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rec.audio.tubes tube tester project, by Steve Bench

Click here for updated
Schematic

Click on here for a variable
voltage filament supply to accomodate odd voltage power triodes (2A3,
300B, 50, etc.)

Background:

There are 3 fundamental Vacuum Tube (Valve) constants. These are
transconductance (gm), plate resistance (rp) and mu. For tetrode and/or pentode
devices, mu is not significant, since the plate resistance is usually much
higher than the load resistance. There is a simple relationship between these:
mu = gm * rp. In a triode, the mu is substantially geometric factor, so it does
not change much as the tube ages. Rather, the gm decreases with time and the rp
increases. Therefore, a measure of the goodness of a tube is generally related
to its measured gm. This is done in a "transconductance" tube tester, but, as
the specific voltage and current used in a particular application is not
possible or practical to set up, this limits the usefulness of the traditional
tube tester. The purpose of the described device is to circumvent these
limitations, and allow evaluation of tubes under operating conditions really
used in your specific application.

DEFINITIONS:

Transconductance:

This is defined as the incremental change in plate current for an incremental
change in grid voltage, with all other parameters (plate voltage, for example)
held constant. The way this is done is to place a small AC voltage (lets say 100
mV) on the grid and measure the output AC current on the plate. In practice,
this is done by measuring the voltage across a small resistor, (lets say 100
ohms) connected from plate to a constant DC voltage source. The current can be
controlled by placing a constant current source in the cathode circuit of the
tube under test, and bypass the cathode for AC purposes. For the example given
(100 mV AC on the grid, and a 100 ohm plate "current sensing" resistor), a
transconductance of 1 mS (1000 micro mhos) would be indicated as a 10 mV signal
across the 100 ohm resistor.

Mu:

This is defined as the incremental change in plate voltage for an incremental
change in grid voltage, with all other parameters (plate current, for example)
mV) on the grid and measure the resulting AC voltage on the plate, with the
plate connected to a high impedance load (current source). For the example
given, (100 mV AC on the grid), a mu of 20 would be indicated as a 2 volt signal
at the plate. Note: The "resistance" of the constant current load must be
substantially higher than the plate resistance of the tube under test for the
results to be accurate. Plate resistance: This is defined as the incremental
change in plate voltage for an incremental change in plate current with all
other parameters held constant. This is not directly measured in the proposed
project (at least initially) but is calculated by the formula rp = mu/gm.

SYSTEM BLOCKS:

Sources:

1. Filament Voltage: Initially fixed at 6.3VAC (no grumbling, we'll
"improve" on this over time). This supply is referenced to about 40VDC to
allow realistic confirmation of (no) heater to cathode leakage or shorts.

2. Plate (anode) Voltage: Lets say 300 VDC, at 50 mA max. Initially this
will be allowed to vary from about 40 volts to about 300 volts, controlled
by a small potentiometer operating a regulated supply. The 50 mA allows both
small signal and power tubes to be measured. This supply is current limited
at 50 or so mA, to handle the defective "shorted tube" case.

3. Screen Voltage: Same as #2, independently controlled. 4. Plate current:
Actually a part of plate voltage control. This is operable only in "mu" mode
to provide a high impedance load as indicated above. The actual tube current
is controlled by the cathode current sink (see below), and this is adjusted
for the test condition voltage.

4. AC: 100 mV sine wave at about 1 kHz. This source is protected against
grid to plate or grid to cathode shorts.

Sinks:

1. Cathode current: Variable from about 100 microamps to 50 mA via a
potentiometer controlling a constant current circuit. This allows gm/mu to
be measured at any desired current level. Combined with the variable plate
voltage source, gm/mu can be measured over a range of voltage and current.
This sink is tied to a negative (about) 60 volt source, to simulate bias
conditions to about -60 volts, primarily for testing of power tubes. Notice
that the actual voltage applied to the tube will therefore be up to about
360 volts.

Controls:

1. (S1) On/Off.

2. (S2) gm/mu switch.

3. (S3) Side1/Side2 switch for switching between "halves" of dual triodes.

4. (S4) Triode/Pentode switch to allow "triode connection" of pentodes.

5. (VR1) Plate voltage control.

6. (VR2) Plate current voltage adjust (mu mode).

7. (VR3) Tube current control.

8. (VR4) Screen voltage control (tetrode/pentodes only).

Indicators:

1. 2 jacks for DMM connection. The DMM measures AC voltage in gm mode, and
DC plate voltage and AC voltage in mu mode.

2. Green Power LED.

3. Yellow LED that illuminates if grid is driven positive.

4. Red LED that illuminates on H-K leakage or short.

5. Red LED that illuminates on high current (plate etc shorted).

Sockets:

1. Octal: (7AC/7S/8EP) Handles KT66/EL34/6L6/6550/6V6-GT (pins 1&8
connected together).

(Uncle Ned notes: 7027 would require disconnecting Pin 1 from Pin 8.
6BG6-G/GA and 6CD6-G/GA/7867 by adding a plate/anode cap. Possibly 6B4-G could
be accomdated too...)

2. Octal: (8BD) Handles 6BL7/6SN7/6SL7/6AS7/6080/6336/6528 etc.

3. 9 pin: (9A) Handles 12AT7/AU7/AX7/ECC81-3/12BH7 etc.

4. 9 pin: (9AJ/9DE) Handles 6DJ8/6BK7/BQ7/BZ7/6CG7/6922 etc.

5. 9 pin: (9V) Handles 417/5842

6. 9 pin: (9BF) Handles 12BY7/12GN7/ etc.

7. 9 pin: (9CV) Handles 6BQ5/6CW5/7189/El84 etc.

8. 7 pin: (7BK/7CM) Handles 6AU6/6AH6/6GM6 etc (pins 2&7 connected
together).

OPERATION:

gm test:

Procedure:

Plug the tube into the appropriate socket, set the gm/mu switch to the gm
position. Set the desired plate voltage and the desired current level. Read
the AC voltage on the DMM.

Reading GM 1 mV 100 umhos (0.1 mS) 10 mV

1000 umhos (1.0 mS) 100 mV 10000 umhos (10.0 mS) etc.

A "constant current" is fed into the cathode. This is bypassed for the
transconductance measurement. This allows the grid-cathode voltage to be
established by the tube itself. There is a warning LED to indicate that the
desired current has caused the grid to go into grid conduction region. This
constant current is one of the "variables" that we can use to evaluate the tube
under test, so that gm can be plotted vs current. A constant voltage is set onto
the plate, and this is the other "variable" we can use to evaluate the tube
under test. A 100 mV AC signal is applied to the grid, and the gm is found by
measuring the AC voltage produced across a 100 ohm sampling resistor. mu test:
Procedure: Plug the tube into the appropriate socket, set the gm/mu switch to
the mu position. This test is only going to work with triodes. Set the desired
level, and adjust the "plate current voltage adjust" to the desired plate
voltage level by reading the DC voltage with the DMM. Then switch the DMM to
AC voltage and read the AC voltage on the DMM.

Reading MU

100 mV 1 V

1.0 V 10

10.0 V 100 etc.

A "constant current" is fed to the cathode. This is bypassed for AC purposes
to allow the mu measurement. This allows the grid-cathode voltage to be
established by the tube itself. There is a warning LED to indicate that the
desired current has caused the grid to go into grid conduction region. This
constant current is one of the "variables" that we can use to evaluate the tube
under test, so mu can be plotted vs current. The plate voltage is established
via a quasi-constant current source whose output resistance is much higher than
the plate resistance of the tube, allowing an accurate mu measurement. This
allows plate voltage to be varied, so that mu may be plotted against plate
voltage. The mu is found by simply measuring the AC voltage on the plate.

CIRCUIT DESCRIPTION

The power supply uses 2 12.6VCT transformers connected back to back. This is
used for the 6.3V for the filaments then provides an isolated (about) 105-110
volts AC. Two DC voltages are developed. The first is a voltage tripler to give
back a loaded voltage of about 330VDC (With no tube load, it provides about 400
volts). This wimpy approach was taken purposely to minimize heat loading on the
"guts" of the circuit under abnormal (shorted tube) conditions. A 2.2 mA
constant current source drives a set of zener diodes, to establish a constant
voltage reference of about 306 volts. This is fed to 2 separate VFET "source
follower" regulators. The gates are simply fed with pots refered to the
regulated voltage. Each regulator is also current limited. The second main
supply is a negative half wave rectified supply that provides 60 to 100 volts
(depending on load current) for the constant current source that drives the
cathode(s). The negative supply has a fairly healthy 20 mA bleeder on it. In
the bleeder string is a 10 volt zener used to provide a voltage reference for
the current source, and a 5.1V zener sitting on the ground side. This is used to
drive a CMOS 1 kHz oscillator.Each regulator is current limited by a simple
transistor "starving" the gate of the source follower. The 22 ohm "sampling"
resistor causes current limit to occur at about 25 mA. This resistor may be
altered if desired. The plate side is limited at 55 mA by using a 10 ohm
resistor. The main tube current source uses a 10 volt zener to establish a
constant gate voltage, adjustable from about 2.5 to about 10 volts. This causes
the 133 ohm resistor in the FET source to provide a constant current of about
0.1 mA to about 50 mA.

A word of caution on the FETs. Make sure the resistor that's in series
with the gate lead is AT THE FET. This prevents the critters from oscillating at
some VERY high frequency. Also, note that although these parts are rugged IN THE
CIRCUIT, they can be blown by static charge while assembling the circuit.

The 1 kHz oscillator is a schmitt trigger oscillator. The "triangle" is fed
through another part of the inverter package, which rounds it a bit more and
then filtered and divided to 100 mV. This produces a relatively pure sine wave
with less than 1k source impedance.

The 6.3VAC is referenced to 51VDC via a 47k resistor and a LED. This provides
indication of heater to cathode leakage or short. Using "universal" 120-240
transformers allows easy build by anyone. Note that the second transformer is
powered from the first one (the 12 volt windings are coupled together) and the
high voltage produced is always wired 120V.

Note however, the first transformer should be wired for either 120 or 240
depending on your high tension source.

CALIBRATION:

After conpleting the unit and finding the 4 or 5 things you did wrong, you
should be pleasantly suprised by the green LED ON.

With NO tubes installed, the following voltages should be present:

Point

Voltage

Notes

A

420VDC

380 to 430 volts is OK

B

306VDC

296 to 316 volts is OK

C

----------------->>>

This will vary from 0 to 300 volts depending on VR1.
If you set this to about 200 volts, then measure current to ground,
you should see about 55 mA (50-65).

D

------------------>>>

This will vary from 0 to 300 volts depending on VR3.
If you set this to about 200 volts, then measure current to ground,
you should see about 25 mA (20-30).

E

-100V

-80 to -110 volts is OK. This is the current source
output.

F

-110VDC

-85 to -120 is OK.

G

-100VDC

Should be 10 volts more positive than F.

H

-4.6VDC

Yeah, I know its a 5.1V zener. Trust me.

J

------->>>>

This will vary form 0 to about 250 mV AC rms 1 kHz.
The frequency ought to be within 200 Hz of 1kHz. Level is controlled
by VR4.

Calibrate Plate Voltage (VR1):

With a voltmeter connected to point C, calibrate VR1. This will be linear
taper. I find I can make minor "ticks" every 10 volts, major ticks every 50
volts from 0 to 300 volts. Since there is no "load" on this point, you could
temporarily place a 100k resistor to ground to provide some load to make the
calibration more accurate.

Calibrate Screen Voltage (VR3):

Same procedure as above. except point D and calibrating VR3.

Calibrate current source (VR2):

Connect a milliameter from point E to ground. You should start to see current
flowing at about 20 degrees of rotation on VR2. If you have to go much more
clockwise to see current flowing raise R15 (270k) to 330k or higher. If you see
more than 100 uA flowing fully counterclockwise lower R15 to 220k or lower. The
220k across the pot (R17) creates a somewhat log taper. I found I could make
minor ticks .1 mA to .5 mA, then 1 mA, then 1 mA ticks from 1 to 10 mA, 2 mA
ticks to 20 mA, and 5 mA ticks from 20 to 50 mA.

Set AC Level (VR4):

Connect an AC VM from point J to ground. Set the voltage to 103 mV +/- 2 mV
with no load otherwise attached. This will make the operating voltage very
nearly 100 mV across the range of currents and voltages. Thats all there is to
the calibration.

PARTS LIST

Most of the parts are available from Digi-key or Mouser. The exception is
the tube sockets, so you'll have to go to Triode.

I have not listed chassis, hardware, knobs, and the like. Use what you like.
I used an old Lafayette (!) rip off of the old Ten-tec boxes that is about
12"x8"x 6" or so. Also, sometimes there's a price break at a larger quantity,
so feel free to order extras for another project. E.g., 1N4007 diodes. I
generally order 100 at a shot, you can use the extras by bending the end of each
lead slightly. They are perfect for hanging ornaments on your (place your
holiday here) tree. Ho Ho Ho!

Ed. Note: We have the sockets
and most of the capacitors
(or higher voltage rating equivalents). There are, I'm pretty sure, SK or
ECG equivalents for most of the diodes & transistors, if you want to buy
them at local distributors.

QTY.

DESC

REF

6

100 uF 350V Elec.

C1, C2, C3, C6, C7, C8

2

47 uF 450V Elec.

C4, C5

1

47 uF 10V Elec or Tant.

C9

2

.1 uF mylar, poly, etc.

C10, C13

4

.01 500V+

C11, C14, C16, C17

1

1.0 uF 50V+

C12

1

.22 uF 50V +

C15

9

1N4007 1A 1KV diodes

CR1-7, CR18, CR19

1

Hi efficiency green LED

CR8

6

51V 5% .5 watt zeners

CR9-14

1

Hi efficiency red LED

CR15

1

5.1V .5 watt 5% zener

CR16

1

10V 1W 5% zener 1N4740

CR17

1

1A fuse-of sufficient voltage rating,ie: not an
automotive fuse

F1

1

fuseholder-depends on type of fuze used.

1

Linecord -- country dependent

1

dual binding post

J1a,b

1

MPSA92 350v PNP TO-92

Q1

2

MPSA42 350v NPN TO-92

Q2, Q3

3

IRF820 TO220 VFET(you can substitute IRF820, 830, 840 or
IRF710, 720, 730, 740)

Q4, Q5, Q6

3 --

Heat sinks for the FETs

Mouser p/n M532-569022B00 or equiv

12

470 ohm 1/4w 5%

R1, R27-37

1

47k 1/4w 5%

R2

1

100K 2W 5%

R3

4

10k 1/4w 5%

R4, R18-20, R23, R26

6

1M 1/4w 5%

R5, R9, R22, R25

1

10 ohm 1/4w 5%

R6

3

100 ohm 1/4w 5%

R7, R8, R10

1

470k 1/2w 5%

R11

1

22 ohm 1/4w 5%

R12

1

200 ohm 2w 5%

R13

1

3.3k 5W 5%

R14

1

270k 1/4w 5%

R15

1

133 ohm 1/2w 1%

R16

1

220k 1/4w 5%

R17

1

200k 1/4w 5%

R21

1

160k 1/4w 5%

R24

1

SPST switch

S1

2

DPDT switch

S2,S3

1

3 position switch

S4

2

120/240v to 12.6VCT 40W mains transformer

T1,T2

1

74HC04 (not HCT)

U1

1

7 pin tube socket

V1

2

Octal tube sockets

V2,V3

5

9 pin tube sockets

V4-V8

3

1 meg lin taper pot

VR1, VR2, VR3

1

2 k trimmer pot

VR4

Hi All, Modifications to the gm/mu Tester - Rev B

1. During checkout, I found one condition of plugging tubes (sideways -
one pin was broken and 2 others shorted) that I could cause the plate regulator
to break, so I've added two zeners to prevent that from happening in the
future.

2. Added a 4D 4 pin socket for 811's etc. This also adds a 5th switch to
"short" heater to cathode.

3. A second schematic page is now available... this adds a variable
regulator to the filament source, to provide a variable filament voltage
from 2.5 to 12.2 volts. This is not required for operation of the basic tester,
but provides coverage for 2A3's, 50's etc. Add it if you like.

4. A "plate cap" is added to the schematic for testing things like 811's,
and 6DQ6 and related 6AM socketed tubes in the 7AC socket.

5. There was one "unclear" portion on the schematic in the tube socket
connections. This is clarified.

6. See below for settings to test a number of common tubes, so you don't
have to look them up.

Page 1 BOM (Bill Of Materials)Changes:

Qty

Description

Ref

Designator

1

SPST Switch

S5

Same as S1.

2

15V .5w Zener

CR20, CR21

1N5235B

1

4 pin tube skt

V9

1

Plate Cap

Page 2 BOM:

Qty

Description

Ref

Designator

2

1000 uF 25V

C101, C102

1

.01 uF disc

C103

5

3A 40V Schottky diode

CR101-CR105

1N5822

1

50 uH 5A inductor

L101

(actually 68 uH)

2

2.2k 1/4W 5%

R101, R102

1

Maxim MAX724

U101

Available from DigiKey

1

Heat Sink

Same as on Pg 1

1

10k lin taper

VR101

Page 2 Calibration Procedure:

With a DVM connected to the output going to the filaments, calibrate VR5 at
2.5, 3, 5, 6, 6.3, 6.6, 7.5, 10, 12, 1.2 volts. Check this voltage with a 50C5
(or 35W4 etc) plugged into the 7BK socket as a load. This voltage should not
substantially change with load. Regards, Steve

Steve's gm/mu Tester. "Standard" Readings for many Tubes.

Note: Vf is 6.3V unless otherwise indicated. To test other than 6.3V tubes,
you must build the Filament circuit shown in the schematics' second page. For
the 4D socketed parts, you must turn ON S5, which connects cathode and
filament. Note the shorted H-K LED will come ON in this case. P after the socket
means use the plate cap to hook up the plate.

THIS LIST IS NOT INTENDED TO BE ALL INCLUSIVE! Many of the tubes not listed
here can be found in the GE Essential Characteristics book.

Vf=Filament voltage. Va= Plate or anode voltage. Vg2=Screen or grid #2
voltage.Does not apply to triodes.Vg1=Negative grid voltage. Only given for
power triodes. Ik=cathode current. gm=transconductance as expressed in
micromhos (typical US designation,1000 umho is equal to 1 ma/V, 1 ma change in
anode current for 1 volt chage in grid (g1) voltage). mu=amplification factor.
Only given for small triodes.

Note that the gm and Ik given are for typical new tubes. Depending on
the tube type, variations of 10 to 20% can be expected to be seen, even for
unused tubes,generally the higher the Ik or gm, the wider the variation that
one can expect. High gm tubes such as 6DJ8, 7308, 12GN7, etc, are often factory
spec'd to as wide as a -20 +40% tolerance in gm. Others may show an increase in
Ik after being "cooked" with plate current for awhile, so if your NOS Mullard
12AX7's test too low, try running them for a while, then retest them.

Tube

Socket

Vf

Va

Vg2

Vg1

Ik

gm-umho

mu

2A3

4D

2.5

300

--

-45

50

4200

6A3

4D

300

--

-45

50

4200

6AG5

7BK

250

150

8.5

5000

6AH6

7BK

300

150

12.5

9000

6AJ5

7BK

30

30

3.5

2300

6AK5

7BK

180

120

10

5100

6AQ8

9AJ

250

--

10

5700

59

6AS6

7BK

120

120

9

3200

6AS7

8BD

150

-45

60

2000

6AU6

7BK

250

150

15

5200

6BA6

7BK

250

100

15

4400

6BC8

9AJ

150

10

6200

35

6BD6

7BK

250

100

12

2000

6BG6

7AC-p

250

250

75

6000

6BH6

7BK

250

150

10

4600

6BJ6

7BK

250

150

12

3600

6BK7

9AJ

150

18

8500

40

6BL7

8BD

250

40

7000

15

6BQ5/ EL84

9CV

250

250

45

10500

6BQ6

7AC -p

250

150

45

5200

6BQ7

9AJ

150

9

6000

35

6BS8

9AJ

150

10

7200

36

6BX7

8BD

250

42

7600

10

6BX8

9AJ

65

9

6700

25

6BZ6

7BK

125

125

18

8000

6BZ7

9AJ

150

10

6800

36

6BZ8

9AJ

125

10

8000

45

6CA7/ EL34

7AC

250

250

45

10000

6CB6

7BK

125

125

17

8000

6CD6

7AC-p

175

175

75

7700

6CG7

9AJ

250

9

2600

20

6CW5/EL86

9CV

170

170

45

10000

6DN7 (sec 1)

8BD

250

8

2500

22.5

(sec2)

250

41

7700

15

6DJ8/ ECC88

9AJ

90

15

12500

33

6EM7 (sec 1)

8BD

250

1.5

2200

66

(sec 2)

150

45

7000

5.4

6F6

7AC

250

250

40

2500

6GM8/ ECC86

9AJ

7

.9

2600

14

6JK6

7BK

125

125

15

18000

6K6

7AC

250

250

37

2300

6L6

7AC

250

250

45

5100

6SL7

8BD

250

2

1500

70

6SN7

8BD

250

9

2600

20

6V6

7AC

250

250

45

4100

10

4D

7.5

300

-20

20

1600

8

12AT7/ ECC81

9A

250

10

5500

60

12AU7/ECC82

9A

250

10

2200

17

12AV7

9A

150

18

8500

41

12AX7/ ECC83

9A

250

1.2

1600

95

12AY7

9A

250

3

1700

44

12BH7

9A

250

11

3100

16.5

12BY7

9BF

250

180

32

11000

12BZ7

9A

250

2.5

3200

98

12GN7

9BF

250

150

35

36000

12HG7

9BF

300

135

35

32000

50

4D

7.5

300

-45

20

3800

275A

4D

5.0

150

-40

17

1600

2.6

300B

4D

5.0

300

-61

60

5500

3.8

350B

7AC

300

250

80

7700

417A

9V

150

26

24000

43

811A

4D -p

300

30

1500

95

5751

9A

250

1

1200

70

5998

8BD

120

45

15000

5.4

6336

8BD

200

-45

185

11000

2.7

6528

8BD

110

45

30000

9

6550

7AC

275

275

45

10000

6922

9AJ

90

12

11500

33

6SU7 8BD see 6SL7

12AD7 9A see 12AX7

12AZ7 9A see 12AT7

12DF7 9A see 12AX7

12DM7 9A see 12AX7

12DT7 9A see 12AX7

12DW7 9A sec1 = 12AX7, sec2 = 12AU7

572 4D -p see 811A

5691 8BD see 6SL7

5692 8BD see 6SN7

5725 7BK see 6AS6

5749 7BK see 6BA6

5814 9A see 12AU7

5842 9V see 417A

5881 7AC see 6L6

5965 9A see 12AV7

6072 9A see 12AY7

6080 8BD see 6AS7

6113 8BD see 6SL7

6136 7BK see 6AU6

6188 8BD see 6SL7

6189 9A see 12AU7

6201 9A see 12AT7

6265 7BK see 6BH6

6485 7BK see 6AH6

6520 8BD see 6AS7

6660 7BK see 6BA6

6661 7BK see 6BH6

6662 7BK see 6BJ6

6679 9A see 12AT7

6680 9A see 12AU7

6681 9A see 12AX7

6851 9A see 5751

7025 9A see 12AX7

7189 9CV see 6BQ5

7247 9A sec1 = 12AX7, sec2 = 12AU7

7308/E188CC see 6922

7581 7AC see 6L6

7728 9A see 12AT7

7729 9A see 12AX7

7730 9A see 12AU7

7867 7AC-p see 6CD6

8431 9AJ see 6ES8

ECC81 9A see 12AT7

ECC82 9A see 12AU7

ECC83 9A see 12AX7

ECC88 9AJ see 6DJ8

E88CC 9AJ see 6922

EL34 7AC see 6CA7

EL84 9CV see 6BQ5

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