
output of up to 10 watts into a 4, 8 or 16 ohm load. The amplifier is built with
meticulous hand construction using our proven point-to-point hard wiring
techniques. Much care has been taken in arranging each aspect of the internal
construction to ensure low noise performance, ease of service and the highest
reliability 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 tube (FT) with a weakly glowing 5V/1.2A
filament developed by Western Electric in the early times of vacuum technology. The
filament of a 300B tube 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 by the
glowing wire itself (directly) and not by a cathode pipe insulated from the filament
as in case of the more widely used indirectly heated tubes. 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 block the 845 tube is DC heated
whereas the cathode point is given by the junction of a two-resistor (RR) voltage
divider, bypassed with a balance 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 balance the cathode point of
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Summary of Contents for THÖRESS SE300B
- Page 1 SE300B Mono Power Amplifiers! THÖRESS Our SE300B Monoblock has been tailored around the highly regarded 300B filamentary power triode in single-ended zero-feedback operation mode for an output of up to 10 watts into a 4, 8 or 16 ohm load. The amplifier is built with meticulous hand construction using our proven point-to-point hard wiring techniques.
- Page 2 the amplifier with respect to the filament of each individual 300B power tube in order to ensure equal current flow through both ends of the tube filament. Optimal cathode point balance is attained when the hum induced in the wanted signal via residual ripple of the heater voltage vanishes (reaches a minimum).
- Page 3 Ud over the winding resistance Rw of the output transformer primary winding on the living object (after the 300B cathode point has been accurately balanced 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...
- Page 4 LOUDSPEAKER LOAD MATCHING The SE300B Mono Amplifier allows for precise 4, 8 and 16 ohm load matching by way of jumpers soldered to the secondary terminals of the output transformer (OPT) according to the patterns indicated below. Each of which pattern corresponds to a...
- Page 5 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.
- Page 6 Do not connect the amplifiers to the mains until steps 1 to 5 have been taken! Do not connect the amplifiers to the preamplifier until steps 1 to 8 have been taken! 1. Make sure that the power switch on the power inlet module is in OFF position on both mono blocks.
- Page 7 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.
- Page 8 (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 tube over case, 595=575+20/speaker binding posts, weight: 14 Kg each Mono Block.
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