"Bias current is 50 mA on both 300B tubes. I have also tried with 80 mA on both 300B tubes.
whether it is 50mA or 80mA makes no difference to the grid DC voltage is stay on -95V."
That does not seem right. To change the plate current, the DC grid voltage must be changed with the bias control.
"DC voltage across the 10R = 2V"
That means the 300B plate current is 2V/10R = 200mA. That is not right.
whether it is 50mA or 80mA makes no difference to the grid DC voltage is stay on -95V."
That does not seem right. To change the plate current, the DC grid voltage must be changed with the bias control.
"DC voltage across the 10R = 2V"
That means the 300B plate current is 2V/10R = 200mA. That is not right.
See attached schematic, and se where R10 is locate. DO YOU MEAN R34!!!!
Attachments
SORRY, my mistake, i had read 10R as R10.
I have 80 mV across R34/R33
R34/R33 is 100 ohms. That's why I was confused when you wrote 10R
I have 80 mV across R34/R33
R34/R33 is 100 ohms. That's why I was confused when you wrote 10R
The input stage will not affect the output stage DC biasing, unless the coupling capacitor is bad.
And you say the 300B grid waveform is good, so the input stage is not the problem.
And you say the 300B grid waveform is good, so the input stage is not the problem.
Yes the 300B grid waveform is good, se post 7, the Blue waveform. What about the voltage off the waveform is the okay?
If this is with an 8R load, that may be at the limit of the amplifier output power, stated to be 6-8W.
Reduce the input by half. If the output waveform good now?
Reduce the input by half. If the output waveform good now?
it is with an 8k ohms load. The output voltage is only 1v when it start. Post 7 shows the voltage is 3v, it's not even 2W!!!!
Either the 300B tubes are weak or misbiased, since the 300B grid waveform is normal.
As mentioned before, the DC grid voltage seems too negative for the value of the plate current.
As mentioned before, the DC grid voltage seems too negative for the value of the plate current.
That's what the bias control is for.
However your negative supply is much too negative, at -278VDC instead of -150VDC as in your schematic.
Is there a problem with the negative supply circuit? That may account for the problem.
However your negative supply is much too negative, at -278VDC instead of -150VDC as in your schematic.
Is there a problem with the negative supply circuit? That may account for the problem.
From the 300B data sheet, the grid voltage should be somewhat less negative.
It lists around 60mA at -75V grid. What brand are your tubes?
It lists around 60mA at -75V grid. What brand are your tubes?