Puzzling Pentode Problem, 1951 radio

Tabby · 2020-08-01T04:23:31+00:00

A Pye P43U 4 tube radio: it works, but all is not well with the output pentode section and I'm not fully clear why. shows the circuit. Problem: Ia should be 48mA, Is 9mA so Ik 57mA. It starts off at 66mA and rises in a few minutes to 77mA, which is the abs max Iq value for the UL41 pentode according to UL41 datasheet. Why is it rising uncontrolled? Grid: goes to 0v dc-wise. There are 2 caps but if either were leaky or even shorted, Vgrid would go -ve not up. This is not the traditional cap from previous anode circuit. Screen: connects direct to the secondary smoothed B+, as recommended in the valve data sheet. Cathode: Rk is 120R nominal, 110R real. This is what I suspect, UL41 datasheet recommendes 170R, which would explain the raised i, but... Why are Pye using 120R? Why is Ia rising? Is it a faulty valve? (I don't have a spare UL41). PS 3 of the 5 power Rs were burnt out, presumably due to overcurrent in the UL41. NT

Aug 01, 2020 40 Replies

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Wond 6 years ago

I too would like to know which _transistor_ radio is "far better"! Or perhaps you were thinking _transformer_?

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Puzzling Pentode Problem, 1951 radio

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