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  1. #11
    Denis R's Avatar
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    digikey

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  2. #12
    Nicholas Lindan's Avatar
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    If the problem with the head is the Vactrol, and if it is an old head it quite well may be as they do burn-out, then...

    The Vactrol used in the old heads had an incandescent bulb and a CdS element. The bulb was driven with the same regulated waveform (at a reduced voltage) as the enlarger bulb -serving as an analog for the lamp in the enlarger. By regulating the brightness of the Vactrol bulb the circuit regulated the light in the enlarger. Naturally the incandescent bulb burns out with time.

    The modern Vactrol replacements use an LED for the light source. These can't be substituted for the original vactrol as: the LED requires a current drive while the lightbulb was driven with a voltage; the brightness/current relationship for an LED is different from the characteristics of a light bulb; the original used a CdS cell, modern vactrols use either a photodiode or phototransistor though EGG may still make them with CdS cells; the dynamic response of a lightbulb is very different from an LED, when fed a chopped waveform a lightbulb will still produce a relatively constant output while an LED will produced a chopped output.

    Beseler at some point changed the design so it could use a modern Vactrol. You may be able to effect the same modifications to your head so it can also use a modern part.

    The original light-bulb Vactrols are in great demand for restoring old electric guitar amps where they were used for the limiting circuit. The new LED Vactrols don't have the same attack/delay times as the originals and the amplifier doesn't sound the same.

    Why someone doesn't go into business making old-style Vactrols, I don't quite know. All there is to them is a lightbulb, a bit of plastic tubing and a CDS cell.
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  3. #13
    clayne's Avatar
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    Quote Originally Posted by Nicholas Lindan View Post
    The original light-bulb Vactrols are in great demand for restoring old electric guitar amps where they were used for the limiting circuit. The new LED Vactrols don't have the same attack/delay times as the originals and the amplifier doesn't sound the same.

    Why someone doesn't go into business making old-style Vactrols, I don't quite know. All there is to them is a lightbulb, a bit of plastic tubing and a CDS cell.
    Even though I don't have this head or even a Beseler enlarger, this info is fascinating Nicholas. I'm always interested in pro audio gear and music equipment - especially the classic analog stuff. Is the issue that the LEDs have too fast a delay or is it that they are now entirely "square-wave" in their response? I would figure a lightbulb would produce a typical knee'd response in it's output and hence result in more musicality for something like a limiting circuit.

    Actually, is what we are describing basically the typical design of an electro-optical compressor?
    Stop worrying about grain, resolution, sharpness, and everything else that doesn't have a damn thing to do with substance.

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  4. #14
    ic-racer's Avatar
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    Quote Originally Posted by Nicholas Lindan View Post

    The original light-bulb Vactrols are in great demand for restoring old electric guitar amps where they were used for the limiting circuit.
    There is one in the tremolo circuit on old Fender amps.

  5. #15
    Nicholas Lindan's Avatar
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    Quote Originally Posted by ic-racer View Post
    There is one in the tremolo circuit on old Fender amps.
    They are voltage controlled attenuators - so if the lightbulb goes to the output of an oscillator and the CdS cell changes the amplifier volume - presto: tremelo...
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  6. #16

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    This has been an interesting thread. I checked with Beseler right away. They no longer supply any parts. I spoke with the two independent repair services that Beseler lists on their site, and for lack of parts, they no longer repair the 45S heads.

    I picked up the rectifier that Nicholas mentioned for 45 cents. I also picked up a capacitor recommended by the retail store that will connect between the output end of the rectifier and ground. This should help filter the signal a little. I could feed a regulated 120 voltage into this small circuit, but I doubt I will. I like the rectifier solution, because I can use the bulbs originally intended for the enlarger. At about $13 each from B&H, I'll get extras. As to installing the rectifier, I'll put it in series with the lamp as Nicholas suggested. The output side of the rectifier should lead to the light. The 100 micropico capacitor will connect this output side to the ground.

    I have the Zone VI Compensating timer, and I've installed an extra sensor into the Beseler head. So, this will regulate the head. Note that I installed the sensor such that it's exposed to the light source, not the filtered light. There's a neat little mirror inside the head that reflects light onto the front dials that also works to direct light to the sensor. (When properly placed.) In this way, I avoid heat issues with the sensor, and it attenuates the bright light so as not to overwhelm the sensor. It appears to work great.

    To anticipate a question, why install the rectifier, if I have the sensor. To answer, I don't like 12 second delays in my printing! In theory, it doesn't matter. But even with the sensor, it makes sense to have the head as consistent as possible.

    As to all the fancy circuitry remaining in the head, it can run the fan.
    Last edited by Neil Poulsen; 11-07-2009 at 04:34 PM. Click to view previous post history.

  7. #17
    Nicholas Lindan's Avatar
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    Do you know the part # of the rectifier you purchased?

    What is the size and voltage rating of the capacitor? 100 "micropico" isn't right (a micropico would by one millionth of one trillionth, or 100 atofarads).
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  8. #18

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    100 Microferads! (Thanks.)

  9. #19
    Nicholas Lindan's Avatar
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    I should update this -- do NOT put a filter capacitor across the lightbulb: it will convert the 1/2 cycle AC at 83 VRMS to DC at ~160 volts (which is 160V RMS, as for DC the peak, average and RMS voltages are all the same). 160V RMS will make for a very short lamp life.

    The rectifier used should be rated at better than 600 volts and 5 amps. A rectifier diode similar to the 6A6, 6A8 or 6A10 would be appropriate:
    http://www.diodes.com/datasheets/ds28009.pdf
    There are many styles of rectifiers available that would work, one is certainly not limited to the above device.
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  10. #20
    Steve Smith's Avatar
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    Quote Originally Posted by Nicholas Lindan View Post
    You don't need a variac - just put a rectifier in series with the lamp as 1/2 wave rectified 120 VRMS is 83 VRMS. Beseler does just this when they put 83V lamps in an unregulated head.
    I assume my LPL 6700 also has half wave rectification as it too uses an 83v bulb but runs on 120v (from a transformer as I am in the UK with a 230v supply).

    I always thought 83v was a funny rating for a bulb..... and I suppose I still do!


    Steve.
    "People who say things won't work are a dime a dozen. People who figure out how to make things work are worth a fortune" - Dave Rat.

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