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Old 12th Nov 2020, 9:32 pm   #1
FERNSEH
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Default Clarke and Smith schools radio receiver.

For a client I'm working my way through a Clark and Smith schools radio receiver.
Set now works reasonably well but some comments about the push-pull amplifier might be of interest.
The attached circuit diagram shows the components in the grid circuit of one of the EL84 output valves.
Just what is the purpose of those components?

DFWB.
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Old 13th Nov 2020, 12:20 am   #2
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Default Re: Clark and Smith schools radio receiver.

Empirical hoot suppression?

Obviously a frequency response shaping circuit of some sort but why in only one side?
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Old 13th Nov 2020, 7:49 am   #3
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Default Re: Clark and Smith schools radio receiver.

Remember that the right hand half of the double triode has feedback around it and that the anode node impedance of the left half gets involved.

I'm not sure whether it's an attempt to tweak the phase of signals arriving at the EL84 grids, or something to keep the phase-balancing loop stable.

It would take a thorough simulation with realistic node impedances from the valves to see.

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Old 13th Nov 2020, 9:09 am   #4
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Default Re: Clark and Smith schools radio receiver.

Is there an overall NFB loop covering more than just the shown components?
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Old 13th Nov 2020, 2:00 pm   #5
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Default Re: Clark and Smith schools radio receiver.

"Is there an overall NFB loop covering more than just the shown components?"

Will check that out later today.
For comparison, the circuit of the Pye G63 "Black Box".

DFWB.

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Old 13th Nov 2020, 5:01 pm   #6
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Default Re: Clark and Smith schools radio receiver.

The 47k and 500pF looks like a high-frequency partial roll-off, possibly C & S had trouble with parasitic oscillations.

The 4.7M and 2000pF will give a lot of bass attenuation, again, it will have a limit as frequency is reduced - until the effects of the 0.1μF coupling capacitor kicks in. But that will be below 1Hz.

It's only on one side, because the pick-off point for the phase inverter is downstream of these two coupling networks so the other valve will replicate what's on the 'upper' grid.

Would be interesting to see the effects of removing, each one in turn!
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Old 13th Nov 2020, 7:43 pm   #7
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Default Re: Clark and Smith schools radio receiver.

Quote:
Originally Posted by kalee20 View Post
It's only on one side, because the pick-off point for the phase inverter is downstream of these two coupling networks so the other valve will replicate what's on the 'upper' grid.
Ah thanks!
I wonder how much symmetrical circuit design is driven by aesthetics, rather than necessity?
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Old 13th Nov 2020, 8:36 pm   #8
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Default Re: Clark and Smith schools radio receiver.

kalee20 wrote: "The 47k and 500pF looks like a high-frequency partial roll-off, possibly C & S had trouble with parasitic oscillations."

The Williamson amplifier has a series connected resistor and capacitor shunted across the anode load of the first amplifier stage.

In the C & S schools receiver the ECC83 phase splitter is preceded by an EF86 amplifier.

DFWB.
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Old 14th Nov 2020, 11:34 am   #9
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Default Re: Clarke and Smith schools radio receiver.

The attachment shows the underside of the amplifier chassis.
The tone corrector components are inside the circle.
The EF86 amplifier consists of a 2.2Kohm cathode bias resistor bypassed with a 100uf capacitor, 1Megohm screen feed resistor and a 220Kohm anode load resistor.

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