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66.74.191.120
There is a surgistor connected to one leg of the primary on the Acrosound UL-120. It's supposed to bring the voltage up to spec in about a minute. The one in mine is shot with corrosion and the gap is way off. What can I use in place of this thing? Also, there is something in parallel with the surgistor that looks like it may have been a resistor of some sort, however it has disintegrated. I see pieces of the sand coating, but it looks almost as if the inside was made of some sort of fiberglass or cloth. Is this part and parcel with the surgistor because I don't see what else it could be. Also, is there a schematic symbol for the surgistor...might it be shown as a choke (1hy@400ma)? The schematic shows this but I don't see a choke in the traditional sense. Thanks everyone!
"I KNOW you can hear it, but are you REALLY listening?"
Follow Ups:
the choke you refer to is built into the power transformer.
I agree with the negative temperature coefficient (NTC) inrush current limiting thermistor idea.
I've provided a link to a data sheet. Pick a part that can handle the power trafo primary current draw and that has enough hot resistance to hold the line voltage down somewhat.
Eli D.
Eli,
How do I determine the Primary current draw? Is the rated fuse a good indicator? This amp requires a 5 amp fuse. Also, do you know what the resistor type thing I described above is...? Thanks!
"I KNOW you can hear it, but are you REALLY listening?"
The resistor type thing you described is a resistor/heater.
A surgistor is composed of two basic parts: a resistor/heater and a bi-metal strip that physically deflects when it's heated. The heater portion is wired in series with the load, usually the primary of the PT for old timey tube amps. The bi-metal strip is set up as a normally open switch parallelled across the heater and in close physical proximity to the heater. When current flows thru the heater it inherently limits max current inrush due to its resistance. It also gets quite warm which in turn heats up the bi-metal strip. As the strip warms up it begins to deflect towards the NO contact until it makes contact. At this point it shorts out the heater thus effectively removing it from the circuit. Since the electrical resistance of the bi-metal strip isn't zero, normal operating current flowing thru it generates enough heat to keep it deflected into the closed position.
About half the surgistors I encounter have failed heaters. They're usually open and look as though they were subjected to gross overcurrent...like someone replaced the fuse with a 15A unit so the surgistor became the defacto fuse. Messed up bi-metal strips are common too as are dirty contacts.
Although ingenious in concept, their Rube-Goldberg nature makes them problematic IMO.
Some assert that a bad surgistor should be replaced with a genuine surgistor replacement part. Apparently they're still available NOS on occasion at that auction place.
Personally, I replace any surgistor I encounter, working or not, with the modern equivalent (sorta), an NTC thermistor or "inrush limiter". These have no moving parts and are generally long lived. Proper sizing requires some knowledge of the operating primary current of the amp but from what small amount I know about the UL 120, I'd be inclined to use the GE (formerly Keystone) CL80 NTC inrush limiter. Looks like a dark grey disc type ceramic cap and goes in series with the PT primary. Mac used the equivalent on all of their MC2xy amps with good results so there is a commercial precedent.
I've been working on an Eico HF-87. Its surgistor was replaced previously by a pair of CL-80 thermistors in series. Seems to work well, although I haven't done any measurements during power up.
. . . Charlie
Steve
I do that same thing you do but what the old reid switch contact type limiters do is keep the voltage down and the IRCL just limits the inrush of current instead of voltage so Im wondering if its feasible or not..Jim McShane an inrush kit and Im thinking that might be a good route to take.
If the power supply waveform isn't pretty,neither is the sound in most cases.
Mike:Jim's kit makes use of an NTC thermistor just as we're discussing. The primary difference I can discern is that Jim has chosen to place the thermistor in series with the SEC of the PT instead of the primary. There are good cases for either location but I prefer the primary because it limits inrush to the PT primary and provides a nice ramp-up of tube heater voltages as well as a B+ ramp-up. The downside IME is slightly degraded voltage regulation. Placing the thermistor in the sec circuit primarily limits voltage surge to the filter caps. I've chosen to live with the minor regulation degradation to gain the benefits of the overall reduction in turn-on surges. Also, PTs with primary side limiting don't ever go "boing" when you turn them on at the wrong part of the AC cycle or when residual magnetization is present.
Edits: 07/21/09 07/21/09
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