plate voltage

[bluerabbitjames]

hello

i have a plate voltage at 370v (pin 3/ground) on v4 and v5
which seems a bit low
I biased to a plate current (pin3/ output transformer center tap) at 47ma
is that correct? with EL34s

[funkmeblue]

Ken wrote in the trainwreck pages that to bias an express amp use the voltage method. -30 volts on pin 5 of the el34's. This allows safe use of both 6v6 or el34. I'm sure if you were sticking with one tube type the bias could be dialed in further. This method works well for me. I dialed it in by ear and it ended up real close to this.

[jjman]

Are you using the voltage-drop method or the shunted-current approach to measure the idle current? I don't like either method but would use both if there are no "current sensing" resistors on the cathodes. Then I would compare the results to verify they are the same so either approach can be trusted. Both approaches are very dangerous BTW and can be way off if the meter has high internal resistance in current mode and/or poor accuracy measuring low dc voltages.

What is the dc voltage on the grids at this time? What is the heater voltage? What is your expected plate voltage based on?

[bluerabbitjames]

hi

yes iam using the output transformer shunt method
the heater are at 6.3v
i expected 415v on the anode but think 370v will be ok
the negative voltage on pin 5 v4 and 5 is -27.5v
with the current at 47ma

[jjman]

It's possible that the tubes are idling hotter than the meter shows. This would cause lower than expected plate voltage. If the meter has significant internal resistance it is artificially cooling down the measured current flow during the shunt. Even a high quality meter can pose a significant skew. You could try the "voltage drop" method assuming the meter can measure low dc voltages accurately.

Is the high voltage winding on the power tranny showing expected ac voltage (under the load?)

Could also be a bad filter cap or some other things too.

[bluerabbitjames]

hi
power transformer secondary is at 578v ac which is ok
but my question was more about the biasing
if i do not harm my tubes with the 370v plate voltage and 47ma current