Watzup with that 25watt 1k resistor in the power supply in the Express? It dissipates much less wattage, no? Probably less than 1watt I would think. Is it similar to the reasons for using high wattage resistors on screens which also dissipate very low wattage? (Don’t want them to blow during shorts elsewhere.)
Sorry if a dumb question but I'm a newbie on these.
25Watt power resistor?
Moderators: pompeiisneaks, Colossal
25Watt power resistor?
If it says "Vintage" on it, -it isn't.
Re: 25Watt power resistor?
That resistor is in the same spot as a choke might be. It has to take all the screen current from the power tubes and all preamp plate current. The math may indicate a lesser resistor would work, but go with the big dog. I had an Express toast a 10 watt ceramic resistor! I wasn't smart enough to track down that burning smell before it opened up.....
Rich Gordon
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"The takers get the honey, the givers get the blues." --Robin Trower
www.myspace.com/bigboyamplifiers
"The takers get the honey, the givers get the blues." --Robin Trower
Re: 25Watt power resistor?
A number of us (myself included) had the same question. Recall that the schemos are listing DC voltage. What drives the need for that high a rated resistor is the AC currrent to all the components the honorable mr. rfgordon mentions.
Previous discussion: http://ampgarage.com/forum/viewtopic.ph ... stor++watt
Previous discussion: http://ampgarage.com/forum/viewtopic.ph ... stor++watt
Re: 25Watt power resistor?
Follow that link if you want to read the whole discussion...gearhead wrote:A number of us (myself included) had the same question. Recall that the schemos are listing DC voltage. What drives the need for that high a rated resistor is the AC currrent to all the components the honorable mr. rfgordon mentions.
Previous discussion: http://ampgarage.com/forum/viewtopic.ph ... stor++watt
Bottom line: 1K dissipates nearly 10W during playing in that spot.
Re: 25Watt power resistor?
In the worst case a EL34's screen may draw 20mA screen current @ about 40-60% of its max. power output, but you have two of it => already 40mA
Now add 1.6mA for each preamp tube system, what's additional 8 mA
48mA across a 1K resistance causes a voltage drop of (I*R=E) 48V
And since I*E=P the max. power dissipation of this resistor would be 2.3 watts
Actually a 5W resistor would be plenty in this spot, but 2.3W is a lot of heat for the small surface of a 5W resistor, what might toast the board area, where it rests and might melt its solder joints...
... so better use a resistor with a larger surface, because it doesn't become far not as hot as the previous one - and your solder joints feel fine, the board area also
Larry
Now add 1.6mA for each preamp tube system, what's additional 8 mA
48mA across a 1K resistance causes a voltage drop of (I*R=E) 48V
And since I*E=P the max. power dissipation of this resistor would be 2.3 watts
Actually a 5W resistor would be plenty in this spot, but 2.3W is a lot of heat for the small surface of a 5W resistor, what might toast the board area, where it rests and might melt its solder joints...
... so better use a resistor with a larger surface, because it doesn't become far not as hot as the previous one - and your solder joints feel fine, the board area also
Larry
Larry's Website now with included Pix's Gallery