Biggest problem with two-by-3-500Z ham linears for AM is the idle dissipation of the tubes. The "Z" suffix letter was meant to indicate that the tube can safely operate with the grid directly grounded and zero bias voltage applied to the tube's grid. It will draw enough current to heat up the anode to nearly half its rating this way with NO drive power applied. This makes for a clean sideband signal, but presents a problem for AM. The heat the tube throws from your carrier alone will push the tubes to near their rated limit before you even say hello.
The SB-220 doesn't operate the tubes with true 'zero' bias. It puts a zener diode in series with the cathode's connection to the HV power supply's Bee-Minus side. Since the grid is grounded, and the minus side of the high voltage supply is not, it has the effect of making the grid about five Volts more negative than the cathode. The Drake L4 doesn't do this, but the high voltage is lower than what's used in the SB220. That makes the zero-drive heat about the same in both models.
We adopted the habit of advising AM operators to increase the bias voltage on the tubes to safely run AM. A circuit board with thirty (yes) 3-Amp rectifier diodes becomes the "zener" diode. A forward-biased rectifier diode has a fairly constant voltage drop of about 2/3 of a Volt each. This puts just over 20 Volts of bias onto the tubes. The idle current is now low enough that the heat from your AM carrier is now manageable and won't melt the solder from inside the tubes' filament pins.
Nothing says this can't be done with a piece of perf board.
I should add that this is the expensive solution. The manual for this kind of amplifier usually tells you to use the "low" or "CW" side of the amplifier for AM. This serves to keep the tubes' heat load at a safe level.
Also cuts your peak power roughly in half. AM operators never seem to find this an attractive option. Adding the extreme bias voltage allows them to use the high 'SSB' side safely in AM mode.
Never did get around to marketing this toy. Was worth making them to save labor servicing this kind of amplifier here in the shop.
If you're willing to hold the peak drive power below 120 Watts or so, you can skip the other "hot-rod" mods. Just keep the contacts on the band selector clean. Simply rotating the band knob a couple of times end-to-end before you use it serves to help with this. The contacts are meant to be what they call "self-cleaning", but only if you turn the knob from time to time. Leaving it in one position defeats this design, and permits silver oxide to build up on the selector's contact surfaces. When it burns out, the 11-meter operator hires someone to bypass it and make it a monoband linear. If you change bands regularly in routine use of the thing, that takes care of the problem. Mostly. But if it's always used on one band, cranking the knob on occasion can prevent it from flaming out.
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