Johnson Ranger modulator Tube Replacement

The Johnson Ranger uses a pair of 1614 modulator tubes rated at 550 volts and 25 watt plate. Some replace with 6L6GC, 6550, KT-88 and EL-34 and believe any of these tubes will perform well. Some internet sites recommend the tubes listed above and others may state not to use a type such as the 6550.

The 6L6GC (not 6L6GB) tubes are normally rated for 450 volts push-pull as maximum volts in the datasheets typical operation however some are rated at 500 volt maximum plate voltage. With about 500-530 volts from the Johnson Ranger I am a little concerned using a 450 volt rated tube. (Do note the first 1954 Ranger uses cathode vs fixed bias thus the modulator tubes operate at about 30 volts less plate to cathode. The 1954 Ranger could get away with 6L6GC tubes). However, the expensive and higher voltage Svetlana SV6L6GC or Sovtek 6L6WXT+ are rated at 500 operating volts to use with -28 volts bias and 300 volts screen. I would question a 450 volt rated tube in the post 1954 Johnson higher voltage circuit. But the forementioned Svetlana or Sovtek should work and are rated 20% more power output vs the 6L6GC. The Russian 6L6WXT+ tube plate is 35 watt that is same as Tung Sol re-issue 7027A 35 watt rated plate. An internet site states the 6L6WXT+ been used for over two years in an Ampeg amp with 550 volts plate and 500 volts screen without issue. This is a very rugged tube. The Sovtek 6L6WXT+ are a reasonable $59 a pair new on eBay. If using a SS 5R4 rectifier, forget using any 6L6 tube with the 570-600 volts. The 6550 is your only choice.

The 6550 has a 600 volt rating but tasks the transformer filament winding with 1.4 more amps per tube pair, but the transformer 6.3 volt filament winding is rated at 10 amps or enough for the 6550 tubes. Using LED dial lamps reduces the extra current to .9 amps and is recommended. Another option is a SS 6AX5 saves 1.2 amps. The Johnson Ranger -28 volt bias is correct for the 6550 with the Ranger other voltages. If you do not want the 1614, this is the best tube for the Johnson Ranger. Using a SS 5R4, I would only use the 6550 due to the higher plate voltage. The most inexpensive tubes are new Shuguang at $86.00 a pair. I do not know Chinese manufacture, but new known quality Russian Sovtek 6550WE are an expensive $144 a pair at the Tubestore. The Sovtek 6550WE are rated at 800 volts vs standard 6550 at 600 volts. And the Sovtek plate is rated at 44 watts! (Note- this 6550 tube info has been edited with newly found 6.3 volt transformer current of 10 amps, not estimated 8 amps).

The recent KT-77 is not a close for replacement. Wants 500 volts screen and -43 volts bias vs -28 volts from the Johnson Ranger.

The KT-88 is expensive and draws same filament current as the 6550. Uses -36 volts bias vs Johnson -28 volts bias. Not a good replacement. Larger diameter glass may hit the shield anyways.
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The EL-34 uses 100 volts higher screen voltage than the Johnson Ranger normally provides plus draws 1.5 amps filament each vs the .9 amp each of the 1614, but the transformer will supply the extra filament current. Also uses -36 volts bias vs the Ranger -28 volts bias however at reduced screen voltage -28 bias may be ok. I do not recommend the EL-34.

The best tube for the Johnson is the 1614 IMO using the 5R4 rectifier tube. If using a SS 5R4 use the 6550 tube. The 1614 in NOS are $21 each at findatube in Ohio.

The tube operating parameters are from the tube data sheets when operated at the approximate 500 volts plate. I am not sure of the resting plate current when a tube is operated at lower than specified screen voltage (not shown in tube data) and -28 volts bias. Best to simply avoid using those tubes.