vacuum tube testing

D

Donald Ennis

Guest
I need some assistance with testing some 6EJ7 vacuum tubes. In the past I
have used several tube testers which are inoperable now. I have access to
power supplies, meters, scopes, fixtures and fixturing equipment and
supplies. I have the specs for the tubes but I'm unsure about setting up
the tests and interpreting the results. If anyone can help please post
back.

Thanks,
Don
 
"Dave" <dspear99ca@yahoo.com> wrote in message
news:6IT1k.1278$Gn.450@edtnps92...
Call your local music store that advertises servicing guitar amps.
Whoever does the work likely has a tester, or knows somebody who does.

I picked up an old tester recently, it'll tell you good/bad, but no good
for gain matching, it's not a mutual conductance tester.

Or just buy one. Never heard of anybody making their own, even the
hard-core geeks on AudioAsylum and AudioKarma don't do it, there's
probably a reason for that...
Click to expand...
Home brew CRT testers were very common in the service trade, the most
difficult part was rigging the interchangeable bases for all the different
CRT types.
 
Ok. A big thanks to everyone for the replies. I have done this before
about twenty years ago but I had the help of an older gentleman who knew
tubes very well. I had notes on how to test tubes using the mfr. specs and
how to interpret the results but can't find them now. Once again I
appreciate your comments and thanks for your help.

Don

"Dave" <dspear99ca@yahoo.com> wrote in message
news:6IT1k.1278$Gn.450@edtnps92...
Call your local music store that advertises servicing guitar amps.
Whoever does the work likely has a tester, or knows somebody who does.

I picked up an old tester recently, it'll tell you good/bad, but no good
for gain matching, it's not a mutual conductance tester.

Or just buy one. Never heard of anybody making their own, even the
hard-core geeks on AudioAsylum and AudioKarma don't do it, there's
probably a reason for that...

I saw an on-line video about Manley Labs (tube amp manufacturer) in
California... they've got a nifty computerized rig where a computer
controls the inputs and monitors the outputs of tubes being tested... I am
sure if tubes were still used in large quantities such software would
probably be available for free on the internet.

Dave

"Donald Ennis" <lisdon@ameritech.net> wrote in message
news:afT1k.4927$xZ.3627@nlpi070.nbdc.sbc.com...
I need some assistance with testing some 6EJ7 vacuum tubes. In the past I
have used several tube testers which are inoperable now. I have access to
power supplies, meters, scopes, fixtures and fixturing equipment and
supplies. I have the specs for the tubes but I'm unsure about setting up
the tests and interpreting the results. If anyone can help please post
back.

Thanks,
Don
Click to expand...
 
Thanks Stephen. This is what I'm attempting to do. I have been varying the
control grid voltage from -2 volts to -3 volts while keeping screen grid at
200 volts. Supply voltage (B+) is 350 vice. Plate resistor is 15K and Grid
is coupled to ground with 10K resistor. According to the curves for pate
current vs. control grid volts I should be getting 10 ma plate current @ 200
volts plate voltage with -2.5 volts at control grid. What I'm getting is
about 187 plate volts and 16 to 17 ma. plate current at this point. On some
of the tubes the current goes quite high and the plate voltage drops to
around 120 vdc with the control grid @ -2vdc. When I change the control
grid voltage to -3 vdc the plate current sometimes drops off sharply to ua
and the plate voltage climbs to around 260 and 280 vdc (I'm doing this from
memory). To me it appears that the tubes are bad but I'm not quite sure how
to interpret the measurements. Any thoughts on this?



Thanks,

Don
"Stephen J. Rush" <sjrush@comcast.net> wrote in message
news:2ZOdnZIZf_gBNNXVnZ2dnUVZ_o7inZ2d@comcast.com...
On Thu, 05 Jun 2008 11:03:32 -0400, Donald Ennis wrote:

I need some assistance with testing some 6EJ7 vacuum tubes. In the past
I have used several tube testers which are inoperable now. I have
access to power supplies, meters, scopes, fixtures and fixturing
equipment and supplies. I have the specs for the tubes but I'm unsure
about setting up the tests and interpreting the results. If anyone can
help please post back.

The basic figure of merit for an amplifier tube is transconductance, aka
mutual conductance. It's the change in plate current for a given change
in grid voltage, and it's usually given in micromhos. The simplest way
to measure it is to set up normal operating conditions and shift the bias
voltage while measuring the plate current. You can get the values from a
tube manual. For audio work, just build a resistance-coupled amplifier
stage around the tube and probe the grid and plate with a scope. Knowing
the plate load resistance, you can easily compute the plate current swing.
Click to expand...
 
Thanks Nick, what a great site. I'll have to take some time and look it
over.

Don
"C. Nick Kruzer" <insula@webtv.net> wrote in message
news:29839-484A3D10-157@storefull-3254.bay.webtv.net...
Hi Don,

I found this which might be of help:

http://www.tone-lizard.com/Tube_Testers.html

insula
Click to expand...
 
Donald Ennis wrote:
I need some assistance with testing some 6EJ7 vacuum tubes. In the past I
have used several tube testers which are inoperable now. I have access to
power supplies, meters, scopes, fixtures and fixturing equipment and
supplies. I have the specs for the tubes but I'm unsure about setting up
the tests and interpreting the results. If anyone can help please post
back.

I can only observe that you may be best served by spending your time
Click to expand...
fixing one of your proven tube testers rather than trying to build one
from scratch.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html
 
Call your local music store that advertises servicing guitar amps. Whoever
does the work likely has a tester, or knows somebody who does.

I picked up an old tester recently, it'll tell you good/bad, but no good for
gain matching, it's not a mutual conductance tester.

Or just buy one. Never heard of anybody making their own, even the
hard-core geeks on AudioAsylum and AudioKarma don't do it, there's probably
a reason for that...

I saw an on-line video about Manley Labs (tube amp manufacturer) in
California... they've got a nifty computerized rig where a computer controls
the inputs and monitors the outputs of tubes being tested... I am sure if
tubes were still used in large quantities such software would probably be
available for free on the internet.

Dave

"Donald Ennis" <lisdon@ameritech.net> wrote in message
news:afT1k.4927$xZ.3627@nlpi070.nbdc.sbc.com...
I need some assistance with testing some 6EJ7 vacuum tubes. In the past I
have used several tube testers which are inoperable now. I have access to
power supplies, meters, scopes, fixtures and fixturing equipment and
supplies. I have the specs for the tubes but I'm unsure about setting up
the tests and interpreting the results. If anyone can help please post
back.

Thanks,
Don
Click to expand...
 
"Tim Wescott" <tim@seemywebsite.com> wrote in message
news:jZSdnWAGPYC5ntXVnZ2dnUVZ_judnZ2d@web-ster.com...
I can only observe that you may be best served by spending your time
fixing one of your proven tube testers rather than trying to build one
from scratch.

--

Yeah, what he said.
Click to expand...
 
On Thu, 05 Jun 2008 11:03:32 -0400, Donald Ennis wrote:

I need some assistance with testing some 6EJ7 vacuum tubes. In the past
I have used several tube testers which are inoperable now. I have
access to power supplies, meters, scopes, fixtures and fixturing
equipment and supplies. I have the specs for the tubes but I'm unsure
about setting up the tests and interpreting the results. If anyone can
help please post back.
Click to expand...
The basic figure of merit for an amplifier tube is transconductance, aka
mutual conductance. It's the change in plate current for a given change
in grid voltage, and it's usually given in micromhos. The simplest way
to measure it is to set up normal operating conditions and shift the bias
voltage while measuring the plate current. You can get the values from a
tube manual. For audio work, just build a resistance-coupled amplifier
stage around the tube and probe the grid and plate with a scope. Knowing
the plate load resistance, you can easily compute the plate current swing.
 
Hi Don,

I found this which might be of help:

http://www.tone-lizard.com/Tube_Testers.html

insula
 
Donald Ennis Inscribed thus:

Thanks Nick, what a great site. I'll have to take some time and look
it over.

Don
"C. Nick Kruzer" <insula@webtv.net> wrote in message
news:29839-484A3D10-157@storefull-3254.bay.webtv.net...
Hi Don,

I found this which might be of help:

http://www.tone-lizard.com/Tube_Testers.html

insula

What a fascinating web site ! I would also like to offer my thanks to
Click to expand...
OP for bringing it to my attention.

--
Best Reagrds:
Baron.
 
On Fri, 06 Jun 2008 22:07:03 -0400, Donald Ennis top-posted:
[top-post repaired below]

"Stephen J. Rush" <sjrush@comcast.net> wrote in message
On Thu, 05 Jun 2008 11:03:32 -0400, Donald Ennis wrote:
I need some assistance with testing some 6EJ7 vacuum tubes.
...
The basic figure of merit for an amplifier tube is transconductance, aka
mutual conductance. It's the change in plate current for a given change
in grid voltage, and it's usually given in micromhos.
...
Thanks Stephen. This is what I'm attempting to do. I have been varying
the control grid voltage from -2 volts to -3 volts while keeping screen
grid at 200 volts. Supply voltage (B+) is 350 vice. Plate resistor is
15K and Grid is coupled to ground with 10K resistor. According to the
curves for pate current vs. control grid volts I should be getting 10 ma
plate current @ 200 volts plate voltage with -2.5 volts at control grid.
What I'm getting is about 187 plate volts and 16 to 17 ma. plate current
at this point. On some of the tubes the current goes quite high and the
plate voltage drops to around 120 vdc with the control grid @ -2vdc. When
I change the control grid voltage to -3 vdc the plate current sometimes
drops off sharply to ua and the plate voltage climbs to around 260 and 280
vdc (I'm doing this from memory). To me it appears that the tubes are bad
but I'm not quite sure how to interpret the measurements. Any thoughts on
this?
Click to expand...
It sounds like it's operating properly - do this with some graph paper
and the curves will look very much like the ones in the book, depending
on how they're arranged.

Good Luck!
Rich
 
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