While trying to place back the mixer and tape delay section of the FBT personal 1002 to its wooden structure a wire was cut accidentally.
It was impossible to figure out where it was going originally so it's other end was traced. And it was connected to the recording head and the bias oscillator output.
This serendipitous accident strongly sujesting that the analog simulation experiments should follow up.
Looking at the schematic of the Italian FBT personal 1002 electron tube PA system the tape delay recording head is connected as follows. At the anode of the recoding amplifier output tube (ECC82 or 83?) is a normal DC blocking capacitor and then a series resistor of 220KΩ! the other side of which connects to the recording head.
By observing the actual equipment the implementation is. The anode connected to a 220KΩ resistor and then a coupling capacitor feeds the recording head.
Electron tubes seem ideal to feed a magnetic head with constant magnetic flux. Electron tubes and JFETs are voltage controlled current sources. Electron tubes have a high output voltage and high output impedance. This output impedance is further increased by 220KΩ in this case. So with the inductance of the recording head we have a great rising voltage responce with frequency (Pleiades filter). Or constant current. So that the tape can approach saturation at all frequencies?
The following questions may be interesting.
How would this tape delay system sound if we forget about tape and just unscrew the recording head and and let it "kiss" at the airgap the playback head?
Should the high frequency bias be on?
What would happen if the halo (echo repeat or feeding back to itself) function is activated and adjusted before oscillation? This time there is no tape delay and the only delay is the time light (electromagnetic wave) takes to travel between input and output of the electron tubes.
(In fact there is positive feedback on the Telefunken V72 preamplifier. By watching the schematic there is a resistor between the cathode of the second stage and the cathode of the first stage).
Are we talking about thousands of decaying sound mirror images happening at extremely small fractions of a second?
To follow up in the analog simulation experiments of analog tape recording the schematic of the Binson Echorec should be examined too.
It sounds great and it was used extensively by Pink Floyd, both Richard Wright and Dave Gilmour.
Pleiades laboratories have 2 circuit boards of Binson Echorec from 2 damaged Binsons.
Should they be used by feeding the output of the recording amp to the primary winding of a transformer, and the secondary winding feeding the playback amplifier?. Using a transformer made of very small volume of iron?
To create a great quality simulation system from scratch (since a transformer must have much less loss than 2 heads joined) would low voltage battery operating JFETs or battery operated (Pleiades V series prepreamps) electron tubes give excellent signal to noise ratio at much less time constant than used at 30ips reel to reel tape recording?
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