Audion Circuit Review - Scott 310 Tuners

Audion

Circuit Review

H.H. Scott Type 310C FM Tuner

The 4310 is the most elaborate example of the 310, but perhaps mostly for its multiplex adaptor which, equipped with vacuum tube diodes, was an early ultimate statement about the art of time switching design. With solid state diodes, the 310E might be seen as a step down, but call that a small step, as its adaptor is also a one-off that may be evolutionary to the 4310, since it was designed in 1963, more than a year after the 4310. But the 4310 is an interesting tuner to look at in an examination of the 310 series of tuners. Circuit-wise, it is identical to a late 310C in its front end and IF stages.

The only substantial difference between the front ends of a 310 and the 4310 is found in the elaborate switching network that Scott supplied to permit switching between 72 and 300 ohms, balanced and unbalanced. As Scott tuner expert Daev Rohr recently noted, the front end of Scott tuners see 72 ohms, regardless of the hookup arrangement that had been supplied at the back of the tuner. The 300 ohm terminal feeds an internal 72 ohm line, and the best way to connect a Scott tuner is with a 75 ohm line with proper orientation to ground. Attaching the bare wires will do it best.

An examination of the antenna matching network shows that the simplest path to the first inductor that the antenna sees is via the 72 ohm, unbalanced path. The 300 ohm network is shorted to ground when the system is switched to 72 ohms.
The antenna connects to the low tap of the aforementioned inductor. The high tap is connected to the regenerative circuit that connects the plate to grid through an r-c network, and feeds the positive signal that energizes the inductor, which along with a capacitively tunable circuit comprises the antenna circuit. For the tuner to work, the correct amount or regeneration must appear at the grid of the tube to reinforce the signal to thousands of times larger than its initial strength. A 310D could supply a useful result in mono to 1.9 uv, but most signals are much larger than this, most being in the 60 to 100 microvolt range at the antenna terminal.
The capacitively tunable circuit mentioned above, includes, importantly, the tuning capacitor, which is controlled by the.. etc... This tunable circuit is triple ganged, supply three identical tuning capacitors to circuits that make it possible for the front end to distinguish and amplify the exact signal that has been selected, and then convert the selected frequency to one that is fixed, regardless of the initially selected frequency. This is called superheterodyning, and the concept was developed by the inventor of generation, Edwin Armstrong.
The only remaining important piece of information there is about V201 is that it is grid-connected to the AGC buss, which is a negative feedback that among other things, curbs regeneration. AGC stands for automatic gain control. The larger the wave that develops across the grid of the 6BN6, the more AGC suppression. It is a rectified signal by the time it hits the grid of V201, and it can be switched to partial suppression to make it possible to obtain optimal stereo performance in poor signal areas. The best way to understand the function of AGC is that it regulates the amplitude of signals across the band, and it curbs superregeneration at the grid of V201, the antenna circuit. Reducing the AGC signal makes it possible to receive weak signals with better retention of the sideband component of the wave that is critical for both reception of weak signals, and for stereo reception, which uses the upper part of the frequency channel centered at 38 kHz. Full AGC suppression can make a weak stereo signal mono, while partial suppression will permit more upper band information to be retrieved.
AGC is not AFC. This needs to be repeated often. All tuners have AGC, second rate, drift-prone tuners can do wonderfully well with AFC, or automatic frequency control. After having tuned a station, the AFC switch will 'hold' the frequency. It is actually possible to detune a signal and have AFC hold the initial frequency. Switch it out and the signal goes to interstation noise. Cute parlor trick.

V201 has done the sturdy job of supplying nonjudgmental amplification to the antenna signal. The task of supplying a strong rf signal falls to V202, a grounded-grid rf amplifier that is married to the capacitively tuned circuit. V202 is cathode connected to V201, but the circuit is decoupled, and on the cathode side of V202 is found the regenerative components. The third capacitively coupled part of the tunable circuit is the superheterodyne oscillator. It is calibrated to produce a signal that, when mixed with the output of V202, will always supply 10.7 MHz. The local oscillator is very similar to the rf circuit. The only pentode used in the front end mixes the rf and local oscillator signals and this output feeds the first intermediate frequency stage, IF1, which can be considered the terminal point of the front end.
The IF stage that follows contains two stages that closely duplicate the function of IF1, and the Foster-Seeley limiter/detector. IF1 differs from IF2 and IF3 in that it utilizes an r-c circuit on the tail of its plate to supply the first stage of limiting that is supplied by the circuit to terminate IF1 in a manner that would permit it to be used exclusively in connection to the limiter/detector, as a complete IF, in other words. The 4310 IF examined here shares its initial topology with all other Scott tuners, and differs only in the bandwidth of the detector. The story of the circuit is probably found in the patent for the 6BN6, a gated beam detector tube that when used in conjunction with a double-sized, balanced secondary detector, delivered 2 MHz bandwidth at a time when 500 kHz was considered to be very good.
Scott first used this circuit in the 310. Although the circuit is much the same as later 310 tuners, the format had yet to evolve, and Scott placed the tubes facing downward. With the addition of a detector tap, it can be used with a multiplex adaptor. The 310B is supplied with a detector tap, along with the 311, and all other Scotts with the exception of those models with their own adapter. The first 310 seems largely inspired by a 1948 Browning FM tuner that must have been considered the reference of its time. It uses what looks to be a similar cascade front end and the Foster-Seeley circuit. With a modernization here and there, the 310 is born. The Browning sat its front end on a silver plate, which Scott began to do, and had the very best IF transformers available, which Scott never used. Look at a Marantz 10B to get an idea of the fit and finish of the Browning Drake.

IF2, the start point of the IF stage, applies AGC to the first grid of its 6AU6 tube, but through a significant value of resistor, in this case 470k. That means the primary AGC function takes place elsewhere. This small signal is a local feedback that acts to suppress the signal that it gets from IF1, but it also acts as a near-intimate tie with the grid signal of V201. Only resistance stands between these circuit points, and it is a far closer electrical connection than exists with the grid input of the 6BN6, or V5, which is the unrectified, coupled output from the secondary of the limiter.
I mentioned above how it would be possible to hotrod a front end to the limiter and get something out of it. Although it stands as the single most effective way to assess a front end - you can safely jumper the connection to judge the result of bypassing IF2 and IF3 - you wouldn't want to use the tuner in this manner. On the transmission line that the IF stage is, each transformer is one further opportunity to lock on to the intermediate frequency and amplify it. The more stages, the finer the result. That's why you pay more to get more.

Series coupling along the B+ transmission line connects the plates of the 6AU6 on a line that employs 220k resistors in series along the line. A further plate resistor varies the plate voltage that IF2-3, and the limiter see. Inductive coupling wasn't employed by any tuner manufacturer at the time. In fact, Scott introduced inductive coupling in its most modest IF of all time, the board that went into the 341 receiver in 1968. The early 312D has them, as does the final model of that type.
Local time correction is applied to the second grid of the 6AU6 in IF2-3. It is negative feedback that is supplied from the bottom of the primary of the transformer that is 'fed' by the plate of the tube.
The third, suppressor grid connects to yet another line, this also connecting the bottom leg of the secondary of the IF transformer. All of these latter components are 'hard wired' to ground. All of this description also applies to the limiter circuit. It adds an r-c network to the tail of its plate, and this signal feeds the AGC line and also supplies a terminated signal to the detector. The only other major difference is that the cathode supplies of IF3 and the limiter have capacitance added in shunt with the cathode resistor. The capacitor's stored charge controls the amplitude of the cathode supply more evenly across the frequency spectrum in exactly the same manner as when the circuit is used in the cathode supply of an audio circuit. It's important to remember that there is no actual difference between rf and af, except for the frequencies being dealt with. The entire description of the function of a 310 tuner could also be well supplied as an explanation for how an AM tuner works. That's because up to this point, there isn't much difference between the two. The 310 tuner is an amplitude tuner so far, not at all concerned with the eventual way that the frequency that is being amplified will be detected.
That's the next story. The detector. In the case of the 310 tuner, much of the magic is found in the Foster-Seeley discriminator. This design made it possible to detect a signal with triple the average bandwidth that a 'decent' tuner might offer. Second-rate tuners could supply no more than 500 kHz. The FCC was convinced that wideband FM tuners would be the order of the day on April 19, 1961, with Docket 13506. Death knell to the junk-makers. The 2 MHz that the Foster-Seeley circuit offered was the ultimate for stereo reception.
The high composite output off the 310 tuner (and any other) will employ a pair of diodes in opposition on each of the high and low terminals of the detector transformer. The output of these signals are accumulated through a balancing network and it is this output, the discrimination of the time differential between a normal wave and the modulated wave that a signal is detected. The high frequency carrier, 10.7 MHz, has been suppressed and the resultant tiny signal is then available to be amplified and equalized for monophonic sound, or it will be further worked on by the next device in circuit under consideration. 310 owners can now attach their 335 multiplex adaptors.

It is a major task to understand the 4310 multiplex adaptor because it has a lot of extra plumbing associated with the multiple diversity circuit. It permitted several diversity-capable tuners to be hooked together. The tuner set to be the master unit would then select from among all the other 4310's hooked up, which supplied the cleanest stereo signal. That was the aim, so the control interface was built across the adaptor, taking the output from the bridge detectors and making some sort of analog-computer judgment about which signal was best. Elaborate relays were used to select the signal that would then feed the audio amplifier and equalization circuits that when applied with intelligence, produce flat frequency response. This is done by de-emphasis, which is a way of saying tailored negative feedback, but it is expressed as a measure of time. In the USA and Canada, the standard is 75 microseconds, in Europe it is 50.
Multiplex began with the notion that the bandwidth allocated to FM stations was not only sufficient for the low-fi background music channels that were a design of the system and kicked in around 60 kHz. That left a space for multiplex at the aforementioned 38 kHz. This value was chosen in concert with 19 kHz, a frequency that would be used to supply a signal that stereo was being transmitted. The 38 kHz oscillator is synchronized through a simple doubling. In order for stereo to be received, the 19 kHz signal had to be present to produce an oscillation out of the 38 kHz circuit that would instruct the time switching bridge when to consider the composite signal to be left channel information, or right.
Time division multiplex is based on very simple math: Consider that FM is comprised of a monophonic sum signal that we hear on a mono radio, and a difference signal that we hear only in stereo. Time switching derives the signals as follows: (L+R)+(L-R)=2L, and, (L+R)-(L-R)=2R. The entire purpose of the adaptor is to produce the result of L and R as separate signals with adequate provision against things like crosstalk. Too much crosstalk and you have mono. The stereo signal is developed across a diode bridge that is usually solid state, but in the case of the 4310 (and the 370), vacuum tube diodes were used instead. This signal that the multiplex adaptor has been developing is based on the high output of the detector. The low output always serves a purpose, either as a meter drive, or in the age of stereo FM, as a benchmark signal to mix with the composite output signals. The low signal is injected into each channel to suppress the common mode of the signal, at some expense of amplitude. This signal is amplified and equalized and if the conditions are right, and you don't walk in front of the antenna, you have nice stereo.
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