Maco 300 sweep tube Linear Amplifier on the bench

[@Snapbeam78]

Had one of these for years!! Great amplifier !

[@tramdr]

Great amps if you don't abuse them.

[@steveroberts3512]

I think I may add one of those field strength meters for fun on my next amp build, thanks for showing it.

[@tramdr]

Go for it!

[@321CatboxWA]

Nice work , thanks for sharing.

[@tramdr]

Thank you! Cheers!

[@bblod4896]

And the diode rectifies the RF to operate the meter. 😊

[@tramdr]

Agreed.

[@1883GotDown]

Very nice!!!!! Gatekeeper said that!!!!

[@tramdr]

Thanks.

[@1965semaj1]

field strength meter

[@tramdr]

Agreed.

[@Snapbeam78]

Mine had 8950’s in it !

[@tramdr]

Yep, but those are very hard to find.

[@charlesbeach4649]

With the diode it is a field strength meter.

[@tramdr]

Yes, but if the gimmick wasn't used to do something, then you wouldn't need the gimmick, right?

[@charlesbeach4649]

That is correct. You will also see “gimmicks” in input keying circuits. Two wires twisted together to form a pf capacitor. Length and twist determine capacitance.

[@ronnielowther2242]

Got a question sir my maco has 2 transformers in it is it possible its a 750 ?? And somebody put the wrong cover on it maybe 🤔

[@tramdr]

The 750 has 6 finals. The 300 has either 4 or 5 finals. They made many versions of the 300.

[@ronnielowther2242]

Yes sir just wanted to say first I enjoy your videos great job! Got a question sir I'm pretty sure you can help I have a maco just like this but mine has 2 transformers in it it was working good till got where it did not want to key I could get it to keyup but now it want key at all any suggestions sir thanks.

[@tramdr]

IMHO when an amp won't key up, meaning engage the relay, it's almost always the keying circuit. Usually, the keying circuit goes when it was driven too hard. Usually its the keying transistor and or the diodes. As in the Maco 300 in the video, I would recommend getting and installing the Nomad keying circuit.

[@ronnielowther2242]

Ok thank you sir I appreciate you taking your time to respond back to me God bless sir.

[@mikesradiorepair]

The worst name ever, "gimmick: or "gimmick capacitor". It's just a capacitor. Two conductors separated by a insulator. Many radios back in the tube era used "gimmick" capacitors for the RF output power meter circuit. Even the Panasonic chassis tube rigs used a "gimmick" capacitor pickup for the power meter. Most of the Lafayette Comstat rigs used one. Just two non terminated wires twisted together. I use the same thing for sampling high RF power circuits when doing diagnostics. Just twist two non terminated insulated wires together 2 or 3 times and you have a simple sample port that is good for 1,000+ Volts.

[@tramdr]

I agree about the name, but I think as you said normal capacitors are 2 conductor devices while gimmick's are single conductor and use something else for the 2nd conductor ie, close to the tank coil. I also think gimmicks have no insulator other than air separation, but they often have an insulator over them as its good practice not to leave bare wires hanging around.

[@johnjones9642]

I have the kw & it can get down.😊

[@tramdr]

I have had 2 and sold them. Great amps. I think the biggest and baddest sweep tube amp made.

[@MM0IMC]

If the transformer is running hot, perhaps having a separate filament transformer would help? 🤔 Something like the Hammond 185G12. You could perhaps run the 12.6V AC to the 8950's and the 6.3V tap the 6LB6's? 🤔

[@tramdr]

Agreed, I have done that on several amps already. That lightens the load of the transformer so it can concentrate just on the HV instead of the HV and the LV. Different but similar, I do the same with my home stereo. Bi-amping it really really opens up the sound. It's interesting to me with stereo as the 2 things that make a huge difference hardly anybody does. Bi-amping and running balanced. The 3rd but not as huge would be running stereo subwoofers.