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Thread: Magnatone M15A Echo click

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    Magnatone M15A Echo click

    I've been slowly working on a bit of a basket-case Magnatone M15A, and I've now got most of it sorted out. The most obvious issue remaining is an annoying click when Echo is used.

    This is one of the models with "Echo" on the reverb, a neon lamp relaxation oscillator in a Fender-style bug with an LDR that only affects the recovered reverb signal. It's a cool-sounding effect, but you get a very audible click in time with the lamp when you turn Echo on. The loudness of the click tracks with the position of the 500k Depth pot.

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    What would help me is to know whether or not they simply came from the factory this way. It seems like Magnatone was constantly revising the reverb circuit during the period when this amp was made (late 1965).

    One thing I noticed is that the 0.22uF capacitor and the neon lamp are actually grounded in different places, and that appears to be factory-original. Someone before me did have to replace the Depth pot, and he had to remove and reattach the neon lamp connections to do it. I assume he put all the connections back where they were, but I can't be 100% certain.

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    Old Timer Leo_Gnardo's Avatar
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    Try swapping a LED in place of the neon lamp. I do this on Fenders all the time - click vanishes. Cheap. My theory is the neon lamp generates a very sharp waveform when the gas breaks down and starts to conduct, and that radiates into nearby circuitry. You may have to add a series resistor to the LED to approximate the action of the original vibrato. I also do this on old Ampegs with failed vibrato lamp/photocell units.

    I sort of doubt you'll find anyone who was listening closely in particular to Magnatone 15 in 1966 and remembers. BUT I could be wrong about that

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    I think the problem with the LED idea is that the neon lamp is not just the light source for the LDR as it is in the Fender circuit. It's also the LFO. There's no separate oscillator driving it.

    I don't think an LED would work as a relaxation oscillator in place of a neon lamp. I know with a neon lamp, the voltage across it has to rise to a certain point for the lamp to fire initially, at which point the voltage across the lamp falls.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Rhodesplyr View Post
    I don't think an LED would work as a relaxation oscillator in place of a neon lamp.
    Right, dang it does require the neon. Oh well... Relaxation oscillors were some of the first circuits I built as a teenage hobbyist, with neon, also with the rarely seen unijunction transistor.

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    Member Jon Snell's Avatar
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    This is not an oscillator.
    The time constant is set by a neon that triggers to a low voltage from around 90volts with the 220n capacitor, setting the charge rate with the high value 6M8 and 5M pot setting the frequency of the charge rate. This process repeats until the switch is opened and the capacitor no longer charges.
    It relies on the LDR portion having a slow reaction giving a modulation effect.

    I would check for leaky decoupling capacitors that will cause any errant DC level to fluctuate and 'click'.

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    Last edited by Jon Snell; 04-09-2020 at 12:23 PM. Reason: More observations
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    This is not an oscillator.
    What do you mean?

    You perfectly described the principle of a classic relaxation oscillator. It just requires a resistance, a cap and a triggering device that doesn't conduct below its ignition voltage, i.e. the neon.

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    Last edited by Helmholtz; 04-09-2020 at 05:26 PM.
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    Member Jon Snell's Avatar
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    I was trying to avoid people getting confused who do not realize that there is no phase shift required as in a standard oscillator, that's all.

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    Lifetime Member Enzo's Avatar
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    Perfect opportunity to educate them that other forms of oscillator exist.

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    Member Jon Snell's Avatar
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    Back in the late 60s when disco lighting was in its infancy, I used a neon, 220n capacitor and pot with a limiting resistor to fire a gate sensitive thyristor in a bridge rectifier for full wave., Triacs were in their infancy and took too much current between MT2 and the gate to work. Not much success with a DIAC.
    It was quite effective and I sold many of them. I quickly moved to a four channel sequencer, using TTL, with sound control which was even more of a success.

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    Lifetime Member Enzo's Avatar
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    I discoverd relaxation neon oscillators early on. A bonus was when I discovered I could parallel sections off the same supply and they would modulate one another.

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    Member Jon Snell's Avatar
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    A company called 10k made a 10 channel chaser using that principal. It was very good.
    Back in the day.

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Jon Snell View Post
    A company called 10k made a 10 channel chaser using that principal. It was very good.
    Back in the day.
    One of my apartment mates in college made a 100-lamp "nothing box" again with the same neon circuit. 100 blinking bulbs with randomly selected timing circuits on a wooden frame, say 40 x 40 cm. With a master speed control - simply dialed the voltage up & down. Back in the day you could buy 100 NE-2 for a dollar from PolyPaks. Fun to watch during late Friday & Saturday night party sessions. He went on to be an astrophysicist .

    Kool idea on the chaser, I could guess neon lamps triggering phototransistors which in turn trigger SCRs or triacs. Good clean fun, and a "wow!" stage spectacle at the time.

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    Lifetime Member Enzo's Avatar
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    I had fun with them, watching the patterns, and then one magic day I coupled the circuit to an amplifier with a cap. Oh the glorious buzzing and whirring.

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    Hey Enzo I heard you were the guy to ask questions to about Peavey VTClassic 50?

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    Lifetime Member Enzo's Avatar
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    Ho, welcome

    Two things:
    1. The way it works is start a new thread for your amp.

    2. I admit upfront I am a nit picker. And Peavey never made a VT Classic 50. Peavey made and still makes a "Classic 50". Fine amp. Peavey also made "Classic" back in the 1970s. A little later they updated that to "Classic VT". So there is a Classic 50 and a VT Classic, but no VT Classic 50. The Classic VT may have been 50 watts, but that is not part of its name.

    Classic 50 is an all tube amp (well except the reverb), and all the plain old Classics are tube power amp and solid state preamp. Very different amps.

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