Resistor choice for 300B amplifier repair

[AndyMcKay]

All,

When restoring an amp and you have to replace a resistor, is there any benefit to replacing a 25W wire wound cement resistor with one of the aluminum chassis mount variety?

I have both cement and aluminum bodied 25W 270ohm 5% resistors I can use.

The amplifier in question is a 300B push/pull monoblock.

Thanks,
Andy

 

[Michael Bean]

Actually the first question is; why do you need to replace a resistor? If it burned out, you need to find out why and fix it. Beyond that, and answering the original question; it shouldn't make any difference which one you use as long as the specs are the same, they are both wire wound resistors, only the package is different, and the chassis mount one when mounted on a heat sink will dissipate heat more efficiently.

Mike

 

[AndyMcKay]

Mike,

Thanks for the info.

A pair of amps came into my possession recently and one of them has an obviously burned resistor. We are in the process of attempting to figure out the "why" as well.

Best regards,
Andy

 

[67339]

Hello,
The French used non inductive wire wounds 50 watts in the past for that application housed in an alumium housing. They will need some serious metal to dissipate the heath.
If you can calculate the dissipation you could also use 2 Mills 12 watt in parallel.
Greetings, Eduard

 

[euro21]

.. 2 Mills 12 watt in parallel..

Rule of thumb: don't use MRA over 50-60% of nominal power rating!

 

[AndyMcKay]

I will be replacing spec w/ spec. The resistor to be replaced is 25W 270 ohm 5% resistor, as noted on its side and the side of the corresponding resistor in the amp's "sibling."

I just wanted to see if there was any benefit to the aluminum chassis mount resistors. Thanks for the help.

 

[grovergardner]

I will be replacing spec w/ spec. The resistor to be replaced is 25W 270 ohm 5% resistor, as noted on its side and the side of the corresponding resistor in the amp's "sibling."

I just wanted to see if there was any benefit to the aluminum chassis mount resistors. Thanks for the help.

As mentioned, they need to be heat-sinked to the chassis to achieve their ratings. I would go with a pair of Mills wirewounds, myself.

 

[audiowize]

While the aluminum housed resistors don't have to be connected to a heatsink, you have to derate them massively if you don't.

For example, the Vishay RH-25 chassis mount resistors rated at 25W get derated to 12.5W in free air. They really do mean "free air", as in no temperature rise from the chassis. If your 300B amp is nice and toasty, let's say 70C, then you get derated to 9.6W. These are specified to be able to operate up to 250C, but I suspect that the power ratings above are for 150C operation, which is still too hot for long operational lifetime.

This is a long way of explaining why you should use the cement bodied parts instead.