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PRNDL 7th December 2006 02:29 AM

Power rating for triode/pentode resistors
 
What power rating is appropriate for triode/pentode resistors in an EL84 push-pull amp.

My initial guess would be to use the same as the rest of the circuit ... 1/4 Watt, but these mod plans show 5 Watts.

http://tone-lizard.com/images/Pentode-Triode_switch.gif

Thanks

tubelab.com 7th December 2006 02:45 AM

Even though it is common practice to use 1/2 watt or even 1/4 watt resistors here, I have had two metal film resistors fail recently (one Xicon and one from Digikey). I have sinced switched to 2 watt carbon film resistors. No problems even when cranked to 11.

I advise people not to flip the switch under power even though I do it all the time. It does however often make a loud pop in the speakers.

HollowState 7th December 2006 03:55 AM

I prefer to use Allen Bradley carbon comp resistors in areas that could experience high current overloads or pulses because they are more durable then most film types. Power supplies and output stages to be specific. Metal films do not withstand overloads very well, even momentary, and will quickly open. That's why they call them flame-proof resistors.

Conversely, carbon composition resistors can be overloaded until they smoke and they'll still function in many cases. Unfortunately they are hard to come by now days. As tubes phased out and solid state miniaturization became the rage, carbon comps fell out of demand. (Luckily I have thousands)

I would use a 1 watt carbon comp or a 2 watt carbon film as a second choice.

Victor

Tubes4e4 7th December 2006 06:30 AM

Re: Power rating for triode/pentode resistors
 
Hi,

EL84 with octal base, huh :xeye:

Quote:

What power rating is appropriate for triode/pentode resistors in an EL84 push-pull amp.
At Va=250V, EL84 screen grid draws about 5,5mA. Voltage drop across a 1k grid stopper is 5,5V. Power dissipation in this resistor would be 5,5V * 0,0055A = 0,03W or 30 milliWatts something. Even a 0,5W type resistors would give more than a 10-fold safety margin :rolleyes:

Tom

tubelab.com 7th December 2006 01:02 PM

Quote:

That's why they call them flame-proof resistors.
The manufucturers have obviously underestimated my ability to blow things up. I saw a momentary fireworks display when one of the resistors died.

Quote:

Even a 0,5W type resistors would give more than a 10-fold safety margin
Under normal conditions this is true. When the big dumb blonde one plugs his guitar preamp into that new SE amp and cranks it to 11, normal formulae do not apply. I do this to "stress test" all of my amplifiers to find weak links. In this case I fried two screen resistors. I have since started using 2 watt carbon film resistors with no issues.

It is possible that an SE amp in normal use would never fail, but I am looking for ultimate reliability. True, carbon comp resistors are much better at forgiving momentary overloads, but they tend to change value with time, heat, and overloads.

I did some investigation and I found that If you drive the amp into hard clipping the screen current momentarily spikes up when the plate voltage drops below the screen voltage. This is primarily a pentode mode thing, but I blew a resistor in UL mode also. The amp in question was a SimpleSE with EL34's. I use 100 ohm screen resistors as a best compromize for use with a wide variety of tube types.

Tubes4e4 7th December 2006 07:15 PM

Hi Tubelab,

Quote:

Under normal conditions this is true. When the big dumb blonde one plugs his guitar preamp into that new SE amp and cranks it to 11, normal formulae do not apply. I do this to "stress test" all of my amplifiers to find weak links. In this case I fried two screen resistors. I have since started using 2 watt carbon film resistors with no issues.
A wise investment in reliability.

I agree with your 100 ohms suggestion for a screen stopper, instead of the 1k the original poster mentioned - as long as it surppresses parasitics reliably, of course.

Tom


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