Monday, April 24, 2017

Vacuum tube curve tracer completed and working perfectly


I installed the barrier strip, insulated joints, tied down the fuse holders and choke inside, trimmed up connections, and put the entire unit together in its final configuration. It sits in the wooden case of a Century Tube Tester that I repurposed to hold my project, leaving some items like the meter and unused sockets because the cosmetics would be worse with all the gaping holes.

Completed curve tracer in its wooden case
The side panel has the connector for an RS232 serial link to the PC, a switch to choose either anode/screen reservoir voltages or the barrier terminal block to connect to special positions 10 and 11 on the rotary switches.

Side connections - serial port, special voltage switch and external heater block
The power brick and the USB to RS232 adapter cords sit in the slot on the left of the faceplate. The nine rotary knobs choose the appropriate connection for each of the 9 possible pins on a tube socket. Only five of the sockets are connected, the rest are dummies.  The black and red top hat connection cables allow grid and anode sources, respectively, to be connected to any tube with a top hat connector. 
Completed and assembled tracer
The Quality light is actually the power on LED showing the circuit board is powered up. The Short light is actually the "high voltage present" LED which indicates that the screen and anode reservoirs have hundreds of volts present. The small 'power' light is not active, nor is the meter on the top.

I fired up the tracer, hooked it to the PC, fired up the GUI control software, and inserted a 12AU7 tube in one of the active 9 pin sockets. I rotated the appropriate switches to connect the target pins to the voltage they represent. 

For example, the second triode in the 12AU7 (it is a dual-triode tube) has the plate connected to pin 1, the grid to pin 2 and the cathode to pin 3. The filament is 12.6V across pins 4 and 5. Thus, switch 1 was rotated to the position for anode voltage (position 8), switch 2 to the position for grid voltage (position 6), switch 4 rotated to filament A (position 2), etc. 

I ran the 'quick test' on the tube, which is set to the voltages from the data sheet where characteristics such as plate resistance, transconductance and amplification factor are reported. In this case, the manufacturer's data sheet showed that with 250 volts on the anode and -8.5 volts on the grid, it should draw 10.5ma on the plate, have a plate resistance of 7.7K ohms, a transconductance of 2200 uMho and an amplification factor of 17. 

I entered the two settings and the nominal values into the quick test form. It showed the plate current a bit high, the plate resistance exactly on spec, and the transconductance and amplification factor about 9% low compared to spec. Reasonable results for a used tube.

I then ran a curve trace to match the conditions for one of the curves illustrated in the data sheet. The curves for 100, 150, 200 and 250 volts on the anode where shown while varying the grid from -18 to 0 volts, displaying the plate current against grid voltage for the four curves. 
The expected performance of a new 12AU7 tube
My curves are similar but not identical, consistent with the degradation of the used tube but similar enough to be a reasonable test of this curve tracer. I could have moved the plate current scale to the right side and adjusted the start/stop values of the axes to line up exactly with the factory drawn curve, if that was important. I can also hide the measurement points and just leave the interpolated curves. 
Measured curve of one of the triodes in my used 12AU7
This project is complete! Next up I started through all the tubes from my Heathkit HW-100 transceiver, to check them against spec. Might as well play with different curve plots and options too.

Saturday, April 22, 2017

Wrapped up tube checker construction - 99% done


This morning while thinking about testing the tubes in our 026 keypunches at the Computer History Museum, I remembered one limitation of the circuit board I built. It can supply filament voltage only up to the supply voltage of the power brick - just over 19V - which is insufficient to test the 25L6 tubes or any others that have high heater voltage requirements. 

The solution for measuring is to hook up external DC for the filament. At that moment, I was faced with new design choices.

I could add in two more positions on each rotary switch to connect external heater power to the tube socket pins, in addition to the two positions I was about to wire to connect the screen and anode voltage reservoirs for continuous measurements. 

Alternatively, I could use two positions on the switches as a generalized set of special sources, with switches to route the reservoir volts and connectors to route external filament power to the same positions.

I liked the new method and set about adding that. I means that I have to find a suitable toggle switch and mount it, to connect or disconnect the reservoir voltages to these special loops. I also need to find and mount some kind of connector for external heater power. 

While I sought these parts and mulled over locations on the panel, I did some remaining tasks - hooked in the top cap connectors to grid and anode voltage loops plus moved the 'power on' and 'high voltage present' LEDs off the board and onto the faceplate.  

I will have a number of connections and a switch to mount, which I decided to place on the side of the cabinet instead of the faceplate due to restricted space up on top. The items to mount on the side are:

  • RS232 DB9 connector for PC communication
  • Switch to connect reservoir capacitors to special rotary switch positions
  • Terminal block to connect external heaters to special rotary switch positions
The switch to install should be a DPDT, wired so that the reservoir power wires are on one side and the terminal strip are on the other, thus having no risk that high voltage will be present on the terminal strip.

The circuit board is now firmly mounted inside the case. Once I have the connectors added to the side, and the power brick mounted inside somehow, there are some ferrite beads and fuses to add to the filament lines before I hook it all together and close it up, plus a fuse and large choke to install on the line from the power brick to the board. 

When I first mounted the DB9 connector, it stopped communication with the PC. Likely this was caused by a short from some of the handshaking bridge wires that are often soldered onto the back pins of DB9 connectors. After inspection and rearrangement, the connector is back in the case and fully functional.

I now have the ferrite beads on the filament lines, the fuses for filament and main power, the main 300 uH choke and the wires coming from the screen and anode reservoir capacitors. I needed to find and mount the switch, connected to the reservoir lines and the new loops I soldered for switch positions 10 and 11 to deliver either reservoir or external heater power to tube socket pins. 

The reservoir lines can carry 300 or so volts each, which is above the voltage rating of most switches, complicating the sourcing. I needed the switch to be on-off-on, stay in any of those three positions, and have twin poles and double contacts for each pole. 

Having found an acceptable switch, I have it wired in place, although it is only an on-on, no center off position. This is okay because the terminal block that will connect to external heater power can be connected to the rotary switch positions 11 and 10. 

The only risk with this on-on switch would be if I switched a tube pin to 10 or 11 and inside the tube that pin was connected to another pin with screen or anode potential. Even there, the high voltage will only be present for a millisecond at a time during testing.

The power brick is installed, although I have to recalibrate slightly to accommodate the slightly different voltage supplied to the board. Mounting and insulating of a few components and connections, e.g. choke and fuses, will wrap up the work. 

Friday, April 21, 2017

Finished main wiring, tested by measuring a 6AU6


I finished wiring the voltage loops for all the regular connections from the circuit board. After very carefully checking every connection with a continuity tester, I discovered that when I swapped out the original selector switch for a new one yesterday, I miswired the filament connections. With that fixed, it was time to run a trace on sample tube, a 6AU6.

Leveraged tester that has become the curve tracer
The controls are simple - there are nine rotary selectors, one for each of the possible pins on a tube socket. They are switched from positions 1 to 11, but only some of the positions are active. Switching the dial for a given pin to the following numbers hooks it up to the associated voltage from the circuit board inside:

  1. n/c
  2. filament side A
  3. filament side B
  4. n/c
  5. cathode
  6. control grid
  7. screen grid
  8. anode
  9. n/c
  10. n/c
  11. n/c
Only five sockets are wired up - one each loctal, octal, and 7 pin mini, plus two 9 pin mini types. The remainder of the sockets on the plate, as well as the meter, is superfluous. The following picture shows which switch controls each pin, highlights the working sockets, and obscures the unused parts. You can see the connections to the circuit board hanging out of the bottom, as I haven't finished the mount yet.

Marked controls and active sockets only
I fired up the 6AU6 and measured curves - I really like the circuit and GUI of the uTracer 3+. I have some more wiring to accomplish, however. First, there are two top cap connectors, for tubes that have the anode or the grid connection on the top of the tube. I have to hook these to grid and anode voltage lines and remember they are also hot during any measurement. Second, I have to create loops for positions 10 and 11 of the rotary switches to carry the screen and anode voltage reservoirs out to tube pins.

The normal mode of operation is for the circuit board to build up the target voltages in the screen and anode reservoir capacitors, but only connect those to the screen and anode pins for milliseconds at a time while making measurements. Certain unusual measurement situations may require a steady voltage, not a short burst, which is the purpose of positions 10 and 11. 

The boost converters can't supply more than 3ma steadily, so this connection to continuous screen and/or anode reservoirs will only work for low current applications. The archetypical case for using the reservoir voltage is to test magic eye tubes. This will not be a frequent mode for me, in fact I may never use it, but the wiring should be in place. 

Thursday, April 20, 2017

Construction continues on vacuum tube curve tracer


I grabbed some lockwashers and other hardware and attempted to install the three remaining rotary switches. Alas, the chassis opening for a 9 pin mini tube socket is slightly wider than the lockwasher. While I can use flat washers to cover the gap, they don't have any grip to keep the switch from rotating when the knob is twisted. Back to the drawing board (hardware store). 

Armed with new wider star lockwashers and a combination wrench, I tightened on the three new switches and began wiring them up. By midafternoon I had seven switches wired. The next two are easier, because both skip the 7 pin mini socket and one of them also skips the two 9 pin mini sockets. 

In the meantime, I started the lines for each voltage, bridging across the various rotary switches on the same position of each, ultimately leading down to the curve tracer circuit board itself. To do this correctly, I had to decide where the board would be mounted. 

The choice is on small standoffs on the bottom inside the case. This complicates things a bit because wire runs have to be somewhat longer to allow the top to be opened for servicing. I want the wire runs to the rotary switches to be as short as I can, in a loop themselves, to minimize inductance. 

I will also run wires out from the PCB to remotely mount the two LEDs under the faceplate, so show the 'power on' and 'HV present' states to the operator. For this I had to desolder the existing LEDs, put on the wire and place my own LEDs on the faceplate. I don't need to worry about wire length for these thus it won't cause problems opening the unit for service.

This will be powered by a 19+ V DC power brick, also installed inside the case. I had one from an old laptop I used to own, which worked out great for this purpose. It will be fixed inside and the AC cable routed out in the case slot that contained the original AC cord. 

By dinnertime, I had completed wiring all nine rotary switches to the associated pins of the five tube sockets on my tester. I had also wired up the two filament loops - wiring looped through switch positions 2 and 3 respectively of every rotary switch. This worked well and lit heaters on real tubes.

I will wire the cathode, grid, screen and anode voltage loops to their relevant positions 5, 6, 7 and 8 on the rotary switches. By the evening, I was done with the screen and anode loops. A terminal snapped off the old rotary switch that I was leveraging from the original Century circuitry, which required me to remove it, drill out the hole to fit my new type of switch, and wire the new one into place. 

After these are seen to work properly, I can bring the two reservoir capacitor lines in a non-loop to positions 10 and 11 of the switches. I will finish up the cathode and grid lines tomorrow, test out everything carefully with a continuity checker, before hooking it all up to the circuit board and testing with a real tube. 

1401 restoration work and continued curve tracer construction

I spent part of the day with the plumber as he finished up the tankless water heater installation and some other work, then spent the rest of the day with the 1401 restoration team. This evening, I attended a lecture by Paul Wesley at Stanford on the history of Silicon Valley. 


I slowly did the tedious wiring of the sockets - loops between each pin number, around all the sockets being used, through ferrite beads and connected to the commutator of the six rotary switches I had on hand. I also prepared the sites for the next three rotary switches to be installed, now that the stock arrived, for controlling pins 7, 8 and 9. 

Before I can mount the next three rotary switches into the holes where I removed tube sockets, I need to get to a hardware store for some regular and lock washers, since a tube socket is larger diameter than a rotary switch shaft. 

After they are mounted, I can wire up those three loops. Actually, one loop goes to all five sockets, a second loop goes to just four sockets, and the last loop goes to only two sockets, the 9 pin minis. 

Another issue is the mounting for the circuit board, power supply brick and the serial port DB9 connector. Besides working that out and doing the install, I have to wire the board produced voltages to the corresponding positions on all nine rotary switches. 

The voltages on the rotary switches are: 
  1. anode
  2. screen
  3. cathode
  4. grid
  5. filament A
  6. filament B
  7. anode reservoir
  8. screen reservoir. 
The regular anode and screen connections are only connected for very brief intervals during measurements, small fractions of a second, otherwise sit at zero. The reservoirs provide the same voltages continuously. 


SMS cards in the 1401 power supply circuitry drive a set of relays which sequence power on and off at startup or shutdown of the system. All the cards we have are scorched black underneath a pair of resistors that are extremely hot to the touch at all times. We investigated to see if this is a result of failed components or 'normal'.

The card schematics for this card matches exactly what is on the card. It is clear that the pair of 2W resistors are asked to handle more than 3.5 W of power, yet are mounted flat to the card with no space for airflow around them. Thus, the scorching and overheated resistors are by design.

More interesting to me is that the resistors in question are simply providing a voltage divider to bias a transistor, switching it on only if the two power supplies that fed the divider were working. They designed this to draw about 65 ma across the 42 V relative voltage, all to create a minute bias current in the transistor.

We then looked at the ALD schematic page, instead of the SMS card schematic, and found that the design had different and much more appropriate resistor values installed. 3,470 instead of 650 ohms, which dropped the power consumption down to a half watt. The schematic pages had the exact same drawing and part number, but the cards as they exist are the scorchers not the design in the ALDs.

We decided to take one of our spare cards and modify it to match the ALD schematic. I found almost every part needed on hand in the workroom, except that I had to parallel up two resistors to create a 43 ohm part. By the end of the day, I was partway through rebuilding the card. The tools on hand for solder removal were inadequate, so I will bring in my own gear next time.

Tuesday, April 18, 2017

Building curve tracer into case


With the circuit board itself working flawlessly, it is time to build out an enclosure with tube sockets and rotary selector switches, mount the board inside, and wire everything up. I hadn't selected an enclosure yet. I do want to move forward on this.

I initially thought to gut a Century tube checker I own, but the panel has dozens each of octal, 7 pin mini and 9 pin mini sockets, whereas I only need a few slots. Further, it has a large meter and other parts installed which are unnecessary, and lacks the rotary switch openings. Still, I decided to hack it up, as the extra socket openings could eventually hold compactron, novar, nuvistor, acorn and other socket types.

I have five rotary switches with 11 settings, not enough to route each connection type (anode, screen, grid, cathode, and one side of the filament) to any of 12 possible pins (most sockets have 7, 8 or 9 pins but there is a compactron type that has all 12). Thus, I will have to connect in the opposite way, using 12 rotary switches to select among six voltages - anode, screen, grid, cathode, filament A and filament B.

The existing sockets are pop riveted in the panel, thus I need to drill out any that I remove. I can then use screw and nut, or new pop rivets if I can find my gun and supplies. First step was to gut the transformer, unused controls and wiring, Next was to trim back the rats nest of intertube wiring.

I have ordered another batch of rotary switches, enough for 10 pin sockets which will cover everything except the compactron (12 pin type). These are almost exclusively used in tube based color televisions, which is not an area where I expect to receive tubes for testing. Thus, I will ignore the compactron type entirely.

There are a wide range of tube socket types, of which I will support as many as I can easily and inexpensively obtain sockets for. The three workhorse sockets are the octal, 7 pin mini and 9 pin mini. I have plenty of these already on the Century unit.

Loctal sockets are similar to octal but have a center pin that snaps into place in the housing. These were intended for use in automobiles and other high vibration environments. I happen to have two loctal sockets on the unit, otherwise I wouldn't have bothered. Again, mobile radio tubes are not what I expect to be testing. Hifi, computers and test equipment will be the primary source of tubes for testing.

There is a 10 pin socket that is like the 9 pin mini but has an extra connection in the center hole. I happen to have one of these sockets, so it will go in. There is a novar type, which is nine pins but thinner and similar to the compactron style. I think I have one of these as well.

Older radios used 4, 5, 6, 7 and 8 pin sockets, UX4, UX5 etc, that I might stick in if I had them cheap and ready to go, but these are mostly from 30s and older era radios and clearly not going to be needed often.

I have the first six rotary switches (one from the original tester and the five I bought for the project) installed in the unit. Next up is to choose locations for the next three switches, which will arrive in two days. That will give me switches for the nine pins of the sockets already in place (7 pin mini, 9 pin mini, octal and loctal).

I began wiring up the first six switches and the five sockets I am using. It is tedious work, building a loop for each pin across all five sockets, including a ferrite bead to soak up any RF oscillation that might try to build, and hooking each loop to a different switch. I can only build the loops for pins 1 to 6 until the new switches arrive and are mounted.

Monday, April 17, 2017

Tube curve tracer board working and calibrating


I resolved the screen voltage problem - turned out to be two hard to spot solder joint issues. The solder welled up on the component lead in a beautiful shiny cone, but had not flowed onto the pad. Looked great until I went to extreme magnification with the stereo microscope, where I could see only 1 or 2 joints at a time. 

Repaired and the hangup of the main logic is gone, as I expected. It goes through a measurement interval, applying anode and screen voltage to the tube under test (but I had no tube connected) while maintaining the voltage reservoirs at their target level.

In addition to the screen circuit problems, now fixed, the grid bias circuit is not working. It should develop a bias from 0 to -50V during the measurement phase of the curve tracer. The microcontroller produces pulse width modulation on a pin which drives a low noise operational amplifier acting as an low pass filter (or integrator). This yields a smoothed voltage between 0 and 5V, which is fed to a second op amp, LM741, wired as a 10X inverter, to produce the intended 0 to -50V.

I zoomed in to this section of the board and looked quite carefully. There was one solder joint that looked as if it would benefit from reflowing the solder to improve pad bonding, but it is for the unconnected pin on the LM741 op amp package thus it doesn't matter. Time to debug the circuit, measuring and observing, until I figure out what is wrong.

The observation steps to make are:

  1. scope on the microcontroller signal that drives this, which is a 19.5KHz signal whose on time is varied (pulse width modulation) to swing the first op amp output between 0 and 5 volts.
  2. if the pulses are good, look at the first op amp to verify it is filtering/integrating and generating the 0 to 5 volt range expected
  3. if the first op amp is working properly, observe the behavior of the second op amp which should be generating 0 to -50 based on the 0 to 5 of the first device.
  4. if the second op amp is working properly, observe the current mirror transistors that produce the actual grid voltage when driven by the second op amp.
  5. If the mirror transistors work, debug the continuity problem from here to the grid connection.
The microcontroller pulses were behaving exactly as they should, varying the pulse width as the target value changed from 0 to 5. We passed test one above. The second test involved watching the output of the NE5535 low noise op amp to see if it did produce the 0 to 5 level. 

The second test failed! The op amp circuit is not integrating nor acting as a low pass filter to produce the target value. I saw a constant output on pin 6, the output pin. There are only a few components in this portion of the circuit, two capacitors and a couple of resistors. If they are good and the connections and solder joints are correct, it points the finger at the chip.

The sequence of tests to perform for this are:

  1. verify that V+ and V- are making it to the chip
  2. verify the proper resistance for the two resistors and two capacitors on the input side
  3. verify that the input pulses are making it to the non-inverting input
  4. monitor the waveforms on the inverting inputs
  5. check for shorts of the output pin
Test 1 failed - no V+ to the IC, nor am I getting +15V to the other three ICs it feeds. Digging further, I found that my 7815 voltage regulator was bad. I had a replacement in stock, put it into the board, and now everything is working properly! Filament, anode, screen, grid and all the measurement logic doing fine.

Now I have a bit of calibration and then have to build it up with tube sockets, switches and all to make a full tester. Calibration is done easily, with a GUI screen to adjust values to match measured results with the DMM. I worked through this tonight as I collect tube sockets and order any remaining ones I will need to install in my project box/chassis. Every is calibrated and ready to install in a box and begin using.