Express Power Filtering Resistor

[bal704]

On the Express schematic, there is a 25w/1k resistor between B+1 and B+2. According to the schematic, there is a 15 volt drop across this resistor.

Why is it a 25 watt resistor? The voltage drop indicates a much smaller wattage rating would work.

Just wondering.

[rock_mumbles]

Because under power the screen current draw goes way up so the voltage drop gets larger ... and anything less than a 25 watt will burn up sooner or later.

[bal704]

Per the math, that would mean a 150 volt drop across that resistor. That would drop the B+5 to around 100V. Does that kind of drop affect the tone?

[BIG Dave]

$1.21 shipped! http://www.ebay.com/itm/NTE-25W210-25W- ... 25810550bb

[bal704]

I was more concerned with the size than the cost. A resistor that big takes up a lot of board space. That's why I was wondering why it needed to be 25 watts.

[katopan]

It's been discussed many times - do a search. But my 2 cents of something that isn't in any of the other threads..... it's way oversize even taking into account running at full overdrive, but being so would result in a lower temperature rise for a ceramic block mounted on a board enclosed in a chassis. Next size down won't fail, but it'll get hot enough to burn the board and maybe damage other nearby components.

Either get the right size, or if you want to be unconventional heatsink a smaller one to the chassis like I'm going to.

[geetarpicker]

In a cranked Express even a 25 watter in that position gets hot enough to burn you, and I wouldn't let any wires touch it as they may melt. In the circuit layout on some original Express amps there are a couple wires that run right under that resistor but don't touch it. The resistor is such a value to contribute sag to the circuit, but in a predictable fashion much like a tube rectifier. In my opinion that resistor helps the clean to mean range of the amp stay more at the same net volume level. This is a very important aspect to the sound and playability of these amps IMHO. Don't mess with the original recipe is my tip of the day!

[Bob S]

I'd highly recommend using a 25 watter too.
And like Glen says - don't touch it soon after using the amp.
It will burn you.
I know this.