[guibo]

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.

[_peter]

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

[guibo]

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.

[_peter]

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