Hey everyone. I have and AC50 MKIII on the bench with a Surgistor that is acting up. For those unfamiliar, this is a mechanical device intended to replicate the soft start of a GZ34 in a solid state rectified amp. Here's a link with a nice look at what I'm talking about.
http://www.voxshowroom.com/uk/amp/ac50dr_hood.html
The Surgistor in the specimen under test is failing. The bi-metallic strip doesn't want to keep contact and intermittently goes open when the amp is being played. What I'd like to know is:
Is it safe to just bypass it assuming modern caps can hold up to the inrush current more effectively?
Is there a modern equivalent that wouldn't require adding an elaborate soft-start circuit?
Should I hunt down a vintage surgistor? It is a very clean vintage amp. Maybe best to keep it original.
What to the Vox Experts out there think?
AC50 MKIII Surgistor
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Stevem
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Re: AC50 MKIII Surgistor
You can do two the things here.
Replace it with a 50 to 100 ohm Thermistor with the needed current rating, or just jump it out but you will need to check the unloaded D.C. Out of the diodes as you may find that you need to series connect 2 filters to handle that surge voltage and or rewire the standby switch such that these series filters have the standby switch after them .
Replace it with a 50 to 100 ohm Thermistor with the needed current rating, or just jump it out but you will need to check the unloaded D.C. Out of the diodes as you may find that you need to series connect 2 filters to handle that surge voltage and or rewire the standby switch such that these series filters have the standby switch after them .
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Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: AC50 MKIII Surgistor
Thanks Steve, there's no standby switch. I could replace the mains switch with an off/on/standby 3- position switch but the original Vox switch with the square-ish plastic bat is part of the look of the amp. Series connection if filters isn't really an option without messing up the layout of the amp. The reservoir and 2nd node filter caps are mounted on top of the lower chassis using isolated standoffs for the hot side. That leaves the NTC thermistor option. I see 22R 2.8A thermistors mentioned as a substitute for the Brimistor which would be found in-between the bridge rectifier and the 1st filter node on the B+ rail. The sugistor is on the primary side of the PT. Any idea how I'd go about calculating a replacement value? I could also jump the surgistor and put the thermistor on the secondary side where the Brimistor would go (this amp never had a Brimistor). Hmmm....
Re: AC50 MKIII Surgistor
With the surgistor jumpered, the B+ at cold startup goes up to about 510v for a sec before settling down to 465v. I'm using 500V F&T caps. Once the tubes warm up the primary side of the transformer is pulling just shy of 800 mA.