Entries from April 2013 ↓


Super IC, this part combines a decade up/down counter, latch, and 7-segment display driver in one package it drives common cathode displays to 25 ma per segment. Add 7 current limiting resistors, a 7-segment LED display, a couple of detectors and SR flip-flop and we have a turns counter for a vacuum variable or rotary inductor.

Soft Start


Soft Start is the term used to refer to the act of limiting inrush current when electronics equipment is first turned on. High inrush currents are encountered in large tube filament and high power applications. A tube filament or incandescent light offers an extremely low resistance when cold. That low resistance becomes higher as the filament temperature increases but the initial low resistance causes high inrush currents that could damage equipment.

High power, power supplies using large capacitor banks after the rectifiers to smooth AC ripple also have high inrush currents due to the fact that the discharged capacitors act as short circuits until they acquire charge. Such shorts put undue stress on transformers and rectifiers.

Inrush current limiting is accomplished by placing low resistance power resistors in series with the load causing the initial current surge. Resistors of 22 ohms and 15 watts are use for many applications. These resistors are only allowed in the circuit for a few seconds after which relays short them out and connect the loads directly to the line. With one 22 ohm resistor in each line the current is limited to a maximum of under 3 amps in the primary.

The soft start circuit below provides several seconds delay after power on. Enough delay to successfully limit inrush current. The circuit can also be used to provide regulated power for other relay and control circuitry.



When V+ is initially applied Vout is just a little over 1 volt because C2 is discharged placing the base of Q1 at ground causing Q1 to conduct which pulls the adj (com) pin to ground.

Even though Vout is only a little over 1 volt, it is enough to cause current to flow through R2 and R3 which starts to charge C2. With the circuit energized, Vout will always be larger than the voltage on the plus terminal of C2. This keeps D1 reverse biased and allows current to flow only into C2. Once C2 becomes charged the voltage on C2 will turn off Q1 and the LM317 will function normally as a regulator with output voltage set by R2 and R1. Reduce the value of R1 to reduce Vout.

When power is turned off (V+ goes to zero), Vout also goes to zero. Now D1 becomes forward biased because of the plus voltage on C2 and C2 is discharged through D1. The only reason D1 is in the circuit is to discharge C2. C2 must be discharged to prepare it for the next time power is turned on again.

The slow rise of Vout when this circuit is energized provides a delay in the closure of a relay energized by Vout. The length of the delay is determined by the time constant set by R3 and C2. Larger values result in more delay.

V+ must be at least 3 volts higher than Vout. In our case we used V+ = 12 volts and Vout = 9 volts. We used 9 volt relays to short across the two 22 ohm 15 watt current limiting resistors in the primary of the transformers. One resistor per primary lead and one SPST relay per resistor to disable the start up current limit.

This soft start method has one very major problem, no feedback from the load. The time delay is generated externally in a supply that is not at all connected with the supply being soft started. If the supply being soft started has a problem, like a shorted output due to tube or component failure, our time delay will still just short across the resistor in the primary circuit and possibly cause even greater problems.

A shorted power supply output will reflect back into the primary of the power transformer. If there is current limiting resistance in that primary, the line voltage on the load side of the resistance will never get to full line voltage. Now if we place the 120vac coil of a 120vac relay across the line on the load side of the current limiting resistor, then use the contacts of that relay to short the current limiting resistor, we have protection for unscheduled shorts of the power supply. The relay will not close unless it sees close to 120vac across its coil. That condition will not occur if the secondary has a short or excessive load across it.

Another possible solution that allows the condition of the transformer secondary load to govern when the soft start relay energizes is to use the LM317 circuit but power it from a spare secondary of the soft started transformer. The advantage of using the LM317 circuit is that the regulator can be set to activate any number of relays without need for dropping resistors.


Here is a circuit, variations of which have been used for decades to limit transformer inrush currents on start up.


R26 and R25 are current limiting resistors placed in series with the primary power to the load. In our case these are 22 ohm 15 watt for a total of 44 ohms series resistance. That amount of resistance provides a 2.72 amp current limit for a 120 vac power input. The circuit above is intended to be used in both 120 and 240 vac primary power applications. That is why the return line powering the relays used to bypass the current limiting resistors is brought out on terminal F. This is to ensure that the relays and their biasing circuitry always see 120vac regardless of the input supply being 120 or 240vac.

Note also that the relay circuitry is placed across the load side of the power source. The load side will see a significant voltage drop on start up due to inrush current being limited by the series connected power resistors.

This initial voltage drop will prevent the relays from closing allowing the current limiting resistors to do their job. Once the start up surge is over, the relays will close and the current limiting resistors will be taken out of the circuit.

I our specific case we are using 9vdc SPST relays with 30 amp contacts because they are inexpensive and will do the job. Their coil resistance is 100 ohms. At 9vdc and 100ohms we have .09 amps of current through each relay coil for the relay to close and stay closed. The relays will be powered from 120vdc coming from the diode. We need .09 amps of current. It will require 1333 ohms to limit the current to .09 amps from the 120vdc supply. We already have 200 ohms (100ohms per relay coil) So we need an additional resistance of about 1200 ohms to make this circuit work.

Actually, the relays are guaranteed to pull in for voltages as low as 6 volts which equates to .06 amps which would call for a resistance of 2000 ohms. We ended up using a 1500 ohm resistor. From IxIxR we see that this resistor will be required to dissipate 10 watts. Since this resistor will be powered up as long as the supply is powered up, we need to select a resistor that can dissipate 20 watts minimum. The requirements on this resistor can be reduced by selecting relays requiring a higher coil voltage.

C8 and C7 across the relay coils are there to prevent relay chatter. Anything from 50 to 100mfd will do and there is no reason to use more than a 25 volt capacitor.

If you build this circuit be sure to measure the voltage across the relay coils after the relays engage. Then adjust the value of the dropping resistor accordingly to make sure that the relays do not see more than their rated voltage.


It was mentioned before that an LM317 regulator could be powered from a supply derived from a filament winding of the transformer and allow the soft start to work off the actual condition of the load instead of a dumb time delay. The circuit below shows such a solution without the LM317 complication.



Once your through playing with circuits and tweaking values here is a sure fire way to get the job done and it only takes two components. Find a relay with a 120vac coil, contacts that will handle 10 amps, and a 50 ohm 10 watt resistor. Wire the resistor in series with one leg of the 120vac supply and wire the relay contacts to short across the resistor when the relay is energized. Connect the relay coil across the load side of the 120vac transformer primary. That is it. You are done. Enjoy the soft start. Now, why didn’t we do this in the first place? Why, because we found all kinds of fancy circuits on the internet that made us think soft start was hard to do. Some people spend lifetimes making the simple complicated and you will almost always find them on the internet.

It is complicated is never an answer. Things become complicated when they are over engineered. Over engineering often suffers from loss of the original intent of the design. Beware of folk who consider things complicated because they do not understand the basics. When you don’t know what you are doing, everything becomes complicated.

General Purpose Linear Amp

Found a computer cabinet that is large and has provision for mounting two computer power supplies. One supply will be a standard ATX supply. The other will be an empty box housing an extra fan and rectifier filter board. The plate transformer is large with an 800 volt center tapped secondary. In a voltage doubler circuit that transformer develops 2240 volts, perfect for 3-500Z or 813 or 572b. In a bridge circuit we can get 1120 vdc. We can also add an isolation transformer and boost the ac voltage to 920 which can develop 1288 vdc. Or we can add the 200 vac transformer for an even 1000 vac to develop 1400 vdc.

Filament power at 5 volts can be obtained from the computer supply to run one 3-500. 811s filaments can be run off the 6.3 volt filament transformer. 813s will need a seperate 10 volt transformer. The neat thing is that the cabinet is large enough to house all these transformers and still allow it to be a desk top unit.

ATX power on switch. Soft start for the filaments using LM317 circuit and 9 volt 30 amp relays. Time delay for filaments to get hot then soft start the HV supply with another lm317 circuit and two additional 9 volt 30 amp relays.

RF deck will be cut out to allow sub assemblies using a variety of tubes to be dropped into place. Use a common plate tank circuit.

Uses for Old Desktop Computers

First off there is really nothing wrong with a working 486 machine. There are still plenty of applications that run just fine on relatively slow 16 bit machines. Using old feature limited Microsoft software for operating systems may not be the best solution but there are lots of Limux distributions that incorporate drivers to support newer features (like USB).

If the slow performance and poor graphics of old desktops becomes unbearable, they can still be put to use as parts sources for other projects, For instance, once the shelves and brackets inside a desk top computer case are removed we have the skeletal framework and finished sides of a very capable electronics cabinet that can be used to build other electronics equipment,

Linear power amplifiers are particularly suited to a re-purposed desk top computer case. Most cases are large enough to house tube and solid state RF decks and power supplies. This would certainly be possible using a larger tower case, but there is no reason the amp could not be built into two smaller desk top cases. The RF deck and filament supply in one case and the high voltage supply in another case. Such dual case solutions would make it possible to have multiple RF decks all powered by the same high voltage supply, one at a time, of course.

Even old desk top computer switching supplies might find use in amplifier construction. One of the more costly items in a tube amp design is the filament transformer. Legal limit tube amps require 5 or 6.3 volts at around 20 amps to light the filaments of the tubes. Computer switching supplies provide 5 volts DC at 20 amps and more. Assuming the filaments can be run off direct current, the computer power supply can be used in place of the expensive filament transformer. Need 6.3 volts? No problem just adjust the regulated output to provide the higher voltage. The computer case is already designed to incorporate the switching supply and the supply can provide 12 volts for relay switching. Or how about a low voltage control circuit to soft start the high voltage plate supply? We could also easily incorporate a delay circuit to allow the filaments to come to full temperature before plate voltage is applied. Ooops…no center tap for grounded grid circuitry. No problem, just tie a couple of 10 ohm, high wattage resistors to ground from each filament supply lead. Use the normally grounded center connection of the resistors as a center tap.

This entire design is most easily implemented using a 120 vac primary power source. Some would argue that a 1kw amp requires a 220 vac primary source. While it is true that there is less IR drop in the primary line using 220 vac. a 120 vac circuit is fully capable of supplying 1kw and more. It is done all the time with 120 vac appliances like hair dryers, toasters, and small ovens.

I will admit that if I could easily wire this application for 220 vac, I would. Allowing primary power to the high voltage supply be routed through the RF head so that only one large 120 vac power connection is needed pretty much dictates use of 120 vac. Okay, I know a 220 vac powered system could be designed but I don’t consider that to be worth the added effort and expense. Engineers often choose the more complicated solutions because they like a challenge. Job security? Just because something can be done does not mean it should be done. Besides, I don’t have a 220 vac service to the shack and don’t have plans to install one any time soon.

There are several items needing attention that have not yet been discussed. Among them are cooling and front panel. You will need a front panel. In most cases the front panel will be installed in place of the formed plastic front that needs to be removed. Exactly how the front panel is attached will have to be worked out on a case by case basis. Perhaps the best general solution is to frame the front panel with angle and allow the case cover to slip under the angle to make for a more finished look. Cooling is best done with simple fans. There are now lots of large and quiet fans available at reasonable prices. Blowers and forced air systems are only required for tubes that are designed specifically for that sort of application. A good place to mount a fan would be in the large open area created with the removal of the bracket that was used to support add-on cards. You also need to allow for air to exhaust the case. Perforated aluminum panels installed over cutouts may be used. Consider such a perforated panel installed across the front of a modified computer case. Air flow would be from the back of the case to the front (which now becomes one of the sides) and the front panel can be installed across what used to be the bottom of the tower case. This assumes the case would be laying on its side rather that in normal vertical tower configuration. The side laying, lower profile is more suited for amp construction.

There are several additional considerations necessary to make these suggestions result in useful solutions. Switching noise may be introduced into the receiver from the switching power supply since the antenna connection will now be routed through the amp. Noise suppression may need to be added. If the switching supply is to be used, make sure that the electrolytic capacitors in the supply are good. If the supply is more than 20 years old, chances are good that the electrolytics will need to be replaced. Make sure that any cable and connector hardware between high voltage supply and RF deck units is adequate for the application. Primary wiring should be sized to minimize IR losses and HV wiring must have adequate insulation. HV insulation requirements can be mitigated by routing HV ac power to the RF head and then doubling and rectifying the ac inside the RF head. This is a real possibility now that electrolytics have gone through a significant size reduction. As a for instance, 1000 vac will become 2800 vdc after it is rectified, doubled and filtered in the RF head. Routing 1000 vac through a cable is safer than routing 2800 vdc.

Linear Power Amps Are Heavy, Dangerous and Often Large

This is particularly true of old tube style amps. The biggest contributor to these disadvantages is the large, heavy, plate transformer. Some amps solve this problem by seperating the power supply from the amplifier. For those building their own amplifiers this can be customized to relocated only the transformers. It is aafer to route 1000vac than 2500vdc. So, locate the plate (and possibly the filament) transformers in an enclosed box under the operating table and run AC power lines to the desktop amplifier which contains the voltage doubling rectifiers and filter capacitors to develop the plate voltage.

On second thought, it is probably better to locate the filament transformer in the desktop unit to avoid IR losses in high current wiring.