Hi,

Ah, I see. But at least I can still get relative readings from tube
to tube, I suppose. (i.e. Get a scale reading for a known good tube,
and compare others to it.) That is of some use to me--if the readings
are somewhat linear.

Yes I think the readings are generally proprtional to Gm, which is
why the good-bad scale works. It's possible however that variations
in the operating point could swamp the Gm variations: in other words,
a tube that happened to draw more plate current for a given grid bias
might test unusually strong even though its Gm was not higher.

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.

73, Alan

In article , Alan Douglas
adouglasatgis.net writes:

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.

I'll bet my 1928 Hickok really "slams" that grid signal. It's so old that it
has a 5-pin adapter with grid cap for them new-fangled screen-grid tubes,
tetrodes or whatever they called 'em.

It seems to use a wattmeter type of meter movement (dual coils) to multiply and
correlate the grid drive nad plate output to compute the gm. Very cute unit.
--Mike K.

Oscar loves trash, but hates Spam! Delete him to reply to me.

Hi,

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.

I'll bet my 1928 Hickok really "slams" that grid signal. It's so old that it
has a 5-pin adapter with grid cap for them new-fangled screen-grid tubes,
tetrodes or whatever they called 'em.

It seems to use a wattmeter type of meter movement (dual coils) to multiply and
correlate the grid drive and plate output to compute the gm. Very cute unit.
--Mike K.

Oddly enough, that model (AC-47) uses a 2.5VAC grid signal. And
yes, it uses a dynamometer meter movement that measures AC
milliamperes directly (6.25mA F.S.). It also has a DC plate
milliammeter.

73, Alan

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