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Arcol power resistors mounting

koi

pfm Member
I have a couple of these in one of my power amplifiers and one was blown I noticed that they were not mounted on heatsinks but just bolted to bottom of amplifier

I have read that they get very hot and was wondering if I should mount them on heatsinks

Any advice appreciated

Cheers
 
Beam echo monos

Think it is to do with fixed bias on el34 valves the circuit diagram is very similar to Mullard 5-20 Amp
 
Like all resistors they only get very hot if you run them at their full rating, bolted to the chassis sounds more than adequate to me if they are for cathode bias.
 
Just a suggestion, but if you are talking about the EL34 cathode resistors, the originals were 470 ohm,10 watt 'Standees' and were not super reliable. But were conservatively rated as they should have 3 watts dissipation in that circuit. (36 volts across 470 ohms)
If they have been replace with 10 watt Arcols and one of those has failed, I would want to check they are not passing too much current. First suspects would be the 2 coupling caps (C6 and C7) for leaking.

Otherwise, Arcols have to be chassis mounted to meet their quoted wattage, if not they usually have about half the rating.
 
If they have been replace with 10 watt Arcols and one of those has failed, I would want to check they are not passing too much current. First suspects would be the 2 coupling caps (C6 and C7) for leaking.

Otherwise, Arcols have to be chassis mounted to meet their quoted wattage, if not they usually have about half the rating.

This is a valid point but even so I`d be surprised if it caused the resistor to fail. I did a test to destruction on several makes of 50 watt metalclad resistors some years ago and after several hours at 150% load on a less than ideal heatsink the Arcol sample popped its end cap but it didn`t actually fail. The others, CGS and Welwyn if I remember correctly were untouched but they were also twice the price at the time.
 
They should be fine then for the use they have been put to.

The MF A370 power amp used these for the soft start circuit, at mains potential, and they were bolted to the rear of the chassis.... one of the common faults was that the metal body would become live and as the amp is earthed... BANG!!
 
They should be fine then for the use they have been put to.

The MF A370 power amp used these for the soft start circuit, at mains potential, and they were bolted to the rear of the chassis.... one of the common faults was that the metal body would become live and as the amp is earthed... BANG!!
Cheers
 
Just a suggestion, but if you are talking about the EL34 cathode resistors, the originals were 470 ohm,10 watt 'Standees' and were not super reliable. But were conservatively rated as they should have 3 watts dissipation in that circuit. (36 volts across 470 ohms)
If they have been replace with 10 watt Arcols and one of those has failed, I would want to check they are not passing too much current. First suspects would be the 2 coupling caps (C6 and C7) for leaking.

Otherwise, Arcols have to be chassis mounted to meet their quoted wattage, if not they usually have about half the rating.

Cheers for information
 


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