Thread: Tone Controls
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Old 13th May 2019, 4:11 am   #23
Synchrodyne
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Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
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Default Re: Tone Controls

A possibly unusual implementation of the Baxandall tone control was in the Armstrong FC38 radio/radiogram chassis, as shown here:

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Name:	Armstrong FC38 Tone Control.jpg
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The tone control circuitry itself looks quite normal, and was built around one half of a 6L19 double-triode valve. The unusual part was that the tone control is completely within the main feedback loop, which also included an active volume control. That loop ran from the 3 ohm output transformer tap back to one side of the volume control, the other side of which took the output from the demodulator or the gram input. Thus all of the AF stages were within the main loop, that is 6LD20 (triode) amplifier, 6L19 (half) tone control, 6L19 (half) phase splitter an 6P25 output pair. One may derive a block schematic showing the Baxandall loop within the main loop as follows:

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Name:	Armstrong FC38 AF Section Block Schematic.jpg
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I am not sure quite what the effect would have been from including the tone controls within the main feedback loop, but perhaps the initial slope of the curves would have been reduced, with marked steepening at the extremities.

The FC38 was released in 1953 June as the direct replacement for the EXP73 at the bottom end of Armstrong’s then chassis range. One of its features was that it had separate treble and bass tone controls, whereas its predecessor had had a domestic radio type single tone control, probably in the amplifier main feedback loop. I haven’t seen the EXP73 circuit details, but the EXP83/3 before it had an AF section comprising 6LD20 (triode) amplifier, 6LD20 (triode) phase splitter, 6P25 output pair. So it followed typical radio set practice in not having a defined preamplifier section. It looks as if Armstrong stayed with that layout for the FC38, simply inserting a Baxandall tone control stage ahead of the phase-splitter, and using a 6L19 in place of the second 6LD20 to avoid an increase in valve envelope count.

Whatever its reasons for the FC38 loop-within-a-loop circuit, Armstrong reverted to more conventional AF amplifier practice for the FC48, which superseded the FC38 in 1954 September. In this case, the control and power amplification functions were separated. The control section comprised an ECC83, the first section as 1st AF amplifier and the 2nd as the Baxandall tone control virtual earth stage. The power amplifier included an ECC81 and a 6P25 pair, with the main feedback loop returned to the 1st half ECC81 cathode. The FC48 was initially advertised as having “heavy negative feedback”, stated as 20 dB in later advertising). Presumably it had quite a bit more than was the case for the FC38, whose rather long loop might have been limiting, stability-wise.

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Name:	Armstrong FC48 Tone Control.jpg
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Quite possibly the FC38-style tone control was also used for the RF41 model, which was available from 1953 September or thereabouts and superseded the RF104, the main apparent difference being that the RF41 had separate bass and treble tone controls, whereas the RF104 would have had a single tone control.

Another apparent tone control oddity from Armstrong was that in the later iteration of the BS125 chassis. I think that it was also used in the EXP125/A and EXP125C models.

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Name:	Armstrong BS125-4 Tone Control.jpg
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At first glance it looks like a regular Baxandall circuit, following an EBC41 (triode section) 1st AF amplifier and using a 6F13 high-slope pentode as the virtual earth stage. But the catch was that the 6F13 cathode was the return point for the main feedback loop. The power amplifier section, on a separate chassis, used a 6LD20 (triode section) phase splitter and a 6P25 foursome in parallel push-pull. Thus the Baxandall loop and the main feedback loop overlapped, as may be shown in block schematic form.

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Name:	Armstrong BS125-4 AF Section Block Schematic.jpg
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This circuitry probably post-dated that for the FC38 by a couple of months or so, and had what might be described as a shorter main loop. The BS125/4 circuit dated from 1954 May. The EXP125/A was available from around 1953 September, replacing the EXP125/3. The EXP125/2 and /3, and the original BS125 had separate bass and treble controls of the switched kind. Whether they were passive or active is unknown, but they were between two 6LD20 triode sections. As far as I know, the EXP125C (from 1954 September) and revised BS125 then survived as such until around 1959 March, when they were replaced by the RF/BS125R/T group, which might have had passive tone controls.

Armstrong’s use of a 6F13 valve in this case followed Baxandall’s original recommendation that a high-slope pentode be used. He used an SP61 in his published circuit, and the 6F13 could be considered as the successor to it.


Cheers,
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