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Pye 350/C All-Electric 3
1. General
The Pye All Electric 3 model 350/C is a three valve, two waveband AC mains TRF receiver.
A separate high impedance loudspeaker is required for the set, which embodies a screen
grid HF stage (V1, AC/SG), a triode anode bend detector (V2, AC/HL) and a triode output
stage (AC/P). A Westinghouse metal rectifier of early design (no cooling fins) is
incorporated in the HT power supply, which operates on a voltage doubler principle.
The HF and detector stages are tuned separately. The wavelength coverage is
approximately 900 - 2300 metres ("long waves") and 210 - 520 metres ("short waves").
2. Circuit details
The circuit diagram is below. The aerial circuit is tuned by L1 or L2 and VC1, which has an
edgewise slow motion drive and drum scale. Two separate aerial input tappings are
provided. V1 is a conventional indirectly heated screen grid valve. This is coupled via L3 or
L4 to the detector tuned circuit, L5 or L6 and VC2. The waveband is selected by the
mechanically ganged switches S1 and S2. The detector is a triode wired as an anode bend
detector. This circuit is unconventional in that reaction is provided via the solid dielectric
differential capacitor VC3 giving feedback to L5 or L6. The reaction winding is itself a part
L5 or L6, and so is incorporated in the tuned circuit.
Neither the aerial nor detector tuned circuits have a DC connection to chassis, as they are
incorporated in the valve biasing arrangements. The detector is coupled to the output
stage V3 with an intervalve LF transformer (T1). A 70 k variable resistor (VR1) connected
across the T1 primary acts as a crude volume control. The output valve is choke-capacity
coupled to a loudspeaker using L7 and C7.
The power supply for the valve heaters is derived from a 4 volt centre tapped winding on
the mains transformer T2. The HT winding is connected to a rectifying and voltage
doubling circuit (C12, C13, MR1). Smoothing is accomplished by the choke L8 and
capacitors C11 and C10. The HT appears across the resistor R1, which is a sectionally
wire wound component. The HT and bias supplies for the valves are picked off from
tappings on V3. Mains voltage adjustment is provided by tappings on T2 primary.
Provision is made for using the set with an electric gramophone pick-up. The intermediate
position of S1/S2, between long and short waves, breaks all contacts except S2g, and the
detector valve bias conditions are altered so that it acts as a conventional amplifier.
Notice that the decoupling components C3, C4, C5 all share a single can, as do the
smoothing capacitors C10 -C13. C12 and C13 consist of pairs of capacitors wired in
parallel.
3. Component values
The component values are given in Table 1. The capacitors were measured from an actual
receiver (serial number C26118) using a Wheatstone bridge followed by "rounding" to the
probable design value. Resistances were measured on a multimeter.
Page 2
4. Voltages and currents
Table 2 gives the voltages and currents found on the same receiver when tuned to the HF
end of the short waveband, but with no powerful signal present. The mains input was
240V. All voltages were measured with respect to chassis (except the AC voltage on the
secondary of T2) using a 20,000 /V meter. The currents are approximate.
5. Operational notes
The operation calls for no special comment. It works well with a contemporary moving iron
loudspeaker. Overloading of the detector occurs on strong signals if the wrong aerial
tapping is used. The volume control VR1 work as well as might be expected (ie not very!).
Most of the effect occurs in the first few degrees of rotation. Reduction of volume by
detuning the aerial circuit is frequently a better way than trying to use VR1.
6. Service notes
On the particular receiver under test, all the fixed capacitors were tested for leakage and
found, rather surprisingly, to be in good condition. The metal rectifier was also found to be
in good condition, though the forward resistance is a trifle high. The secondary winding of
T1 was open circuit, as was the wire-wound resistor R1. Both were re-wound.
Notice that the spindle of VR1 is at HT potential. It is insulated from the metal front panel
by means of an ebonite bush, which must be carefully replaced if removed. Care should
also be taken if the moulded knob is removed! The moving vanes of the reaction capacitor
are also at HT potential, but in this case they are insulated from the spindle.
The moving vanes of VC1 and VC2 are not earthed. Do not attempt to dismantle the slow
motion drives or remove the knobs - they are very difficult to re-assemble without damage.
Page 4
Table 1 - component values and functions
Capacitors
C1 1.0 AF V1 screen decoupling
C2 1.0 AF V1 bias decoupling
C3 1.0 AF V1 anode decoupling
C4 1.0 AF V2 anode decoupling common can
C5 1.0 AF V2 bias decoupling
C6 0.0003 AF V2 anode HF bypass
C7 2.0 AF Loudspeaker coupling
C8 0.002 AF V3 anode HF bypass
C9 1.0 AF V2 bias decoupling (gram)
C10 1.0 AF Smoothing
C11 1.0 AF Smoothing common can
C12 2 x 1.0 AF Reservoir
C13 2 x 1.0 AF Rerservoir
C14 0.001 AF Mains RF filter
Resistors
R1a 3k
R1b 2.7 k R1 a-h is the HT and bias
R1c 29 k potential divider.
R1d 18 k
R1e 48 Total resistance = 53.5 k
R1f 28
R1g 70
R1h 680
R2 100 k V2 bias decoupling
R3 100 k V2 bias decoupling (gram)
Others
VR1 70 k Volume control
T1 Pri 1000 approx V2 - V3 coupling DC resistance
Sec 12 k approx DC resistance
L1 - L6 Tuning coils
L7 400 approx V3 output choke DC resistance
L8 700 approx HT smoothing choke DC resistance
S1 RF band switching S1 and S2 are
S2 Detector band switching mechanically coupled
Page 5
Table 2 - voltages and currents
Valve V1 (AC/SG) V2 (AC/HL) V3 (AC/P)
Anode voltage (tap on 120 133 155
R1 to chassis) (V)
Anode curent (mA) 1.9 2.5 7.8
Screen voltage (V) 35.5 - -
Screen current (mA) 0.9
Bias voltage (tap on R1 - 0.76 - 2.4 - 12.5
to chassis) (V) (- 1.2 on gram)
Voltage between C11 and C12 junction and chassis (V) : 166
Voltage across secondary of T2 (V rms) : 130 (AC)
Total HT current (mA) : 15.1