Hello All:

I have an I-177-A tube tester and a Munston MX-949 A/U adapter, which
has a more modern 9-pin socket. I also have a full set of maunals
thanks to Nolan Lee and his website.

The I-177-A tester gives readings for 6L6-type tubes that are right in
line with what I would expect. (Around 5000-6000 micromhos.)

Using the MX-949 adapter, I am now trying to test several 12AX7,
12AY7, and 12AT7 tubes. But here's the problem: With the dial
settings specified in the test data tables, I seem to be getting Gm
readings that are not close to the readings given in various tube data
sheets.

Here are the dial settings I am using for 12AX7 tubes:

Fil: 12.6 volts
A: 4
B: 2
L: 0
R: 19
MMHOS: 3000

At these settings, a known good 12AX7 gives a Gm reading of around 700
micromhos for each of the two sections of the tube. That seems too
low.

If anyone has information on what a typical correct Gm value should be
for a 12AX7, 12AY7, 12AT7, and 12AU7, I would appreciate your
response. I would also appreciate input from others with an I-177
tester and MX-949 A/U adapter as to what the correct L and R dial
settings are for these tube types. (The manual could be wrong.)

Thanks for the help.

--E

Hi,
You won't get any useful Gm figures for a 12AX7 from an I-177. Its
5VAC of grid signal is way more than a 12AX7 can take without driving
the grid positive.

73, Alan

Alan--

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.

Oddly, the "Good/Bad" reading seems to work properly for 12AX7's.
(Known good tubes test in the midle of the green range, and a couple
of known weak tubes test just below green or into red.) Can you
comment on that?

Questions:

Do I risk damaging these 9-pin tubes by testing them in the I-177?

Is there a preferred Gm-capable tester for 9-pin tubes?

Thanks,

--E


P.S. I forgot to mention that my attempted Gm readings were done with
the "L" dial set to "Gm" (i.e., 60), as is standard practice.

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