RAT Construction Project - A gm and mu tester you can build. 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) held constant. The way this is done is to place a small AC voltage (lets say 100 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 0 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 55 mA, to handle the defective "shorted tube" case. In the mu mode, the requlation is converted for AC signals into an AC current sourse, DC voltage source. (That is, the output impedance is low for DC, providing good requlation, and increases with frequency so that at 1 kHz, it is hi impedance). 3. Screen Voltage: Same as #2, independently controlled. Current limited at 25 mA. Does not have modification for mu case, as mu is not a practical tetrode or pentode measurement. 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. 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. This is a 3 position switch, with g2 either connected to its own source, or to the anode, or to the control grid. 5. (VR1) Plate voltage control. 6. (VR2) Tube current control. 7. (VR3) Screen voltage control (tetrode/pentodes only). Indicators: 1. 2 jacks for DMM connection. The DMM measures AC voltage used to indicate gm and mu. 2. Green Power LED. 3. Red LED that illuminates on H-K leakage or short. Sockets: 1. Octal: (7AC/7S/8EP) Handles KT66/EL34/6L6/6550 etc. (pins 1&8 connected together). 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/7BD) Handles 6AU6/6AH6/6GM6/6JK6/EF86 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 AC bypassed for the transconductance measurement. This allows the grid-cathode voltage to be established by the tube itself. 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 voltage and current levels and read the AC voltage on the DMM. Reading MU 100 mV 1 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. This 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 by turning the DC voltage source into an AC 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 Ned or someone. 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. In the parts list below the "M" means I found the part shown in Mouser. 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 tree. Ho Ho Ho! QTY. DESCRIPTION REF DESIGNATOR Source P/N 6 100 uF 350V Elec. C1, C2, C3, C6, C7, C8 M 140-XRL350V100 2 47 uF 450V Elec. C4, C5 M 140-XRL440V47 1 47 uF 10V Elec or Tant. C9 M 581-47K10V 2 .1 uF mylar, poly, etc. C10, C13 M 146-400V.1K 4 .01 uF 500V or 1kv disc C11, C14, C16, C17 M 140-CD500Z9-103Z 1 1 uF 50V mylar etc. C12 M 140-PF1H104K 1 .22 uF 50V mylar etc. C15 M 140-PF1H224K 9 1N4007 1A 1kV diodes CR1-7, CR18, CR19 M 583-1N4007 1 Hi efficiency green LED CR8 M 606-CMD41101UG 6 51V 5% .5 watt zeners CR9-14 M 583-1N5262B 1 Hi efficiency red LED CR15 M 606-CMD41104UR/A 1 5.1V .5 watt 5% zener CR16 M 583-1N5231B 1 10V 1W 5% zener CR17 M 583-1N4740A 1 1A fuse F1 M 5760-12001 1 fuseholder -- M 504-HTB62I 1 Linecord -- country dependent 1 dual binding post J1a,b M 565-4243-0 1 MPSA92 350v pnp to92 Q1 M 333-MPSA92 2 MPSA42 350v npn to92 Q2, Q3 M 333-MPSA42 3 IRF820 TO220 VFET Q4, Q5, Q6 M 333-IRF820 (you can substitute IRF820, 830, 840 or IRF710, 720, 730, 740) 3 Heat sinks for the FETs -- M 532-569022B00 12 470 ohm 1/4w 5% R1, R27-37 ANYWHERE 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 M 108-MS500KCL-BLK 2 DPDT switch S2, S3 M 108-MS500FCL-BLK 1 3 position switch S4 M 10YX043 2 120/240v to 12.6VCT 40W T1,T2 M 553-VPS123400 1 74HC04 (not HCT) U1 M 570-CD74HC04E 1 7 pin tube socket V1 A 2 Octal tube sockets V2, V3 A 5 9 pin tube sockets V4-V8 A 3 1 meg lin taper pot VR1, VR2, VR3 M 31VA601 1 2 k trimmer pot VR4 M 594-63X202T7 Modifications to the gm/mu Tester - Rev B 1. During checkout, I found one condition of pluging 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 socketted tubes in the 7AC socket. 5. There was one "unclear" portion on the schematic in the tube socket connections. This is clarified. 6. There is an attached file (gmmu2.txt) that provides settings to test a bunch of common tubes, so you don't have to look them up. Page 1 BOM Changes: Qty Description Ref Designator Part Number 1 SPST Switch S5 Same as S1 etc. 2 15V .5w Zener CR20, CR21 1N5235B 1 4 pin tube skt V9 1 Plate Cap Page 2 BOM: 2 1000 uF 25V C101, C102 Mouser 140-HTRL25V1000 1 .01 uF disc C103 5 3A 40V Schottky CR101-CR105 Mouser 583-1N5822 1 50 uH 5A ind L101 Mouser 542-5248 (actually 68 uH) 2 2.2k 1/4W 5% R101, R102 1 Maxim MAX724 U101 Available from DigiKey 1 Heat Sink --- Same as on Page 1 BOM 1 10k lin taper VR101 Mouser 31VA401 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.