Build a Tube Type Linear Amplifier For CB Or Other HF Bands Part 4 How To Unlock An MOT

[@MM0IMC]

I used an old MOT to build a 300W amplifier, using a GI-70Bt triode tube. I tested it for a while and had no problems with heat from the transformer. I removed the magnetic shunts as per your instructions.

[@radiomedic111]

I have been very surprised lately when testing some of the newer transformers. when I put 12 volts AC in on the primary to do a safe test, I am getting over 200 Volts out and that relates to over 2000 volts when you apply a full 120 Volts ac in on the primary and some of the transformers even went over 260 Volts with 12 volts on the primary. For those that have not watched the video where I explained about how to do a safe test on MOT's, you connect a Variac to the primary of the MOT and an AC volt meter and run the variac up to 12 Volts and then unplug the variac from the wall outlet without touching the control and move one lead of the volt meter to the MOT secondary output tab or wire and the other lead to the base or frame of the MOT making sure to scape away the varnish where you are making the connection, then switch the volt meter to read one of the higher volt ranges like maybe 500 or 700 volts then plug the variac back into the outlet and look at the volt meter... the reading that you see will be 1/10 of the full output when 120 volts are applied to the primary of the MOT

[@thomasking5970]

Hi! Any reason not to use an autotransformer to reduce the primary voltage, instead of taking wire off the secondary?

[@linearone]

What about that thin lead riveted to the laminations. Isnt that a center tap that should be removed if using it with a full wave bridge?

[@MfxRocca]

Can you make a video on how to make a solid state amp for 11m with components from old stuff? I really enjoy your videos, I am going to start to build an amp with 6je6b tubes. Thank you!!

[@radiomedic111]

Yes you could do that but you would need a Vairiac to get the exact volts that you need and at this age of the game they are very pricey. Also the other reason that is not a good idea, is that it limits the "Stiffness" of the supply so that you end up with more sag in the high voltage when you start drawing plate current because you are adding resistance to the primary and you are not removing resistance from the secondary by not removing the turns of wire. I hope that makes sense.

[@desdedesde790]

Unfortunately in the latest ovens, there are no transformers! Switching high voltage power supply😢

[@radiomedic111]

That just might not be a bad thing except for using them to power 4CX250b's ect. it would be perfect for tubes like the 3-500Z, 4-400, GS-23B and 3CX800 Ect.

[@micro4329]

I saw a video on mots somewhere said you could remove some primary winding to get the voltage down on secondary what do you think of this method

[@radiomedic111]

No...That would actually raise the secondary voltage, as you would be changing the turns ratio. It would be the same as using a higher voltage on the original primary. To step up, you would use more turns on the secondary and to step down, you would use a lower number of turns on the secondary than the number of turns on the primary. Thank You for asking that question!

[@micro4329]

Hi glad you posted this video been wanting to learn to build amps with tubes I have an amp wit a 4cx250 that needs a powersupply I been wanting you buil one from mots that I have hope you will make a video of the full build so I could follow along and build one for my 2 meter amp. Thank you so much for sharing your knowledge . and the wonderful videos. Ke4pmq

[@luckydubeinrc5165]

These transformers heats up fairly quickly( the core), so maybe for a short duration amp, like 30 minutes , but i would suggest you research MOT a bit more.

[@radiomedic111]

Hello...I am not sure where your reference of that comes from but I have been building amps with MOT's since about 1982 and just this summer I built 8 more and yes they do get warm but in 1999 I built a two tube 4CX800/ GU74 and at that time I tried something new...I put a 3 min. timer on the step start hold circuit and that monitored the key up circuit and if no transmit after 3 min the timer would shut the MOT's down and after that the transformers barely got much more than warm and when the next time the rig was keyed up the step start circuit would bring the MOT's back online...the main problem with the MOT's these days is that they are wound with aluminum wire. If anyone wants the schematic to that hold circuit just send me an email at: [e-mail]

[@UDX-21]

Thanks.