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Thread: Fender 75 Zener Diode...

  1. #1
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    Fender 75 Zener Diode...

    I couldn't decide if the Fender 75 is a vintage amp or not. Well, it's about 35 years old now so I guess it might be considered vintage, certainly one of Fender's last point-to-pont amps.

    The Fender 75 schematic shows a zener diode (6.2 volt, Fender part # 015005) in the bias area but also for the vactrols, probably for regulation. It's the one and only zener in the whole amp.

    I bought one of these amps recently and my very clean and unmodded version (almost looked new inside - I don't think it's had any repairs) did not have this zener diode present. I worked another Fender 75 about two months ago and this version also did not have the said zener diode in the circuit. Any other Fender 75 owners out there who've noticed this small discrepancy? I'm wondering if I should add it now, although it's made it for 35 years quite well without it. I guess it probably helps the vactrols with their switching function. Any comments?

    Thanks for your participation,

    Bob M.

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    Senior Member Enzo's Avatar
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    The circuit uses the same negative supply as the bias uses. After all it matters not which polarity DC lights an LED.

    The zener caps the LED supply at 6v. IF Fender left it out, my first thought is they decided it was not needed, or that it caused unintended problems in certain situations.

    My first question is this: WHY do you want to ad one? You said it already, the amp has made it 35 years without it so far.

    Resist the urge to install things you see in schematics but not in the amps. Usually it just represents changes. Circuits change all the time, and often they do not publish a new schematic after each change. It is especially hard for people when they see empty spots on a printed circuit board. "But, but, it says right here there should be a cap." If we add in the missing part, we may also overlook changes elsewhere that made it unnecessary, and we may add problems by returning the part.
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    Interesting - any other deviations from the schematic? http://schems.com/schematicheaven.ne...r_75_schem.pdf
    The upper smaller fainter one in the link, dated 10/23/79 is more accurate to the ones I've seen.

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    Senior Member Enzo's Avatar
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    Without laboriously studying them part by part, what are the differences between the two drawings?

    To be honest, unless I was having a problem on that supply line, I would not likely notice the diode is not there.
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  5. #5
    Old Timer J M Fahey's Avatar
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    That line feeds 2 Vactrols through 270 ohm resistors.

    Start by remembering that Leds are current controlled devices, not voltage controlled, so rather than slamming any specific voltage to them, we wire series resistors and connect them to a supply source; Ohm´s Law will define what current is passed there.

    I will round off all values to within 10% for easier calculation, no need for better precision here.

    *With* Zener diode, we have 6V supply, 2V drop across Leds, 270 ohm resistor, so when active we send 4000mV/250Ω=16 mA to each LED.

    *Without* Zener, main resistors are 820+390 ohm ones, going from -56V to roughly -6V, so they will pass some 50V/1200 ohms=40 mA ... for two Leds, so some 20 mA each.

    Now hitting a Led with 20mA instead of 16 mA is almost imperceptible.
    To boot, those Vactrols are not used as linear controls but as switches, so even less important.

    In a nutshell; I bet Fender found those Zeners were pretty un necessary, so .....
    Which is confirmed by having *two* unmolested Factory ones without them.

    IF any problem at all, they "should" added the letters n.u. (not used) or n.f. (not fitted) at the schematic just by them.
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    Juan Manuel Fahey

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    Quote Originally Posted by Enzo View Post
    Without laboriously studying them part by part, what are the differences between the two drawings?
    I think it's mainly just around the volume control, with a bigger 500pF bright cap and a treble peaker formed by the previous 120pF bright cap and a 820k connected in parallel, feeding the volume pot.
    Last edited by pdf64; 08-08-2017 at 05:18 PM.

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    That's a great analysis of what's going on with the Vactrols, LEDs and their respective power supply as to current and voltage by J M Fahey. Enzo, I'm a 'if it ain't broke, don't fix it' kinda guy. I was thinking someone might talk me into using the zener diode in this circuit or show me why it should be there. As we see, the opposite has happened. But you make a strong point about unintended consequences, something to keep in mind in many situations.

    I did some net research and found that some are of the opinion that there are two (official) schematics for the Fender 75 circuit. I have only seen one schematic but here's a summary of changes made to the second one. I have not verified any of these against the actual amps but I'm just passing this along so it might need some verification for accuracy:

    There are two official schematics for this amp, the first dated drawn on 28th October 1979, the other revised version without a date. The only differences that I can spot between the two are;

    1. The removal of the 820K resistor from the signal path between the Treble pot wiper and Volume pot top. This obviously leads to more gain, hence the second difference;
    2. The given input test voltage is dropped from 8mV. to 4mV. resulting in the measurement on the plate of tube V1A having 220mV. instead of 440mV. With the 820K resistor removed, the voltages remain the same in the rest of the amp.
    3. A different arrangement around the Bright Switch; A 500p that bridged the 820K resistor would have added quiet a presence boost, it is removed, and the 10M resistor that suppresses the switching noise that the bright caps can cause, now discharges to ground rather than across the switch. The revised version is more like a standard Fender Bright Switch.
    4. The wiring to the tip and ring of the Effects Jack have been reversed.
    5. The voltage given for the cathode follower V3B. Bias has been corrected from 200V to 50V.

    Like pdf64 says, most changes are mainly around the bright switch and volume.

    Thanks for the comments,

    Bob M.

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