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Pride KW-1, the Havana Taxi lives.

nomadradio

Analog Retentive
Apr 3, 2005
6,938
11,063
698
Louisville, KY
www.nomadradio.com
Here is a derelict Pride KW-1. And that's the only kind of KW-1 I have ever seen. This one was cheap, since it was missing the small transformer, had a blown (substitute) HV transformer, and the power-supply pc boards were just gone. The relay board was typical, burned in places and missing various foil traces. The board with the dummy-load resistors was just gone.

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The plate choke was a bit on the large side. The Load control was a bit on the small side.

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The fan was a commercial "Rotron" brand replacement. Sketchy TIG on the adapter plate. This blower has a recangular outlet. The original one had a round flange. It works, so that's one less thing I need to replace.

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This is the origin of this amplifier's nickname, the "Havana Taxi".

Like a 50 or 60 year-old american-made car in Cuba. Fewer and fewer original parts every time it gets repaired.

This amplifier ends up with only the chassis, Tune control, tube socket, meter and coax sockets still original. Oh, and the serial number plate.

I put it back together as a DX300, with a stripped-down relay board. Has no preamp, and only the 10-meter input coil.

Fortunately a proper Load control is no big deal. Not original, but sufficient for 10 meters.

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The new HV transformer is a toroid type from Antek Inc. It says it's rated for 400 Watts at 50 Hz. At 60 Hz, it's closer to 500 Watts. This is the largest transformer of this type that will fit, and costs a LOT less than a custom-made copy of the original. The output voltage from two 400-Volt secondaries requires a voltage doubler circuit, rather than the full-wave bridge used in the original setup. No big deal to do this with the HV boards we sell. You do need to use bigger filter caps than the original 100uf parts. Six 220uf caps provide the same filtering using this circuit.

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Like I said, it just barely fits.

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The smaller transformer to the right of the blower is for the heater/relay and bias voltages. That's the one I get asked about the most. This one is Antek type AS05T240. Costs half what ARF Parts wanted for the original until they ran out forever.

Just one problem. It provides only 240 Volts AC, not 500 Volts center tapped like the original. Our low-voltage board adapts to use this transformer by boosting the original 40uf bias-filter caps to 150uf parts. The tube's screen voltage is slightly reduced, but not enough to worry about.

Mounting it may be done more than one way. This time I used a flat bracket of 1/8-inch aluminum.

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The only reason a bracket this thick fits under the cabinet top without interference is that there is a gap that thick built into the rear half of the chassis. They used this gap to clear the heads of hex bolts. Came in handy for me.

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A steel "L" bracket can be used for this, but not this time.

One of the two 25-ohm 30Watt resistors is the surge limiter, between the HV transformer and the voltage doubler. The other one is a "glitch" resistor between the high-voltage output and the tube's plate choke. Just didn't want to leave that out.

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The original "hair pin" parasitic suppressor choke is now a more-conventional type, with a bracket to hold the tube in the socket. The new plate choke is the one we make and sell on FleaBay.

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So now it's a somewhat cut-down DX300. The tube's heater voltage comes from the small transformer, but it's not 6.3 Volts AC like the original. This one is six Volts AC. Six point zero. Doesn't sound like a big difference, but it's a part of what made the Pride a hot rod. The tube is rated for six Volts, not six point three.

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Leaving out the band selector enables one feature that can be handy. The bias control is mounted where the "Band" knob can be mounted to it by an extension shaft. This puts a carrier-control knob on the front panel. Handy if your radio doesn't have a carrier-power knob on it.

So, the high voltage is only about 2250 Volts DC instead of 2450 or so. Not a big difference, but this reduces the max peak power a bit. And the lower heater voltage reduces the peak power a bit as well. This box will comfortably show 500 Watt peaks with a good tube, but won't deliver the 600 or more you may see with the stock components.

At the very least it does demonstrate the substitute transformers will work, even if the performance is compromised by ten or fifteen percent.

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And if the front weren't so butt-ugly I would probably try to sell it.

73
 

Question on the amp. I see you have a glitch resistor in series on HV. I normally do this on triodes. But on tetrodes I've heard this is a bad idea. I know it needs some kind of resistor in series to limit current if shorted. It should be shut down by some other method or hold just long enough to trip fuse/ breaker. But don't want it to explode or their is a possibility of damaging screen on tube? Just wondering what you thought. Using one of those $300 Tetrode boards isn't really worth it in a amp like this. Since the board cost more than rest of amp. Lol
 
Any time the anode and screen supplies come from two separate sources there is the risk that the anode potential can shut down and leave the screen grid energized.

This is widely acknowledged to be a bad idea. It can fry the screen grid in a perfectly-good tube. This is one feature of the expensive Tetrode Board designs. Definitely worth it if your tetrode costs more than that board. Insurance only pays for itself when it protects you from a loss larger than the price of the premium.

The use of a tiny one or two-Watt resistor as "glitch" protection leaves open this possibility. That resistor is a fuse, nothing more. Dentron amplifiers using grounded-grid triodes used this method. No screen grids to worry about there.

The 30-Watt resistor in the picture should hold long enough for the 7.5-Amp breaker to trip out. In this case the glitch resistor is not meant to vaporize and serve as a fuse. It's purpose is to set an upper limit on surge current while the breaker gets around to shutting it down. It's supposed to be big enough to survive and fight another day. Changing the main fuse to a breaker should make this more predictable.

Since the small breaker that protects the small transformer is being fed from the big breaker, both the screen and anode supplies should shut down together if there's a fault.

But that's the underlying plan.

Just hearing an echo of the old wisdom "No battle plan survives first contact with the enemy".

73
 

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