Old Radio Circuits page

Welcome to my Old Radio Circuits page page. Last updated 10th November, 2004..

A number of people entering this hobby (or are more experienced experts at cabinet and appearance restoration) regularly pop up with questions as to the nature of the circuits of the old grand and what might appear electrically enigmatic instruments we have at hand. It is with such folks in mind, together with younger people developing an interest as well as those retiring who finally have enough time to get down to this work, that I present a couple of circuits from a long-forgotten valve manual, together with a few comments as to their operation. Very many such amplifiers or radios had similar circuit arrangements, differing only by the addition of a tone control, single-ended or push-pull audio outputs, a gramophone input, or parallel / series heater chain connections. Some ran straight from a D.C. mains supply or a battery, some had mains transformers (or "line transformers") for A.C. use only, and some had rectifiers and resistor "droppers" for use of either A.C. or D.C. at different line voltages.


The first thing to remember is that a valve/tube (I shall use the term 'valve' from hereon in since that's what I grew up using, being English!) is a voltage-operated device: that is to say that the output at the anode (anode current) is usually dependent upon the voltage applied at the control grid(s), in many cases g1, with respect to the cathode. Basically, the more negative the control grid voltage with respect to the cathode, the less anode current flows. This is what the 'mutual conductance', expressed in mA/V, tells us: for example a triode with a 'mu' of 2mA/V will reduce its' anode current by 2mA for each volt of extra negative grid voltage (within normal operating conditions). We can stll make this into a voltage amplifier by connecting the anode to (e.g.) a 200V positive rail via a resistor (or inductive load with a high impedance).
Let's use a triode that draws 5mA current at a g1 negatve voltage (with respect to cathode) of -1V. If we connect the anode to +200V via a 20,000 (20K) load resistor, tha cathode to the chassis/0V rail via a resistor of 200 ohms, and g1 to the chassis/0V rail via a high value resistor (e.g. 500K) since input impedances are normally very high, then we have the following situation (neglecting the 1% error introduced by the extra 200 ohms in the cathode): the anode will be at a potential of +100V due to the 5mA being drawn through the 20K load resistor. The cathode will be at +1V due to this same 5mA passing through it; since g1 is effectively at chassis/0V, meaning it is at -1V with respect to cathode. So we have a stable operating condition. For those who are interested, we arrive at this from switch-on due to the cathode heating up, allowing anode current to start flowing, which in turn increases the cathode voltage until we reach this stable situation.
Now let's say our triode has a mutual conductance of, say, 2mA/V. If we make g1 go to -250mV instead of 0V, then the anode current will reduce by 0.5mA (2mA/V = 0.5mA/250mV) That reduction in anode current will lead to a rise in anode voltage of 10V, so we have a voltage gain of 10/0.25, or 40. Now in actual fact, the cathode voltage will also reduce a little, negating some of the signal input change on g1, but we shall still have made a voltage amplifier with a very significant gain. If we 'decouple' the cathode resistor with a high-value capacitor (e.g. 25uF) and 'couple' the input signal from an audio source like a gramophone pick-up via a series capacitor of some 0.05 uF, the changes in cathode voltage will not happen since they are smoothed out by the capacitor, so now we have a practical audio preamplifier if we take the output from the anode also via a 'couling capacitor', typically 0.1 uF.

This is not intended to be an exhaustive, or overly accurate, tutorial, just an introduction into the ideas behind some of the circuts that follow, so I'll put my (un-earned) teaching hat away for now.


First we'll take a look at a push-pull typical amplifier circuit using 6V6's in the output stage. It is very representative of such amplifier circuits, this one producing some 12 watts output, but others at lower powers and higher powers (e.g. those using a higher H.T. voltage and a pair of 807's) with minor differences in the way the two phases of audio are produced. Here is the diagram:
6V6 push-pull audio amplifier
This amplifier has two inputs, a microphone input and a gramophone input. The microphone input has its' own preamplifier input based on a low noise pentode operating as a voltage amplifier V1. The output from this is capacitively coupled to the first half of a double triode (V2) which operates with a common cathode: the gramophone input is capacitively coupled to the second half. The cathode is not decoupled, therefore the effect of one half's input going positive is effectively the same as the other half's input going negative since the cathode follows the input(s). The common 2K cathode resistor, passing both anode currents, develops the negative voltages on the grids with respect to cathode.
The outputs at the anodes of the double-triode are therefore out-of-phase, which is what we want in order to drive the output valves V3 and V4. Each output valve handles one half of the audio cycle, feeding a centre-tapped balanced audio ouput transformer which matches the anode-to-anode load of 10Kohm to a loudspeaker output impedance which in this example is selectable via tappings on the output winding. V5 is a full-wave rectifier valve fed by a centre-tapped mains transformer; not the separate heater winding for this valve since it is held at the H.T.+ voltage level. A tone control is effected by varying the amount by which the 0.002uF H.F cut capacitor is introduced across the total audio signal fed to the output valves.

Next we'll look at a simple portable radio intended for battery use. It uses only 4 valves (no rectifier being neccessary of course). The fewer valves in such receivers, the less current drawn, meaning longer battery life. It operates only on the one band, at that time there were not so many stations spread over medium- and short-waves as we are faced with today! Let's first see the diagram:
Portable receiver diagram
Being a portabe receiver, it was not designed to have a connection for a long-wire or other external antenna, so the first part of the set is a loop aerial which is tuned in step with the local oscillator so as to be effective wherever the set was tuned to. Its' output is connected to the signal grid g3 of the mixer valve V1, but is not grounded at D.C., rather it is decoupled at R.F. by the 0.05uF capacitor at its' 'earthy' end; more of which later. The g1 of V1 is connected to the oscillator coil primary, the secondary of which is connected to g2 (and g4); g2 acts as the oscillator 'anode' and varies in frequency due to the ganged tuning capacitor across the coil primary.
In this way, in the one valve, we have both the wanted signal and the oscillator signal (separated in freqency by the intermediate frequency where we perform some amplification) and the difference (intermediate frequency, which is a constant) appears at the anode where it is selected (and other products rejected) by I.F. transformer T2. This is double-tuned for extra selectivity and for ease of coupling, tuning being effected by ferrite or iron-dust cores with a screwdriver slot. Never force these, they break easily, and do not actually use a screwdriver because of its' magnetic prperties: use a non-ferrous tool.
The output of T2 feeds g1 of V2, the I.F. amplifier, the 'earthy' end of T2 secondary again being decoupled rather than connected to ground/chassis. V2 is a straightforward I.F. amplifier, a pentode for lower noise and AGC here (see later) feeding a transformer T3 which is very similar to T2.The output of T3 secondary is rectified by the diode part of V3, developing a negative voltage which is fed into the 3.3megohm resistor and then the bias for V1 and V2. Now these valves have a useful characteristic not common to all valves: the more negative their control grid voltages, the lower their mutual conductance. So if you have a very strong signal, possibly too large for one of these valves, then the negative voltage produced by the rectified I.F. will increase, duly increasing the negative bias on these valves and thereby reducing their mutual conductance, and hence their gain. In this way, we have 'automatic gain control' (agc) sometimes called 'automatic volume control', and that is why the grids of V1 and V2 are not grounded to chassis/0V but decoupled by that 0.05uF capacitor. The reason for the 3.3 megohm series resistor is, in conjunction with the 0.05uF capacitor, to avoid normal audio amplitude variations affecting the gain - it forms a low-pass filter.
The audio component of the rectified I.F. is smoothed by both 100pF capacitors and is fed to the volume control which feeds the audio pentode preamp in V3 acting as a voltage amplifier, the output of which feeds the output tetrode V4 which operates in single-ended mode. The grid bias for this output valve is provided for by applying the H.T.+ for the rest of the set via a 820 ohm resistor in the negative line: this has virtually no effect for the rest of the set but develops enough negative voltage with respect to chassis/0V for biassing the output valve, which feeds an output transformer matching the 5000 ohms of the anode to the loudspeaker.

The last circuit shown here is a mains-operated set, using miniature (younger) valves. The basic principles are the same as in the portable set described earlier, so I shall not go into so much detal; rather I'll just mention the more important differences. Before the circuit, some basic aspects. Since this is a mains operated set, there is far less importance placed upon power consumption: it uses some 4 times the H.T.+ voltage at only marginally less current. Also, since it would 'live' in the living room or the study, a permanent aerial/antenna connection woulb be expected to be used. Also, since by this sets' time there were far more stations on more bands, there are three switched bands, eached tuned, rather than just the single knob. Here is the circuit:

Mains operated radio

The first thing to note is that the mixer valve is a double-valve, a triode-heptode. The triode functions as a stand-alone oscillator but its' g1 is internally connected to g3 of the heptode. Aerial signals, from the switched coil, are coupled to g1 of the heptode. Some bias is obtained from a cathode resistor, bypassed at R.F. by the 0.02uF capacitor. In other ways the heptode of V1 can be likened to the heptode of the portable set.
Likewise, the I.F. amplifier V2 has the same functionmality and nearly the same circuitry as that in the portable set. Automatic gain control is still obtained from rectified I.F., but in this case is coupled from the primary of the last I.F. transformer to a separate diode detector in V3 - in this set it is said to improve the performance of AGC when there are very close interfering stations (the selectivity of T2 secondary no longer being of influence). Amplitude-modulation audio is taken from T2 secondary in the same way as in the portable set described above. The audio voltage amplifier in this set is a triode rather than a pentode, but is still operated as a voltage amplifier feeding the output valve V4, a tetrode. Bias for this tetrode is developed by its' own cathode resistor, which is decoupled to audio by the 25 uF parallel capacitor.
The rectifier valve used here is again a full-wave type but with separate cathode and heater connections: the heater-cathode voltage rating is sufficient not to require a separate heater winding as was the case with the 6V6 amplifier unit.

I hope that the above notes and descriptions have been of some help to some of you - compiling some of this has been something of a mammoth manual task so I do take a day or so's rest now! All the best in your restoration work.

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