I recycled many cassette and VHS tapes, and I want to clear them up for avoid noise or distortions in future recordings. I know that tape recorders have built in erase heads, but I want to keep my tapes nice and clear. What's the best way to erase them? Maybe a motor or ballast. I've tried with magnetron magnets, but they seems to not clear the tapes. 

Big strong neodymium magnets would do the trick, there are also device you can buy to cleanly clear the heads.

Maybe rotating the magnetron magnet with a drill will simulate a high intensity AC field. Maybe getting a bunch of insulated copper coil and making one, which is used for demagnetizing CRT TVs. Buying a tape cleaner is not an option, because those things are history now and so expensive, and I can damage the device in the process, so the best options are the DIY ones.

My father used to have a magnet in a box with a slot on each end. You slid the cassette in one end and it came out of the other, getting erased on its way through. When I worked in a radio studio, we had a big electromagnet which was mains powered that we used to erase reel to reel tapes, but you really had to watch you didn't leave tapes you wanted anywhere near it, it was powerful 

I tried rotating the magnet with the drill, but it didn't work. I shaked the magnet around the tape and it seems to work, but it takes a lot of work to clear it up, seems like my magnets are not strong enough, so I'll need to use some sort of electromagnet or a coil without core running AC current, but my meter can measure inductance so I don't know how much turns of wire to make.

You need VERY strong magnetic field to erase them, something around 0.4T.
The erasing head in the recorder may look tiny, but it has just a narrow gap, so although not that high power, the field is concentrated into the gap and so becomes very strong there. Usually it is a ferritte core assembly, driven to saturation by the circuit (an LC oscillator with the head being the resonator tank; assume a decent quality HFAC erase and bias).

I wont be magnetizing it with anything strong, that may actually leave it magnetized, so bring way more noise into the recording (the tape irregularities are causing a multiplication style noise on top of the magnetization the tape carries; with just AC magnetization, the noise is then inherently following the actual signal strength, so protrudes less; but with DC bias recordings, the random multiplication of something in the half amplitude range is causing the noise to correspond to full signal, although the real signal may be there very quiet).

Strong permanent magnet based erasers were used to destroy the records for security reasons, but using these has made these tapes very noisy afterwards (from the parts of the tape between the opposite direction tracks, not erased by the recorder).

For low noise I would leave them alone, otherwise you need a special electromagnet supplying first strong AC, then gradually (few percents per cycle) reducing the amplitude till zero. But I doubt it would help in any way for the noise, unless someone had magnetized them before. But then I would expect a lot of residual noise anyway (due to the storage with the magnetic forces within the reels).
The eraser head does just such demagnetization in its slit during recording (that is, why it is there after all), you should just make sure the generator in the recorder is strong enough and has clean supply (so no EOL electrolytic capacitors with high ESR,...)

@Medved a fancy AC frequency controlled coil is not needed, just by starting the coil in front of the tapes and then slowly pulling it back in circles is enough. In This video he does the same but with a CRT TV, so the effect is more evident, and he just uses a bare coil connected to the 50Hz 220V mains. After all the process no magnetism seems to be left in the TV, so the same would happen and the tape irregularities multiplication effect should be none or very little, and the tape should be ready to use after the process.

Radio Shack used to sell a device known as a bulk tape eraser for this purpose. Personally I just wipe them with the recorder. If I'm being extra cautious, I'll run them once and record silence (turn the level all the way down or engage rec mute) on the tape before making my actual recording.

Radio Shack used to sell a device known as a bulk tape eraser for this purpose. Personally I just wipe them with the recorder. If I'm being extra cautious, I'll run them once and record silence (turn the level all the way down or engage rec mute) on the tape before making my actual recording.

That's basically what we used in the radio station. It was an electromagnet and you just rotated the spool of tape on it, it looked something like this:

@sox35 is that coil tape eraser destructive or just cleaner? I mean if after the process the tape can still be recorded without major problems.

Oh yes, it's not destructive, it's just a hoofing big electromagnet 

The thing is, if you are too quick to pull the tape away, the tape will become noisy.
These devices were designed to get rid of the records, not that much to clean the tapes from as much noise as possible.
For that you really can not reduce the field any faster than few percents per cycle, so we are talking about 10's seconds to gradually pull the tape from that electromagnet.
But on the other hand if your recorder uses permanent magnet erase and mainly DC recording bias, it does not matter anymore, the noise would be there.

That's true, you do have to remove the tape from the magnet gradually while it is still powered up.

The process might take 2-3 minutes, but I can do it with many tapes at the time displayed in a table. I think I should start with a rather fast movement and then slowing down as I move away, I think if I start slow I'll get the wahwah effect.

The process might take 2-3 minutes, but I can do it with many tapes at the time displayed in a table. I think I should start with a rather fast movement and then slowing down as I move away, I think if I start slow I'll get the wahwah effect.

If you are slow enough, thre is no "wah-wah" at all. That tends to happen when you are too fast with one side of the reel (as the tape unwinds,thenoise varies). And the "wah-wah" is then a clear signal of too quick, so an improper demagnetization (some sections remained magnetized)...

It is because of how strong magnetization you may accidentally create when being too quick, instead of getting rid of the small remaining magnetization, why I would not recommend it for a noise sensitive tapes, even when it istheoretically possible. A decent recorder will have proper and strong enough demagnetization erase head, it will get rid of the old recordings.
And if something is to remain after passing the erase head, it is more likely the strong tape magnetization from the improper demagnetization attempt (and the consequent noise), than the weak noise fromthe previous use.
You may "record a quiet" on the recorder the first time to get rid of the magnetization properly, that I see as way better chance to get low noise afterwards.

But in theory it should work...