I got a working px18, Bias is set around 60mV, tested working well with a 2x5.6 ohms resistance in series (20w each) and quickly tested with a testing speaker (6ohm) and a guitar.
I have got a doubt about the impedance selector, how i can measure it to find the 4 ohms and the 16 ohms selection?
I have done some measure in AC and with the speaker plugged i get:
A) 6.6 Kohms ; 10V
B) 6.2 Kohms ; 11V (middle selection so the 8ohms selection)
C) 5.5kohms, 11.25V

So which one is the 4ohms?
Thanks for your help.

regards
Antoine.

Hi Antoine,

with the amp switched off, disconnected from mains and discharged caps:
Apply a low ac voltage (say 10V) across the primary of the output transformer (connect it to the plates of the output tubes).
If the speaker jack is a switching type that shorts to ground, insert a plug into the jack so the jack is open.
Read the ac voltage across the jack for each position of the impedance switch.
The lowest is 4 Ohms, second 8 Ohms, highest 16 Ohms.
The voltage in the 16 Ohms position should be around double the voltage in the 4 Ohms position.

Best,
Peter

Just done it, works perfectly ! its very low voltage but i was able to read the diff, so i found the 16 ohms selection
Thanks you, really appreciate the tip.

regards.

Hi,

yes, the voltage is low because of the transmission ratio of the output transformer.
But for this purpose it is perfectly fine and safer than using a higher voltage.

Best, Peter