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Does anyone know why Radford used a 6U8A triode-pentode as a long-tail pair phase splitter? It occurs in the STA25, for instance, and it seems a very strange arrangement to me but perhaps I'm missing something?
Follow Ups:
Hi Ray,This circuit was described by A. R. Bailey in a three page article, entitled "New Phase-Splitter" appearing in the September 1962 issue of Wireless World. The author addresses the high-frequency roll-off displayed by the ECC83 in the conventional long-tail-pair phase-splitter (28db gain, but 2db down ~ 12khz). He achieves 24db gain with the ECF82 tube, but the high-frequency roll-off does not commence until 40khz, and does not fall 2db until after 100khz....
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Thanks, Graeme, that makes sense, especially if the input stage to the splitter has a high OP impedance. The additional loading on the first stage of an ECC83 splitter's Miller capacitance could also cause distortion, I suppose. Still, I wonder how Radford achieved balance with such an unlikely arrangement and why no other designers ever used this idea?
Are you sure it's a LTP? Scott used pentode/triodes for gain and splitting in a paraphase circuit.
Eli D.
Yes, I checked for that. The second (triode) section of the splitter has a grounded grid and is cathode-coupled to ther first )pentode) section. It looks like a classic LTP, except for the fact that one half is a pentode and the other half is a triode.I've never seen this design anywhere else. Radford could have used a triode-hexode frequency changer tube for this purpose, I imagine!
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