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| Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets. |
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1st Aug 2025, 9:03 am
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#1 |
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Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,788
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Mystery Valve
Hi folks, I have a mystery Valve that has lost its markings. It appears to be an audio pentode on a B7 base, with a hefty heater and I can see a mesh anode plus 3 grids inside. Top cap is grid 1 and pins 4 and 5 are definitely heater. The heater glows dimly with a 4v supply. It looks very much like a Mullard Pen4DD but, when I test it on the VCM163, using cold leakage tests it shows a short to ground on grid1 and a"d. Any ideas if it might be a different valve, or have I most likely got a Pen4DD with internal shorts? Jerry
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1st Aug 2025, 9:18 am
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#2 |
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Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,788
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Re: Mystery Valve
I might have answered my own question. I reset thumb wheel #1 (pin1) from a"d to grid1 and fully tested the valve. It gave an anode current of 40mA (spec 36) and gm 10mA/v (spec 9.5). a'd tested 95%. So it looks like there is an internal short between grid1 and the second diode. I guess it could be made to work in a set using a semiconductor diode to replace a"d. If anybody wants it for £4 postage then drop me a PM. Jerry
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