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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 Jul 2020, 10:48 pm
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#1 |
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Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Mullard PM12 valve.
This early RF tetrode has become de-based, it's easy to identify the filament leads but which one is the control grid?
To find out which wire it is, a 1.5volt cell was connected to the filament leads. Next, connect a 390Kohm resistor to the positive terminal of the cell. The positive lead of a digital meter was connected to the free end of the resistor and negative meter lead to one of leads on the valve. One lead gave a reading of 0.052volt and the other gave a reading of 1.43volt. The one with the lowest reading has to be the control grid. Actually, the lead with the sleeve on it was most likely to be the screen grid but one has to make sure. Now the base can be refitted. DFWB. Last edited by FERNSEH; 1st Jul 2020 at 10:56 pm. |
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1st Jul 2020, 11:21 pm
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#2 |
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Dekatron
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
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Re: Mullard PM12 valve.
Correction to my last post. Instead of connecting the test meter between the resistor and one of the free leads on the valve, the resistor was connected direct to each lead in turn and the voltage across it measured. One wire gave hardly any reading at all and the other showed 0.943V.
The higher voltage across the resistor confirms which wire is the control grid connection. So in fact it is the wire with the sleeve that is the control grid. DFWB. |
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