Protecting AC Transformers in old tube receivers and transmitters

[K1DEU]

  To protect our power, filament and plate transformers I locate a slow MOV and fast transient absorber (both in parallel) near and across (in parallel) transformer primaries.
For 120 VAC protection I Use the following peak voltage devices;
A  Panasonic ( Matshuta ) 220 Volt Bi Directional ( AC ) Transient Voltage Suppressor rated at 1500 Watts   
  Digi-key PN # 1.5KE220CALFCT-ND
And A Panasonic ( Matshuta ) 220 Volt Surge Absorber ( MOV i.e. Metal Oxide Varistor ) rated at 6,500 Watts
  Digi-key PN #  P7230-ND
For 240 VAC Protection  I use;  devices with the same wattage and rated at Twice these voltages I. E. 440-450 Volts.
See   http://hamelectronics.com/k1deu/pages/ham/general/pages/surge_suppression.htm

Making sure there is a fuse in series with the hot AC input nearby. If I cannot drill a hole for a 3AG holder I glue a snap in holder on the Chassis.

   In the 40's through 60's In most all cases our gear expected to be supplied with 110 VAC or 220 VAC not our more common current Mains of 120 or 240 VAC.  In my DX-100 with 120 VAC supplied my filament supplied voltage was nearly 7 volts. No way Jose! Timmy WA1HLR had an excellent idea. He stated simply take the unused (if u solid state the 5volt LV Rectifier) winding and buck the LV, filament, bias transformer Primary. Sounded great except I was using this winding and another in series for some low voltage DC for a quiet 12 volt computer fan on my 36LW6 finals. Oh Well.
    Well this started a new different addition to the MOV, Transorb and fuse above.
    Starting backwards, when I snap my Receivers or Transmitters on everyone notices that the dial lights do Not flash on but gently brighten and my B+ comes up slowly, with less strain on the rectifiers. No, Not step start but a big series resistor in the primary. In my trusty ole DX-100 I mounted a apx 20 ohm 20 watt ceramic power resistor on top of the chassis with teflon wire in series with the hot AC line cord and fuse. So much for a narrow brain that had always read that only real men had stiff voltage regulated supplies, I now feel I never want to be a Real Man again!

    Also, take the classic case of the little or big bang coming up the power cord. Yes it is clamped across the primary and easily absorbed by the lossy slow big resistor. Never adding to the dielectric break down of our old magnet wire transformers. No doubt to me how long my dial lights, filaments and old AC transformers are going to last.
     In a plate supply the primary being clamped helps the discharging choke, which could also have a diode across it for some standby problems..  sorry but I don't type and greatly prefer voice..  73  John, K1DEU

http://www.spiritualpeaceproject.com/  and  http://hamelectronics.com/k1deu/

[Bacon, WA3WDR]

That's a good idea.  Evidently an MOV can handle a lot of surge current, but it responds a little slowly.  So the additional faster-acting protection catches the leading edge of the transients, for better protection.

For inrush current limiting, I have used 'Globar' resistors - negative temperature coefficient resistors that start out pretty high in value, and drop rapidly in resistance as they heat up.  Placed in series with the AC power input, they greatly reduce the turn-on surge, without adding much resistance to the circuit in normal operation.  However, they do not provide as much assistance as a series resistor if the AC power flickers, because the resistance of the Globar unit will be low as the power goes on and off.

These things operate HOT - so they need to be mounted appropriately, and wires and parts around them must be dressed away from them, so that they do not get scorched.

I don't see Globar in Google searches, but Digi-Key carries Cantherm inrush current limiters, which appear to use the same negative temperature coefficient concept.

http://www.cantherm.com/products/thermistors/cantherm_mf72.pdf

MF72-003D9 - 4A, 3 ohms room temp, 0.12 ohms hot - Digi-Key #317-1156-ND
MF72-004D9 - 3A, 4 ohms room temp, 0.19 ohms hot - Digi-Key #317-1157-ND
MF72-005D9 - 3A, 5 ohms room temp, 0.21 ohms hot - Digi-Key #317-1158-ND
MF72-006D9 - 2A, 6 ohms room temp, 0.315 ohms hot - Digi-Key #317-1159-ND
MF72-008D9 - 2A, 8 ohms room temp, 0.40 ohms hot - Digi-Key #317-1160-ND
MF72-010D9 - 2A, 10 ohms room temp, 0.458 ohms hot - Digi-Key #317-1161-ND

http://www.digikey.com

I have seen solid-state rectifiers with a turn-on delay.  If this delay is based on the applied filament voltage, this would be very good for tube equipment, because it would allow the tube cathodes to be warmed before applying operating voltage.

[WU2D]

Hi John,

I found out why I kept cutting out when I was talking to Dirk and yourself from the mobile somewhere in the Green Mountains yesrterday.

The confusing part was that the HV stayed on and I only lost relay and fillament voltage.

The Heath HP-13 has two circuit breakers, the main breaker 30 Amps and a second breaker 10 amps which services the fills. It was the 10 Amp that was popping when she heated up. It was only after the box heated up that it would pop - it had been fine all morning on the trip. I suspect a weak or corroded connection to the case because I am not drawing more than 5 Amps total on the relay and fills.

Oh and the box is laying on the insulated floor of the rear seat - not too swift.

Nice rant on the MOV's yesterday - glad you brought it to the forum.

Spiritual Circuit Breaker - Mike

[K1DEU]

Hi Bacon & Mike.  Glad u solved your mobile intermittent, Mike. Vry interesting Bacon, I developed a bad taste for Thermistors in the 60's and 70's in color Tv chassis. It was commonplace to check every chassis all brands coming into the shops for visual thermistor problems. In large color chassis they were the size of three quarters in dia and thickness. One or both sides would be unsoldered and sometimes they were blown apart due to lightning. A wide Majority of manufacturers stopped using them. But looking at the Cantherm data sheet I would not be afraid to try a big one sometimes. In most cases as the mains supplied to our old rigs is too high I would prefer to use a big ceramic, constant loss resistor. Many of us destroy our old transformers unknowingly when our freezer/refrigerator compressor cycles off , or we are turning a large drill, water pump, etc. on/off elsewhere in our house. Also Electric Utility switching capacitor banks (canceling long wire inductance) in our neighborhood often cause bad switching spikes when they correct the Power Factor (phase) for different demands.

In the early 80's I started experimenting using bi-directional 1 picosecond TranSorbs. Which are back to back zenier diodes in a cheap, neat package. They sure protected the transformers but in a medium or larger bang they shorted and required replacement. Surprisingly, adding the Slower MOV in parallel meant only resetting the circuit breaker or replacing the fuse.

Many of our old transformers were pre-formvar dielectric era. And the new transformers do not use expensive thermalize. When the magnet wire was formed around a 90 degree corner the dielectric coating wrinkled creating a weak spot. Weak spots or not many small hits over the course of many years take the toll in the same weak areas and will eventually puncture through. 73 OM's John, K1DEU

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