20-04-2018, 09:56 PM
Thanks for those.
The Murphy circuit certainly does have some unexpected features.
The use of a 6C12 (ECH81) for the second conversion is as expected, although I had wondered whether one might find a 6C9. Murphy had hung on to the latter – understood to be quite robust – until quite late in its radio receiver circuits, after all of the other signal valves had been changed to novals.
But the use of a second 6C12 to provide the second sound IF gain stage and the first audio stage was initially a surprise. It does make sense, though. Murphy required a remote cutoff valve for the 2nd IF stage, and an additional triode for the AFC DC amplifier. So a double valve was a logical. And whilst another triode-pentode might be the first port of call, such types with remote cutoff pentode sections would have been very scarce when the circuit was designed. On the other hand, the 6C12 heptode was remote cutoff, and was designed for use as a gain-controlled IF amplifier as well as a gain-controlled mixer, so it was in fact a good fit. And the 6C12 triode was designed to be used as an AF amplifier as well as an oscillator, so it too was a good fit. In respect of the disposition of the two triode section, that in the 30PL1 had a separate cathode, and so was a better fit for the DC amplifier role, where the cathode operated with divider bias.
The 6C12 heptode as operated as an amplifier with AGC bias on TV sound (AM) and as a delayed limiter with standing bias on FM. As the anode and screen potentials were not switched between the two modes, presumably the AGC bias was arranged to keep the valve away from limiting on AM. The AGC bias was derived directly from the demodulated AM signal, and so was not delayed. It was also applied to the mixer grid on AM only. On FM, AGC bias for the RF amplifier was obtained from the limiter grid.
Given that the 6C12 was operated as a limiter on FM, grid detection for AM would have been possible (and was used as a source of RF AGC bias on FM), but it is clear that as two gain stages were required on AM, this option was precluded.
That a DC gain stage was required for the AFC bias possibly reflects the fact that a ratio detector was used for FM. Had a Foster Seeley discriminator been used, then maybe there would have been enough AFC bias without a DC amplifier. But that almost certainly would have required an additional IF stage, to give two gain stages and a full limiter, meaning an extra valve envelope. In that case, though, AM grid detection would have been possible.
The AFC system was unusual in that it was a long loop, including the second frequency changer stage. Presumably the second oscillator was sufficiently stable that a situation where the AFC was correcting for second oscillator drift by pulling the first oscillator out of correct tune did not arise in a material way.
A neat feature was the use of the AM noise limiter diode to provide sound channel muting during warm-up.
In the main AGC system, the IF bias was clamped at the point where the RF bias started. This was different to the usual Mullard curves where the IF bias continued to rise after the RF bias started, with the curves crossing at the points of respective maximum bias. Apparently, it was preferable to ensure that the IF bias did not take the valve(s) to the bend in the curve (with resultant severe non-linearity) before the RF amplifier got there, the more so if sharp cutoff valves were used in the IF strip. Evidently Murphy ensured that condition by simply clamping the IF AGC bias at the appropriate point.
That Murphy circuit was interesting enough that in retrospect, it’s surprising that Murphy did not “plant” a full article in Wireless World at the time. But both the use of dual-conversion and the subsequent addition of AFC were noted in brief items:
Cheers,
Steve
The Murphy circuit certainly does have some unexpected features.
The use of a 6C12 (ECH81) for the second conversion is as expected, although I had wondered whether one might find a 6C9. Murphy had hung on to the latter – understood to be quite robust – until quite late in its radio receiver circuits, after all of the other signal valves had been changed to novals.
But the use of a second 6C12 to provide the second sound IF gain stage and the first audio stage was initially a surprise. It does make sense, though. Murphy required a remote cutoff valve for the 2nd IF stage, and an additional triode for the AFC DC amplifier. So a double valve was a logical. And whilst another triode-pentode might be the first port of call, such types with remote cutoff pentode sections would have been very scarce when the circuit was designed. On the other hand, the 6C12 heptode was remote cutoff, and was designed for use as a gain-controlled IF amplifier as well as a gain-controlled mixer, so it was in fact a good fit. And the 6C12 triode was designed to be used as an AF amplifier as well as an oscillator, so it too was a good fit. In respect of the disposition of the two triode section, that in the 30PL1 had a separate cathode, and so was a better fit for the DC amplifier role, where the cathode operated with divider bias.
The 6C12 heptode as operated as an amplifier with AGC bias on TV sound (AM) and as a delayed limiter with standing bias on FM. As the anode and screen potentials were not switched between the two modes, presumably the AGC bias was arranged to keep the valve away from limiting on AM. The AGC bias was derived directly from the demodulated AM signal, and so was not delayed. It was also applied to the mixer grid on AM only. On FM, AGC bias for the RF amplifier was obtained from the limiter grid.
Given that the 6C12 was operated as a limiter on FM, grid detection for AM would have been possible (and was used as a source of RF AGC bias on FM), but it is clear that as two gain stages were required on AM, this option was precluded.
That a DC gain stage was required for the AFC bias possibly reflects the fact that a ratio detector was used for FM. Had a Foster Seeley discriminator been used, then maybe there would have been enough AFC bias without a DC amplifier. But that almost certainly would have required an additional IF stage, to give two gain stages and a full limiter, meaning an extra valve envelope. In that case, though, AM grid detection would have been possible.
The AFC system was unusual in that it was a long loop, including the second frequency changer stage. Presumably the second oscillator was sufficiently stable that a situation where the AFC was correcting for second oscillator drift by pulling the first oscillator out of correct tune did not arise in a material way.
A neat feature was the use of the AM noise limiter diode to provide sound channel muting during warm-up.
In the main AGC system, the IF bias was clamped at the point where the RF bias started. This was different to the usual Mullard curves where the IF bias continued to rise after the RF bias started, with the curves crossing at the points of respective maximum bias. Apparently, it was preferable to ensure that the IF bias did not take the valve(s) to the bend in the curve (with resultant severe non-linearity) before the RF amplifier got there, the more so if sharp cutoff valves were used in the IF strip. Evidently Murphy ensured that condition by simply clamping the IF AGC bias at the appropriate point.
That Murphy circuit was interesting enough that in retrospect, it’s surprising that Murphy did not “plant” a full article in Wireless World at the time. But both the use of dual-conversion and the subsequent addition of AFC were noted in brief items:
Cheers,
Steve
