Erase head amplifier

[mrmagnetophon]

I am working on designing a Erase head amplifier for a r2r machine. The head is an old style Ampex head, and will need approx 3 watts of power) so my friend says. I am going to also need to design a Oscillator to provide the erase oscillation aswell. At this point would i be better to just use a DC supply and build a DC eraser?
ßChris

[Andrew2]

AC erase will provide lower residual noise AFAIK. Do you want to use a valve circuit or transistor? A valve such as an EL84 or 6V6 will easily provide the power in self-oscillating mode without an amplifier.

[Herald1360]

So would a reasonably heatsunk TO220 power transistor.....

[julie_m]

Even a TDA20xx-type power op-amp should have enough gain to oscillate at tape recorder bias frequencies.

But whether you use silicon or valves probably will ultimately be determined by the impedance of the head (low or high, respectively).

[Ed_Dinning]

Hi Gents, some of the cheaper 60's tape recorders switched the output stage as bais oscillator during record, some even used a tap on the op trans to match to the erase head.

Ed

[julie_m]

That was pretty much standard practice with 2-head recorders. The common valve line-up was ECC83 (2 stage mic / head preamp), ECL86 (monitor / playback preamp and erase oscillator / audio power amp) and some sort of indicator tube (EM84 / EM85, DM70). The record / play switch would perform a pretty severe rewiring job.

[mrmagnetophon]

Originally Posted by Andrew2

AC erase will provide lower residual noise AFAIK. Do you want to use a valve circuit or transistor? A valve such as an EL84 or 6V6 will easily provide the power in self-oscillating mode without an amplifier.

I've got enough 6v6's around here and plenty of sockets.
Will i need some type of coil? Or could i provide some type of a howling style feedback using capacitors,etc?
-Chris

[Herald1360]

You could certainly build a phase shift oscillator round an output valve. Coupling the signal to the head without a transformer or anode feed inductor of some sort might be rather inefficient, though.

Take a look at a few old domestic tape recorder circuits for inspiration.

Snippet below for starters. V4 is EL42 but 6V6 would no doubt work quite happily.

[Radio Wrangler]

The interesting thing in the diagram Chris posted is that the heads are switched. One, the other or both can be in play. So when both are going, the inductance is effectively halved.

The coupling of them to T3 has to be fairly light so the oscillation frequency doesn't change too much, because it is usually used as the recording head bias source. When I built a tape recorder, I used a master oscillator for the bias feed, driving two power amplifiers, driving resonant matching circuits into each erase head winding. One oscillator meant everythiing was on the exact same frequency and so there were no beat notes. The amplifiers buffered the oscillator from the loads, and the resonant match meant I could drive high-Z heads with transistors.


David

[Herald1360]

The TK9 was a two track mono machine with the interesting capability of playback and record in either direction

So I suspect the switching was one or the other but not both.

So I suspect the switching was one or the other but not both

I doubt it would matter as long as there was enough erase current.

[mrmagnetophon]

The problem is going to be, I have no coils to use, how do I bypass that?
-Chris

[mrmagnetophon]

Isn't there some type of self oscillating 555 timer chip that I could amplify to drive a head, the head would need say 5 watts probably, at 50 volts though?
-Chris

[Refugee]

I have a Mullard book that has a bias erase circuit that does not have a transformer in it.
I have not had time to look for it over the past few days due to a crime having been committed in our street. Too busy copying CCTV images onto CDs and giving statements

[ricard]

You initially mentioned an erase head amplifier. Does that mean that you actually have an erase oscillator and just need to beef up the output? In that case a 6V6 connected as an amplifier would do just in fine.

In the valve days, the oscillator usually drove the heads directly, as in many transistor machines, however, I've seen other (usually more upmarked) transistor machines where the oscillator actually drives a small power amplifier which in turn drives the heads.

[Amraduk]

Originally Posted by ricard

You initially mentioned an erase head amplifier. Does that mean that you actually have an erase oscillator and just need to beef up the output?

No, he doesn't.

Originally Posted by mrmagnetophon

I am working on designing a Erase head amplifier for a r2r machine. The head is an old style Ampex head, and will need approx 3 watts of power) so my friend says. I am going to also need to design a Oscillator to provide the erase oscillation aswell?

Regards,

Dave.