Early 1954 Johnson Ranger Low Modulator Power Fix

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I thought this would be important enough to post.

This 1954 Ranger has cathode bias vs fixed bias. The cathode bias produces less power output vs later Ranger fixed bias and is also about 30 volts less modulator tube plate voltage to cathode vs later Rangers. The Ranger 1954 modulator looks to be about 30 watts output including transformer losses or not enough for 100% modulation in stock condition at full Ranger RF output. Later Rangers with fixed bias produce about 45 watts modulator output. To increase the output power with cathode bias I would bypass the 200 ohm cathode resistor with 47uF @ 50 volt cap vs stock 10uF cap to reduce losses. I may increase the screen from 300 to 384 volts and use an array of 24 volt 5 watt transorbs (zeners) to regulate the screens to keep the power up. The 7027A show higher screen voltage vs 300 volts. The 6550 with cathode bias have screen voltage rating of 300 volts and a 170 ohm cathode resistor for about 45 watts out. I will verify if the Sovtek 6L6WXT+ listed below are more of a 7027A or 6550 operating parameters by measuring the plate current. The 6AQ5 clamper tube uses 300 volts that shares with the 1614 tube screens. A second resistor tap will be required for higher voltage screen and this tap will be rewired to supply about 400 volts to the 1614 zener regulated 384 volts to the screens rewired directly thru switch SW4B. 384 volts is not good idea on a 6AQ5 that is rated at 250 max volts. The new tube screen 384 volts should produce about 45 watts out with fresh tubes. If the tube plates operate at dull red glow at idle, reduce the screen to 360 volts. The 7027A tube shows higher voltage screen vs the 6550 tube.

The data for 45 watts power out is looking at the 7027A tube that is a heavy duty 6L6GC with cathode bias using same 200 ohm cathode resistor as the Ranger but operate at 425 volts plate and 425 volts screen on the data sheet. 500 volts plate with reduced screen from 425 volts to 384 volts would yield about same results. The 1614 tubes would be way overtaxed at this higher screen volts with the 25 watt rated plates. The tubes would be dissipating about 70 watts plate (two tubes) set up with 500 volts plate and 384 volts screen with the 200 ohm cathode resistor. Although the 7027A tube would be great for this application they are very expensive and needs a socket pin out change. I would use Sovtek 6L6WXT+ tubes with about 35 watts plate dissipation each that is close to same as the 7027A tube. 6L6WXT+ uses same 35 watt plate as the re-issue Tung Sol 7027A with a 35 watt plate. The Sovtek tubes are rated at 500 volts and the 1954 Ranger provides about 500 volts plate to cathode. There is a report on the internet the Sovtek 6L6WXT+ replaced 7027A tubes as used in an Ampex guitar amp with 550 volts plate and 500 volts screen and has worked well for a few years. The Sovtek 6L6WXT+ are $59 a pair new on ebay.

Power supply consideration. The 5R4 tube is rated at 250ma current. However into a choke at 250ma current the tube can supply 340ma current out the choke per power supply simulator PSU2 due to no rectifier peak current vs a capacitor input filter. No issue with the 5R4 tube.

The 1614 draw 60ma at idle and peak at 120ma (160ma in later Rangers) and the 6L6WXT+ draw 150ma at idle and peak at 190ma. Then the rest of tubes draw current, but the 1954 Ranger has same power transformer size as the 1955 and later Rangers with two more tubes and 160ma peak modulator current. The power transformer will be loaded heavier at idle, but capable of the higher 1614 160ma peak current it was designed to supply. The transformer will operate a little hotter, but should be good. The transformer HV is rated at 260ma continuous duty, should be ok at 50% duty cycle. If it fails, I will buy a Heyboer M6 lam heavy duty transformer capable of 1/3 higher HV current.

Note- I assume the Ranger tiny mod transformer can modulate about 100% at 400Hz with the Sovtek 6L6WXT+ tubes. In my application I will be using the Heyboer heavy duty modulator transformer that can take 3x the stock Ranger mod transformer current and produce 100Hz at 100% modulation.