Don, what would you recommend be used in place of the "Brimistor" immediately after the diode bridge on a late '64 AC50MKIII? I guess it's a thermal protection device of some sort. Is it a Varistor? If so what is the modern value to use?
I see the later AC50 circuits indicate a 22R/6Watt resistor in this position. Is this an alternative?
Replacment for "Brimistor" in AC50 MKIII
Moderator: jingle_jangle
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toneman
Glen; The Brimistor was used as a thermal device in those amps. A Varistor basically protects against voltage spikes. Two different things. I would go with the resistor. Although a 6 watt type maybe a bit hard to come by. Go to a 10 watter. I have seen many (including my own AC-50 smallbox) that don't even had one.
Thanks Don. I think I'll go for the 22R resistor. Not only for the inrush limiting but to give the power supply a more valve rectifed feel.
I've since found out that the modern equivalent of the Brimistor is an NTC Thermistor. My AC50 doesn't appear to have ever had the Brimistor but I'm keen to build in some protection. You can still get thermistors with sufficient current rating, but you need to know the room temperature resistance to select the correct one.
I also have a question about the choke on this amp. On my inductance meter the one that came with it measures at about the right value, (6 H) but the dc resistance seems really high - reading nearly 19K.
I have another choke that looks almost identical. It reads about 4 H inductance but only 100ohms dc resistance. Which one is correct for an AC50?
I've since found out that the modern equivalent of the Brimistor is an NTC Thermistor. My AC50 doesn't appear to have ever had the Brimistor but I'm keen to build in some protection. You can still get thermistors with sufficient current rating, but you need to know the room temperature resistance to select the correct one.
I also have a question about the choke on this amp. On my inductance meter the one that came with it measures at about the right value, (6 H) but the dc resistance seems really high - reading nearly 19K.
I have another choke that looks almost identical. It reads about 4 H inductance but only 100ohms dc resistance. Which one is correct for an AC50?
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toneman
Glen; The resistence on the choke seems awfully high!!!
I just checked the AC-50 schematic and it calls for an 8 H choke. I have an AC-30 choke(schematic says "choke, anywhere between 10-20 H"...LOL!) and a few Marshall 10 H chokes. The AC-30 one reads 276 ohms and the Marshall one's read 450 ohms. These chokes are from Mercury Magnetics.
I would say a choke that reads anywhere between those two would work for the AC-50.
I'd say you have a shorted choke and at 19K is sucking down alot of B+ to the following stages of that amp.
I just checked the AC-50 schematic and it calls for an 8 H choke. I have an AC-30 choke(schematic says "choke, anywhere between 10-20 H"...LOL!) and a few Marshall 10 H chokes. The AC-30 one reads 276 ohms and the Marshall one's read 450 ohms. These chokes are from Mercury Magnetics.
I would say a choke that reads anywhere between those two would work for the AC-50.
I'd say you have a shorted choke and at 19K is sucking down alot of B+ to the following stages of that amp.