Alan Douglas adouglasatgis.net wrote:

Any tester made after, say, 1950 will probably use a lower grid
signal than 5V. The lower the better, for low-bias tubes like the
12AX7.

Two more questions:

1) Is there a risk of damaging a 12AX7 by testing it in the I-177?

2) Is there an easy way to lower the voltage of the grid signal in an I-177?

Thanks.

Hi,

Two more questions:

1) Is there a risk of damaging a 12AX7 by testing it in the I-177?

I doubt it.

2) Is there an easy way to lower the voltage of the grid signal in an I-177?

A resistive voltage divider at the 5V transformer winding would
lower the signal to (say) 2.5V, which is just what Hickok did in some
later models. *Theoretically* that would halve the Gm readings, but
in practice, you'd have to measure some known-good tubes to get a new
bogey value.

73 , Alan

Is the modification of the transformer done on the adapter unit or the I-777?

I'm illterate as to the construction of this tester but I just picked one up and would like to have a way to accurately test 9 pins.

thanks
Jonathan