1K 25 WATT resistor

[Mook]

The Trainwreck Express and Liverpool both use a 1 watt 25 watt power resistor in the place that other amps use a choke.

Why 25 watts?     Seems to me a regular 5 watt will work, here.

Discuss?


Mook

[jjasilli]

The Blue Guitar has updated DIY schematics showing 25W, 5W or a choke.  I agree that 5W should be more than adequate.  But, it depends on how hot the Resistor actually gets.  Resistance increases with heat.  If a 5W resistor gets hot, its resistance will change.  So the tube's performance, and tone, will vary -- possibly as a result of something so mundane as how long the amp was left ON.  A 25W resistor is less likely to vary its value due to the watt it dissipates in this circuit.  That's the only explanation I can think of.

[vibrolax]

I agree with your analysis.  In my TW Express build (6V6's) I used a 10W cement resistor for the R between the plate and screen node.  I calculated about 6 mA running through that node at idle.  Maybe 12 mA at peak.  So I * I * R = 0.012 * 0.012 * 1000 = 0.14W. A 1W resistor would be sufficient.

Working it backwards, for a 5W resistor:
5W = I * I * 1000
I * I = 5 / 1000,
I * I = 0.005
I = sqrt(0.005)
I = 70.7 mA

So a 5W 1000 ohm resistor is rated to dissipate heat from 71 mA current.
Using a safety factor of 4, that's 18 mA.
I don't know what the screens currents in a cranked pair of EL-34's are, but it can't be anything remotely close to needing a 25W resistor.
The preamp section draws a few mA.

If anyone can explain the justification for 25W resistor here, other than blincly copying what Ken Fisher put in his amps, I'd be most curious.

Regards,
Jon

[vibrolax]

The Blue Guitar has updated DIY schematics showing 25W, 5W or a choke.  I agree that 5W should be more than adequate.  But, it depends on how hot the Resistor actually gets.  Resistance increases with heat.  If a 5W resistor gets hot, its resistance will change.  So the tube's performance, and tone, will vary -- possibly as a result of something so mundane as how long the amp was left ON.  A 25W resistor is less likely to vary its value due to the watt it dissipates in this circuit.  That's the only explanation I can think of.

Interesting theory.  I have another 1K 10W cement resistor.  I could check the temperature coefficient from the datasheet, then measure how the resistance changes with temperature, but that will have to wait for a day when I have my temp measurement setup assembled.

Jon

[vibrolax]

Referering to this data sheet for Xicon cement resistors: http://www.mouser.com/catalog/specsheets/XC-600041.pdf

The temperature coefficient is +/- 350 ppm / degree C.  Hmmm, the sign is +/-, so it doesn't say whether the resistance increases or decreases with increasing temperature.

I calculate a resistance change of about 25 ohms between 30C and 100C.  That's about 2.5% for a 1000 ohm resistor.

The heat rise chart shows a 70C rise occurs at % rated load of about 20% for 15W & 25W resistors, and 40% of rated load for 5W.

I wouldn't be bothered by a 2.5% resistance change.

Of course, there may be other factors involved, but I'll admit ignorance.

[vibrolax]

So a 5W 1000 ohm resistor is rated to dissipate heat from 71 mA current.
Using a safety factor of 4, that's 18 mA.

Mistake! Power dissipation increases with the square of the current.  So to limit the dissipation by a factor of 4 = 5W / 4 = 1.25W
I = sqrt(1.25 / 1000) = 35 mA

[Mook]

another funny question regarding this is:

Mark Abbott drew the schematic for the Liverpool as well.   In his comments, he said that in order to use 1/2 power (2xEL84) the resistor should be changed to a 2K.   But, I don't see the reason for that.....that's barely dropping any volts.


Mook

[2deaf]

I don't know what the screens currents in a cranked pair of EL-34's are, but it can't be anything remotely close to needing a 25W resistor.

Screen current in overdriven EL34's might surprise you.  I use 10W 1K screen resistors in my big EL34 amps, and I ain't the only one.  That guy probably knew what he was doing when he used a 25W in place of the choke.