Kathode Resistor

[Demsy]

Would it be alright to substitute the 2x8 ohm, 10W, with a single Mills MRC50 a 4ohm, a 50W one. Pcx has them for $5.95 a piece.

[Paul Birkeland]

You could do that, but be very aware of the dimensions.  (The 50W part is likely to be significantly fatter and longer than the two 10W parts)

-PB

[Demsy]

Yes Paul, it is significantly bigger than the 10W ones. Mills also have the 25W version but Pcx doesn't carry it. Looking at the picture of the underside of the base plate, the 50W will fit, though.

[Paul Birkeland]

I looked into this a little more for you. 

The body of the Mills resistor is wide enough that it will interfere with the input wiring.

The body of the mills resistor is considerably longer than the terminal strips we provide with the kit.

If you can't mount the resistor to the chassis, the datasheet indicates that the power dissipation drops a lot (looks like to about 35% of its rating).  This means that your 50 watt resistor is now effectively a 17 watt resistor.  This is actually a bit below our safety margin, so we can't recommend this.

I've seen other folks find different combinations of Mills 12 watt parts that can be substituted for the parts we provide.

-PB

[Demsy]

Thanks a lot Paul. I was just measuring the resistor against the picture of the bottom part of the BeePre in the manual, proportionally. Measuring it that way looked like the 50W resistor would fit easily. I will go and look for the 25W version which physically is way smaller.
Could you tell me which part or parts contribute the most heat to the base plate. I'm going to use laser cut stainless steel again instead of the stock aluminium plate. I could easily have extra 3mm holes cut to it for attachment screws of the aluminium bodied resistors. But if there were other parts producing higher temperature to the base plate than the resistor, then I guess it's not a good idea after all.

[Paul Birkeland]

The heat contribution by the resistors is proportional to their size.  The two pairs of 10W resistors produce the most heat, followed by the two individual 10 watt resistors.  The heat contribution of the rest of the resistors can be ignored.

The power transformers and tubes themselves also run pretty toasty.

-PB