Skipper 300

nomadradio said:

I feel like my idiot light just came on.

For those parts to immediately blow up the way they did makes me suspect that your 300A has a mismatched transformer on it.

The power transformer was made in two versions. One of them supplied about 560 Volts AC to a full-wave bridge rectifier. Took the form of a small rectangular block of black epoxy with four wire leads out the bottom, mounted alongside the two big filter caps.

Your RF deck was built to take the OTHER version of the transformer. It supplies about half that much AC voltage on the high-voltage winding, around 280 Volts AC. Your amplifier clearly has a full-wave voltage doubler circuit to rectify and filter the 800 Volts DC needed to power the tubes. The four individual 1N5408 diodes tell us that.

If you plug the higher-voltage transformer into the lower-voltage type power supply, it always goes "SLAM!" right away.

Haven't seen this happen in quite a while, but there are two possible ways to mismatch the amplifier and the power transformer.

If your RF deck had been built for the 560-Volt type, and you plug the lower-voltage transformer into it, the tubes will always be running at half the design voltage. You get an amplifier that is perpetually on "Low" side, no matter which way you flip the High/Low switch. No matter how good the tubes are.

And if you mismatch it the other way, the two big filter capacitors now will see nearly twice the voltage they are rated to tolerate. Causes them to break down almost instantly.

This is the only way I can see to burn up the two bleeder resistors, by putting twice the correct volage onto them. Those bleeders almost never overheat with the correct voltage on them.

Shoulda caught on to your remark about burning up the bleeders. Can't see any other way to make them overheat like that, except to use the wrong transformer.

No, the two versions of the transformer have *NO* markings to identify which one you have. And no markings inside that I have ever found.

They appear completely identical in every way, until you measure the voltage from the HV winding.

Safest way to tell is to find a 12-Volt transformer. Hook the transformer up to 120 Volts AC, and Gator-clip the 12-Volt side to the amplifier's power plug so that you are now feeding it 12 Volts into the amplifier's power cord and not 120 Volts. This makes it safer to put your AC voltmeter onto the HV winding's hookup on the power supply board and measure it.

Just measuring 560 Volts AC directly is above the safe limit for many inexpensive multimeters. If you get a reading of 56 Volts or more you have the "High" version.

And if the meter shows under 30 Volts, you have the right one.

And if I'm right that you have the High version transformer and the Low version RF deck, this is the only way to perform that measurement without blowing something up.

Darn! Wish I had caught on to this earlier.

73

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Tokin said:

Thats not the wrong tubes but the accepted conversion to get away from the very expensive 8950 tubes. The 6lf6 has 40 watts plate dissipation. If I were you I would leave it wired for the 6v tubes as nos 6lf6 tubes can be had for $40 and the 6kd6 for $34. The 6lb6 is another option.

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