[AMPS] Creepy Amplifier

[Peter Chadwick]

Mark said:

>Have measured the grid bias, no change under "heat up conditions." 

I'd be inclined to suspect tubes producing some reverse grid current. However,
have you measured the bias at the grid pin? (Put a series resistor or an RF
Choke in series with the meter. A digital meter will get away with a series
resistor, but may tell total lies with the RF applied. An old fashioned moving
coil meter really needs a series RF Choke) It's possible that the resistance in
the return from grid to bias to cathode has gone too high, and a smidgeon of
reverse grid current  can cause problems. It may be that getting the resistance
in the grid as low as possible will allow the use of possibly dodgy tubes. One
trick suggested for 4CX1000 s (which are rated for no grid current at all) is to
use a diode to clamp across the grid feed resistor so as to prevent the grid
showing signs of going positive under reverse grid current conditions.

The reverse grid current could be due to gas, or it could be that some cathode
material has sputtered over onto the grid. I surmise that if the tubes are
fairly old and have seen a lot of service, this could happen. Worth checking if
both tubes do it, or only one - pull each tube in turn.


73

Peter G3RZP



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