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PSE002 Parallel Single-ended 300B mono-block amplifier. 300B single-ended circuit. 2 stages, transformer coupled, fixed bias output stage. Combination valve/solid state rectified high voltage supply with choke smoothing. Choke input filter supply for input/driver supply.

PSEOPT125 1K25:0-4-8 Single-ended output transformer


SEIT100 1:1 inter-stage transformer


2 x 300B









7 8 2.5V 190V

47R, 2W




100k 470uF 10V Bias test points 1.5V 10R

470uF/250V + 100k, 2W x2


47K 2W

47uF 250V

47uF 100V

10K 5K 10 turn 2W -100V


GZ37/GZ34/GZ32 5AR4/5R4/5U4G


CH100 3H,200mA

8 2 1N4007 2n2 2kV Ceramic disc 4



470uF/250V + 100k, 2W x2

1N4007 6


Black 0.1uF 250V AC X2 SPST



2K2 47uF 450V

CH107 20H, 25mA 47K 2W 2 x1N4007* + 10n 1kV


10k Carbon comp




Purple 47uF 250V 4.7K 47uF 250V 2 x1N4007* + 10n 1kV Purple


FLTX030 Filament transformer 300B heater supply 0.82R 5W *** Green 4 x 1N5401** +10nF Green L 0.1uF 250V AC X2 N DPST E 7.5-0 4A


5V 10,000uF 10V



Input valve heater supply.

Blue Black





3.15 SPST 0.1uF250V AC X

* 1000V soft recovery types can be substituted ** hexfreds or soft recovery diodes can be substituted. Schottky types can also be substituted but an increased resistor value will be required. *** If an external PSU is used, this resistor value will need to be adjusted to achieve 5V on the 300B filament TENT labs filament supplies can also be used directly with this filament transformer. If an external supply is used, place the TENT supply in the audio chassis.

Notes. PSE002 is a mono-block parallel single-ended 300B design using a pair of 300B's in each channel and is transformer coupled. Whilst not as pure as a single 300B design it still has many of the prized single-ended qualities and with a 18W/channel it has the ability to drive a much wider range of loudspeakers. Audio Circuit description: 6C45pi A Russian 6C45pi combines input and driver duties and drives a pair of 300B's via an inter-stage transformer. One of the so called `super triodes' the 6C45pi has very high transconductance which makes it an ideal transformer driver and its high gain makes it possible to construct a 2 stage amplifier with reasonable sensitivity. The high transconductance means a grid stopper is essential to prevent oscillation and a quick look at the data sheet shows there are multiple grid and cathode connections. Some designers advocate connecting all the grid connections and cathode connections together whilst others advocate separate grid stoppers and cathode resistors (plus by-pass capacitors) to each connection. The simple scheme shown in the diagram gives excellent sonics but the keen experimenter might want to try out different grid and cathode wiring schemes. Whichever scheme is eventually chosen the lead length of the resistor-valve base connections should be kept as short as possible and direct mounting of the resistor to the valve base pins is advised. Inter-stage transformer The inter-stage transformer is a very wide bandwidth design (-3dB 8Hz to >110kHz rp=2k5, Ia=10-15mA) and does not ring therefore no Zobel network is required across the secondary. Using the driver transformer allows excursions into Class A2 positive grid which will increase power output on peaks. This amp should provide 18W/channel in Class A but there is quite a lot more available if a blind eye is turned to distortion. Gain and Feedback This is a low gain design needing approx 1V RMS to generate full power output and should ideally be partnered with an active pre-amplifier. This design has has no negative feedback and feedback cannot be applied. Biasing The 300B's are operated in fixed bias which is more efficient than cathode bias and also gives tighter bass. There is only one bias supply for each pair of 300B's meaning only a single adjustment is required but this does mean that matched pairs of 300B's must be used. Heaters: A separate filament transformer is provided and each pair of 300B's are DC heated from a single resistor-capacitor supply. There is no need for individual heater supplies in a fixed bias design as both cathodes are at the same potential. Each diode in the DC heater circuit is `snubbed' with a small value capacitor. The output from the filament transformer is specified to allow the use of a TENT labs filament supply, something we endorse highly. The input/driver valve is AC heated. Power Supply Driver supply H.T. and bias supply. Single ended designs work the power supply hard so a separate supply is provided to the input stage to immunise it from the excursions of the output stage. This supply uses choke input filtering for good regulation and noise suppression. The input/ driver H.T. and bias supplies use solid state rectification for reasons of ruggedness, economy and longevity. The diodes are by-passed with snubbing capacitors to remove high frequency edginess. It is remarkable how smooth solid state diodes can sound when these measures are taken. A 10 turn pot is recommended for the bias supply to make accurate adjustment possible. Output stage H.T. The output stage supply is unusual as it uses a valve rectifier in parallel with solid state diodes to combine the low impedance and high current capability of solid state with the good sounds of valves. This allows a relatively large input capacitor to be used, something which is not possible with a valve rectifier alone and is good for regulation and therefore bass performance. If the arrangement looks unusual it is, and there are no doubt sceptics that will doubt it's efficacy, but it does sound like a valve rectifier with good bass. It is possible to run without the valve rectifier to save on costs in which case some more snubbing caps will be a good idea. Turning on. The separate filament transformer allows the filaments to be warmed before the HT is applied as using a valve rectifier in this configuration does not give a slow start. A few seconds wait is recommended before turning on the HT.



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