Chris Tran GM3WOJ wrote:
>
>Hi Mike et al
>
>Interesting problem - are you measuring the grid bias
>and screen volts at the socket pins, or remotely (e.g.
>some test point) ?
>
Good point - and also, how accurately are you measuring the grid and
screen voltages? Are you using a DVM or do the voltages only *look*
kind-of constant on an analog meter?
If you had not said that the screen voltage remains "constant", I would
have guessed at secondary emission from the screen, causing negative
screen current (out of the tube) that drives the voltage upward. That
causes more output, but also more secondary emission, higher voltage,
more output... until something gives.
Zener stabilization is not a proof against this, especially if there is
significant series resistance between the zener stack and the screen.
Please let us all know how accurately you're measuring the screen and
the grid voltage - right at the pins, as Chris points out, or through a
*verified* low-resistance connection.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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