Fusistor replacement

[Kamakiri]

Got this suitcase record player with the cheapest amp I have ever seen. I need to replace a 220 ohm 5 watt sand fusistor. I replaced it with a standard ceramic, and it started burning.

Oddly enough, I had this problem in a Predicta Holiday and I went back and replaced it with an NOS fusistor, and no smoke this time....which tells me that I'm doing it wrong.

What do I replace this 220 ohm 5 watt with, or does anyone happen to have a good one of this type?

[old_coot88]

Are you sure it's 220 ohms? Fusistors ('Gravel Gerties') were usually very low ohms values, like 4.7, 5.6, etc.

[zeno]

It is an odd value for a fusable. If it is 220 ohm I
would put a fuse in series with it. Odds are if its burning
some things shorted. If its SS look for rectifier or
output xistor. If tubes rectifier or filter caps.
If you give me model & make I will look it up & check
the specs if I got the manual.

73 Zeno

[old_coot88]

Forgot to ask if the amp is tube type or SS.
But since a Predicta was mentioned in context with it, i assumed tube type.

[Kamakiri]

Tube amp. Just 2 tubes, a 50C5 and a 35W4. I assume the resistor does a lot of voltage dropping, hence the 220 ohm rating.

[jr_tech]

35V + 50V = 85V (tube heater total voltage)
117V - 85V = 32V (voltage drop needed to supply heaters from line voltage)
32V/0.15A = 213 ohms (resistance needed to drop 32V)
0.15A x 32V = 4.8 Watts (power dissipated in resistor)

So the resistor seems to be used for the heater supply for the tubes and the right size and rating... I don't understand why a fuse resistor was chosen here, but your replacement would appear to be correct.... you must have a short somewhere.

jr

[Kamakiri]

Actually, the amp was working with the old one in there. The whole reason I'm replacing it is because it looks like someone snipped a lead to test it at some point, but cut the stupid lead so close to the body of the resistor that the solder joint there was questionable at best, and there's not much left to solder on to.

Maybe I just need to get one with a higher heat tolerance, like a Dale. Problem is, there's really no chassis to sink it to.

[NoPegs]

Originally Posted by Kamakiri

Actually, the amp was working with the old one in there. The whole reason I'm replacing it is because it looks like someone snipped a lead to test it at some point, but cut the stupid lead so close to the body of the resistor that the solder joint there was questionable at best, and there's not much left to solder on to.

Maybe I just need to get one with a higher heat tolerance, like a Dale. Problem is, there's really no chassis to sink it to.

Four 900 ohm 2 watt jobbies in parallel perhaps?

Alternatively change things up a bit and use a cap to drop the heaters and then just fuse the B+ end of things? I only suggest this because in this scenario 4.7µF is PRECISELY the calculated value for 120v input. Finally a preferred number is useful!

[jr_tech]

Originally Posted by Kamakiri

I need to replace a 220 ohm 5 watt sand fusistor. I replaced it with a standard ceramic, and it started burning.

Couple of things... Assuming that you have no shorts anywhere and the value/rating of the resistor are indeed as stated, and not miss-marked.
The current drawn by the tube heaters at turn-on will be considerably higher than the spec. value of 0.15A for a few seconds, until the tube heaters start to warm up. The 5 watt resistor rating was likely exceeded for a few seconds, but this is usually no problem. New power resistors may also smoke for a while when first used.
Did it actually "burn up" or just smoke a bit? Did the tubes begin to light up, or remain dark?

jr

[Kamakiri]

Tubes lit up, and I could see a wisp of smoke coming from the resistor, which had begun to brown slightly.

[jr_tech]

Any sound or hum from the speaker?

jr

[old_coot88]

Some super-cheapo record players used a tapped motor winding to get the heater voltage.
Barring that, 220 ohms for a dropper sounds just about right. It'll dissipate around 5 watts. I'd use at least a 10 watt rating for replacement.

[Celt]

That's what I was going to say. I had a cheap Japanese record grinder in the 60's with those two tubes and the BSR changer had a second winding on it to supply the 85 Volts.

[Kamakiri]

The odd thing about this is that the amp works, but very low volume. So low, in fact, that I got nothing out of it with the turntable. I did manage to get some decent low volume out of the amp by hooking up a different source via the RCA jack.

Figured replacing the resistor couldn't hurt, since I've replaced everything else

[old_coot88]

Those cheapo players with no gain stage used a ceramic cartridge with 2 - 3 volt output to drive the 50C5 directly. But the cartridge element always dies an early death.