[Amps] 2M 8877 Amplifier: No grid or plate current

[jeff millar]

Sounds like something wrong with the cathode circuit.

Suggest the following tests.

Inspect the path from cathode to ground, including the cutoff bias resistor, key relay, and bias 
circuit.  Maybe the circuit never got assembled correctly

With the power off, measure the resistance from cathode to bias circuit.  It should run about 10K 
ohms due to the cut off bias resistor.

With the amplifier High Voltage off, apply 12V through a 1K resistor to the cathode pin and look for 
  voltage on the cutoff bias resistor and zener diode (or other bias circuit).  Key the amplifer and 
verify about 5 V on the bias circuit.

With power off and the cathode bias circuit shorted, activate the key relay and measure the 
resistance from cathode to ground.  It should be zero

Measure the cathode pin voltage with the amplifier powered but not keyed.  It should run 10-50V 
indicating enough current flowing to generate cut off bias.

With the amplifier keyed measure the cathode voltage, it should be about 5V, but will probably be 
10-50V indicate the tube is still biased to cutoff.

Hope this helps...

jeff, wa1hco

W.F van Wyk wrote:
> Greetings,
>  
> I am attempting to commission an 2 meter 8877 home built amplifier.
>  
> As I did not do the construction I have to reverse engineer.
>  
> The mechanical and electronic construction do look sound and according to
> the schematic I have with the amp.
>  
> There is about 2,200 Volts on the anode and 4,7 Volts on the filaments.
>  
> Problem is with or without drive I get to see not Grid or Plate current on
> the meter (which is functioning).
>  
> Also I get to see the RF through but when I initiate the AMP the RF through
> decreases and no setting will get any response.
>  
> Other than the tube being damaged what else can I look for to be the
> problem?
>  
> Any tests I can do on the tube?
>  
>  
> 73
>  
> Wynand
>  
> ZS6ARF
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