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Synchrodyne · 27th Feb 2017, 10:32 pm

Rather belatedly, I thought to actually look at the circuit for the PCL83 version of the Black Box (BBA et al), and that did provide further insights.

It was actually somewhat unusual, consisting of half an ECC82 as an input stage, followed by the other half of same as phase splitter, with each PCL83 triode section acting as driver for its corresponding ultra-linear connected pentode output section. The main feedback loop went to the cathode of the input triode, but there were subsidiary loops, one to the grid of the phase splitter as part of the treble control, and another, which I haven’t figured out, but which looks as if it had both positive and negative elements, rather like the damping control loop on the Pye Mozart amplifier, but without the variable control.

That made it look somewhat like the Williamson amplifier, whose input, phase splitter and driver stages were based upon the 6SN7GT medium-mu double triode. And the ECC82 was, in a proximate sense, the miniature successor to the 6SN7GT. Not only that, but the PCL83 triode was very similar to that in the ECC82, same mu and slope, but having a higher allowable anode dissipation, 3.5 W as compared with 2.75 W.

So, even if the use of the PCL83 were effectively a forced choice for availability, inventory and/or commonality reasons, Pye evidently put some thought and effort into the design of the circuit that used it, such that it was not simply a misfit in a circuit that would have done better with a different valve.

Re the initial Black Box BBH series, I suspect that the use of a triode-connected EL42 pair was quite unusual; most car radio applications probably had them pentode-connected. Similarly their use with an ultralinear connection in the BBH Mk II series was probably equally unusual. In unusuality stakes, Pye must have been a rare case of an equipment maker who used both the conventional (Blumlein; Hafler & Keroes) ultra-linear circuit, as in the BBH Mk II and BBA, and the Walker partial cathode loading circuit, as in the Mozart HFA10 amplifier.

Cheers,

27th Feb 2017, 10:32 pm	#13
Synchrodyne Nonode Join Date: Jan 2009 Location: Papamoa Beach, Bay of Plenty, New Zealand Posts: 2,944	Re: The Mullard EL42 Rather belatedly, I thought to actually look at the circuit for the PCL83 version of the Black Box (BBA et al), and that did provide further insights. It was actually somewhat unusual, consisting of half an ECC82 as an input stage, followed by the other half of same as phase splitter, with each PCL83 triode section acting as driver for its corresponding ultra-linear connected pentode output section. The main feedback loop went to the cathode of the input triode, but there were subsidiary loops, one to the grid of the phase splitter as part of the treble control, and another, which I haven’t figured out, but which looks as if it had both positive and negative elements, rather like the damping control loop on the Pye Mozart amplifier, but without the variable control. That made it look somewhat like the Williamson amplifier, whose input, phase splitter and driver stages were based upon the 6SN7GT medium-mu double triode. And the ECC82 was, in a proximate sense, the miniature successor to the 6SN7GT. Not only that, but the PCL83 triode was very similar to that in the ECC82, same mu and slope, but having a higher allowable anode dissipation, 3.5 W as compared with 2.75 W. So, even if the use of the PCL83 were effectively a forced choice for availability, inventory and/or commonality reasons, Pye evidently put some thought and effort into the design of the circuit that used it, such that it was not simply a misfit in a circuit that would have done better with a different valve. Re the initial Black Box BBH series, I suspect that the use of a triode-connected EL42 pair was quite unusual; most car radio applications probably had them pentode-connected. Similarly their use with an ultralinear connection in the BBH Mk II series was probably equally unusual. In unusuality stakes, Pye must have been a rare case of an equipment maker who used both the conventional (Blumlein; Hafler & Keroes) ultra-linear circuit, as in the BBH Mk II and BBA, and the Walker partial cathode loading circuit, as in the Mozart HFA10 amplifier. Cheers,