1. Original design
Lafayette HE-30 (similar to Trio 9R-59) is a basic single conversion tube semi-professional short wave communications receiver. It covers 0.55 to 30 MHz in four bands. It has bandspread function by way of a small variable capacitor connected to each gang of the maim tunung variable capacitor.
As many other similar designs, it has one RF stage (6BA6), local oscillator (6BE6 in triode), mixer (6BE6), two IF stages (6BA6), AM detector and first audio (6AV6), output audio (6AQ5), rectifier (5CG4). This model also has a Q-multiplier (6AV6) which can work as a BFO if driven into continuous oscillation. BFO signal is injected into the 1-st IF input via stray coupling.
Original HE-30 receiver has several drawbacks. To name a few, it has less than desired frequency stability at the top band, low audio quality, and almost useless Q-multiplier. Some simple modifications, described below, can greatly improve performance of this vintage radio.
2. Voltage regulator
In the original design all the LO, mixer, IF stages, Q-multiplier and even the screen of the output audio stage are powered by an unregulated voltage about 110...140V. Needless to say, local oscillator frequency depends on the mains voltage -- frequency sometimes jumps with turning on of an air conditioner of even a microvave oven in the household. Sometimes signal strength affects the LO frequency. Automatic gain control changes current draw of the RF anf IF stages, which causes LO supply voltage to change -- hence frequency drifts. Besides, unregulated voltage supply to the IF counteracts AGC operation.
Adding a voltage regulator for the RF, LO and IF stages solves the above problems. An example of such a regulator is shown below.
Regulated voltage of about 114V is close to optimum. A Zener diode is used as a reference. A high transconductance Q1 MOSFET 2SK4100 provides good load regulation. Suppressor resistor R5 is essential. It shall be connected directly to the gate pin of Q1. Another Zener D2 is for the MOSFET protection. R4C8 filters Zener noise and ripple.
High voltage Zener diodes are very noisy in the wide frequency band. For that reason, to filter RF component of the noise it is very important to connect a shunting low inductance disk ceramic capacitor C7 as close as practical to the Zener body and on the shortest leads, for example, as shown in the picture below.
It is also advisable to mount the Zener D1 away from the sensitive RF or IF nodes, to avoid the residual noise leakage into the receiver signal or IF path. In the given example the regulator is located in the corner of the chassis near the phones connector -- far away from any RF circuity. It is very convenient to choose Q1 MOSFET in a fully insulated TO-220F package and mount it directly to the chassis for a heat sink.
Now when the HF stages get regulated supply, the other section of the C4 electrolitic capacitor can be used solely for filtering of the supply to the AF amplifier: the first stage (6AV6) and the screen grid of the output stage (6AQ5) . Filtering resistor R1 can now be larger, giving an excellent hum reduction. Besides a higher voltage (175V as compared to the original 110V) is available on the 6AQ5 screen grid, which increases audio output power.
It is also recommended to replace the original 5CG4 rectifier with a 5Z4GT, which has a lower voltage drop, or even with the silicon diodes 1N4007. C1, C2, C3, C6 are optional to reduce the modulation hum (tuned hum), but with the vacuum rectifier it is virtually non-existant anyway. An optional dropping resistor R6 may be used if the mains voltage is consistently higher than nominal, like say in Australia, where it is usually 240...250Vac most of the time.
3. AM detector
In the original HE-30 design the AM detector is AC loaded by the AGC filter R18C3. It limits the maximum undistorted modulation handled to about 75%, whilst nowadays MW and SW broadcast stations use up to 100% modulation. A relatively large loading capacitor C20 (100pF) further contributes to distortion at higher AF frequencies.
To improve audio quality it is recommended to:
- separate AGC detector and audio AM detector;
- reduce capacitive loading on the AM detector;
- use a sensitive Schottky diode instead of a vacuum detector (6AV6).
A picture below illustrates these changes.
A 1N5711 Schottky diode in fact has so small "knee" (about 50mV at those sub-microamp currents) that it works better than a vacuum diode at low IF levels. Its low capacitance (1.5pF) helps reduce IF losses in C2. Capacitor C3 may be omitted, as the capacitance of the shielded cable to the volume control potentiometer gives enough filtering. A small "knee" of about 0.5V of the AGC detector silicon diode D2 provides a mildly "delayed" AGC action. Total grid leakage R4+R5 resistance is lower than the original 2.2M, which makes the circuit more forgiving to old and gassy tubes. Besides, now the diodes in 6AV6 are no longer used, hence the cathode of 6AV6 can be lifted of the ground and can be used for a negative feedback from the secondary winding of the output audio transformer.
4. Q-multiplier / notch filter
A Q-multiplier, which was intended originally to pull weak ham AM signals, in modern days is almost useless. It narrows down the bandwidth, making the sound of the AM stations even more "low" and muffled. Besides a Q-multiplier working close to the oscillation point is prone to distortion, "ringing" and instability.
To reduce interference, it is far better to suppress an interfering signal, rather than peak on the signal of interest.
AM detector is fundamentally a non-linear device. If two signals are present at its input, on the output there will be not a sum of both demodulated envelopes. Instead, a stronger signal suppresses a weaker one. Suppose the weaker one is the interfering signal. It will turn into a "whistle" or a "monkey-chatter". At the same time, the weaker interference signal becomes a "floor" for negative modulation excursions of the signal of interest. Thus an interference does not only add to the signal of interest as a "whistle", but it also limits and distorts the main signal. For that reason such interference can not be simply removed at audio frequencies by a treble tone control or an audio notch filter. Such technique might reduce the "whistle" but will not eliminate distortion and partial loss of the signal of interest.
It is more useful to try to remove the interference in front of an AM detector. A narrow-band notch filter can be used for such purpose.
Converting a HE-30 original Q-multiplier into a notch filter is easy -- one has to couple a Q-multiplier to an IF tank not in parallel, but through a small (15...22pF) series capacitor. In this case, a sharply tuned Q-multiplier LC tank at its resonant frequency will start to "suck" energy from the IF path, creating a notch.
An implementation of such a notch filter is shown below.
Original circuit, mounted in an enclosed metal compartment, does not change. Connection to the IF transformer changes. To switch between the normal peaking mode and new notch mode, a former noise limiter ON/OFF switch (SW1) can be used. When C3 is bypassed, the Q-multiplier works in the normal peaking mode, when C3 is in circuit -- in the notch mode.