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It's a finely-balanced design. The driver is a 500-Volt tube with 1200 Volts on the plate. By rights it should go poof the first time it's keyed. Had a memorable conversation with an "engineer" at Wawasee Electronics many decades ago. Asked him how they get away with that. He explained that...

The series filament connection lets you use wire half as thick on the cathode choke. Also shrinks the wire size needed to wind the filament secondary on the power transformer. Just be sure to put a fan of some sort just below the tube sockets. Doesn't have to be big, only needs a breeze not a...

If memory serves, the AMC on that radio was a retrofit. When that model first hit the market, a limiter hadn't yet been required. Some time around 1974 a limiter got grafted onto the radio's audio circuit. Seems to me it worked by pumping positive voltage into the emitter of the mike amp...

A resistor in series with that 10pf cap will extend the upper power limit. It's a compromise. Make the resistance too high and bareftoot radios won't produce decent output to the 'scope. Too low and you roast the diodes. This is probably the only application where I would recommend a...

I use an envelope detector on the dummy load. Can't watch the RF output on the antenna this way, but that's not an issue. The Heathkit cantenna has this circuit built in, meant to insert a meter probe. Changing a capacitor in it allows a fine view of the demodulated audio, and makes triggering...

After the 40-channel rumors started, manufacturers were desperate to talk you into buying a 23-channel radio. Most of them were mortgaged to the hilt getting radios into the sales pipeline. The 40-channel announcement from the FCC stopped up that pipeline. Buyers sat on their hands waiting for...

Here's what we use. https://www.ebay.com/itm/116472537974?mkcid=1&mkrid=711-53200-19255-0&siteid=0&campid=5336136228&customid=&toolid=10001&mkevt=1 Kinda pricey, but maintains the full modulated peak output of the radio at all carrier settings down to a half Watt. Designed to save labor, not...

The lesson is never accept the offer of a ride in a home-built submarine. Airplane, maybe. Sub? No. 73

Screw should be an american thread #4-40 thread pitch. Looks like it was driven with a radio almost as big as the amplifier. 73

A MOV is only safe to use if there is proper fuse protection upstream from it. 73

Odds are the linear will sound better with around 2 Watts carrier from the radio. You could skip changing the low-side resistors in the amplifier and install a carrier control on the radio. That would let you get the carrier level down to the comfort zone around 30 Watts on High side without...

I would start with the two 2-Watt resistors just below the preamp relay, the black one that says "Sigma". These are the low side resistors. The one on the right is marked 27 ohms, red-violet-black. Changing it to 47 ohms at 2 Watts might get your carrier into the sweet spot around 30 or 35 Watts...

I won't be buying one, but it looked amusing at the least. 73

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Insurance doesn't come any cheaper than that. 73

How much carrier comes out of it with the High/Low switch set to Low side? 73

Saw a technique in UTube packing a 3D print item into a can packed with table salt, then packed as tightly as possible. Goes into an oven set just below the melting point and the print lines go away. Maybe worth the trouble, maybe not. 73

Um, okay. Wouldn't have thought of that. If you don't have a scope handy, my advice isn't so useful. I'm surprised the Simpson will read RF voltage on the AC scales. Following it downstream from the radio socket to the tube cathodes is still what needs to be done. 73

Something is blocking the tracks. This is where a 'scope is your friend. Pulling the plug makes it safe to poke inside. A handy bench-type 12-Volt DC supply is used to energize only the amplifier's relay circuits. A current limit if 1 Amp or other protection is a good idea. Putting the...

Seriously? John quit making his MMM board? Hadn't kept track. I acquired a broken one years ago. Had been dipped in some kind of rubber like you use to dip a tool handle into. Peeled right off. Couldn't resist tracing down the schemo. Had exactly one conversation with John almost ten years...