Driving a tape recording head directly from anode (part 5)

At your risk. Taking all safety precautions is very important.


The Pleiades V6 schematic is used. Anodes directly connected to Tascam Porta 03 recoding heads. 2 amplifiers for 2 of the 4 tracks. Cassette running at normal speed.


For the time being DC tape bias is used (the anode current itself). It gives thicker noise. But it sounds nice. Like a vinyl record fff... sound when the song comes to the end.


A bias current of about 300μA-400μA seems for the time being desirable.


One of the main problems is designing an amplifier that has a quiescent class A current of 400μA while at the same time able to give much voltage which will be needed for the high frequencies. The head is an inductive load so it will be driven by constant current. This means the driving voltage will increase by 6dB per octave with rising frequency as the amplifier is less and less loaded.


So what drives the magnetic head will have to be of high enough impedance. Electron tubes seem to be perfect for the job.


So far the EF183 electron tube had been tried at up to 24V plate supply. A Pleiades bias or pull up bias resitor from anode to grid makes operation possible. Open grid was also trisd, see previous posts.


The problem is that Ia=400μA cannot be much exceeded or tape will be over biased. But for example EF183 gives much more output and less distortion at high level at Vb=12V when bias is such that anode current is say over 1.5mA. 1500μA can be achieved by a 10MΩ resistor from anode to grid. And in fact connecting the HD580 to anodes gives an excellent sound.


But when 400μA is desired the Vb would better be lower say 5V-6V. Rag can be 4MΩ. Connecting the HD580 headphones at such operating conditions gives a nice sound but with distortion if one wants higher output. When connecting the heads the treble immediately rises or bass decreases due to the constant current inductive effect of the head. In fact the head become like a low cut inductive filter connecting across a microphone (see Pleiades filters).


But the distortion further increases as heard on the headphones. Nevertheless it is surprising that at such conditions a relatively nice recording can be made to cassette tape. Modulation goes up to about +3 VU at playback. Warm noise or hiss is buried in music. Guitars have a high class quality that reminds 60's electron tube recordings such as CBS, for example Jim Reeves recordings. But there is still treble missing.


It became apparent that a different electron tube would be better if it can give less current at higher voltage.


Today the EF89 was tried. Anode current when Philips electron tubes were used was indeed lower. But the sound on the headphones while listening to what will go to the heads sounded much more distorted and much less loud. The EF89 is less sensitive needing a higher signal driving voltage. This was tried too.. Recordings with EF89 were much more distorted Unless very low modulation was used. Treble was higher. Signal was lower. Bass hiss this time could be heard along with the music.


So the setup was returned for the time being to EF183.


After relistening to all recordings done so far the best seem to be those done as follows. EF183, with 1.5MΩ plate to grid resistor, plate supply Vb=3.6V, Ia=250μA. More modulation but less treble is when Vb=5.5V, Ia=500μA. Possibly aiming for a DC bias current between 250μA and 500μA. Voltage headroom of amplifier needs to be increased but without increasing Ia if possible?


Another candidate electron tube may be the EF86.


And there is a gut feeling that the ECC83 will be best. It can have an operating point of 400μA at high voltage. It has a high voltage amplification factor. It has a high anode output impedance. It may have to operate at a high plate supply voltage. Or lower if Pleiades bias is used.


Another consideration may be whether an electron tube can cope directly connected to an indicative load such as a recording head coil. It may. If not the usual high series resitor would have to be used between output and head. See nearly all schematics of recording amplifiers, electron tube or transistor.


Another consideration is whether what has already been done (deliberately not using extra HF boost) is adequate and all now needed is to connect the EF183 anode to the head of a machine running tape at 7.5ips. Eg Uher 4200. Or even at higher speed.