Tube guitar amp help needed

[mich]

i've built the P1 amp from ax84.com (SE EL84 with one preamp tube),
and im trying to use the backward trannie thing for the B+.
so i have 6V @ 1A trannie for the filaments of 12AT7+EL84 ,and 2 transformers 9V @ 1.3A for the B+.

i calculated 40mA for biasing the output tube over 250V wich gives
10W of power needed,and the transformers can supply 1.3A x 9V=11.7W

is this correct?

im having problems getting more then 23mA on the cathode of EL84...

i have 250V B+ and 245V on plate,and im using 4k to 4R OT with 4R 10W
resistor as load.

any help would be apriciated....thanks

 

[mich]

i'll refrase my question...

when using the "back to back" arrangement with power transformers,e.g:
230:9 - 9:230 with 1A rating for both transformers (like on jakob's tube designs) - what will the current rating be?

is it 0.7A beacause of the rectification?

 

[Boswell]

I'm not 100% sure what you are asking, but I would allow for only half my mains input power to be available at the d.c. B+ line. If you want 250V 40mA there (10W), I would use 20W transformers.

Assuming sufficient anode volts, the current through the EL84 is governed by the grid-cathode voltage, which in self-bias configuration is determined by the value of the cathode resistor. Try reducing the value of the cathode resistor to about 70% of whatever it is now and see what current you get.

How are you measuring the current, anyway? A meter in series with the cathode will give a false low value because of the voltage drop across the meter terminals. With the power off, you should use the meter to measure the value of the cathode resistor in Ohms with one end disconnected from the circuit, then measure the volts across it in normal operation. Ohm's law gives you the current.

 

[mich]

i calculate current draw of anode by measuring cathode resistor while the amp is off,and then voltage on cathode divided by this value minus the screen current wich i measure by dividing the voltage drop across the screen resistor by its value.
i dont lift one leg of cathode resistor ...

i lowered the cathode resistor value to 1 ohm and still the tube draws about 30mA max.
i dont know how to get this up to 50mA....what else could limit this current other then the power supply?

 

[shabtek]

what else could limit this current other then the power supply?

what is the resistor from input/grid to ground? i think this will also determine quiesent current...i am thinking of output tubes that are biased with a negative voltage--more negative=more cut-off/less current

try reducing the value of this and see if idle current goes up

 

[mich]

what is the resistor from input/grid to ground? i think this will also determine quiesent current...i am thinking of output tubes that are biased with a negative voltage--more negative=more cut-off/less current

try reducing the value of this and see if idle current goes up

its 2.2M wich is in paralel with the tone stack (250K+1M+25K)
ill try reducing it and report back-thanks

 

[mich]

tried lowering the grid to ground resistor till 0 ohm with no increase in idle current....any other idea?

i would love to know if its my back to back power supply arrangement causing this-but have no tube PT to subtitue and verify......