Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on November 30, 2010, 05:40:29 pm
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In a Fender SF DR, is there any reason I cannot use 1k instead of 470 ohms on the screens? It is using 6L6 tubes instead of 6V6. I have the 1k bu not the 470 at the moment. I suppose there would be a few more volts dropped with the 1k, but that is not a big deal is it?
Thanks
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That'l work fine......... :wink:
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In a Fender SF DR, is there any reason I cannot use 1k instead of 470 ohms on the screens?
None whatsoever. 470R is a minimum, 1K and up to 2K work perfectly with 6L6GCs and similar tubes - EL34s and other pentodes need higher Rs.
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Thanks for the quick replys.....I really appreciate it.
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Keep wattage rating in mind. Often 5 watt resistors are used with 1K and up screen resistors.
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Keep wattage rating in mind. Often 5 watt resistors are used with 1K and up screen resistors.
I will keep that in mind. I was going to use 1k 3 Watt. If I am doing my math right....even if it drops 5 volts at 425 volts it would be just over 2 watts. Is that right?? I had to install new tube sockets, so I do not even know what the plate/screen voltage is. The amp belongs to a friend, so I am not very familiar with it. It had 1 watt resistors in it for 35 years. But I will definitely heed your advice and measure the voltage drop when I am done. If my use of 3 watts was a bad call, I will fix it for sure.
Thank You
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Note:
1. measure the voltage drop across the screen resistor = supply voltage - screen voltage
2. the voltage drop, if any, measured @ idle is irrelevant to wattage dissipation here. @ idle the screen draws almost no current. So, there is no voltage drop across the screen resistor. Therefore @ idle the screen resistor dissipates approx 0 Watts. But with power amp overdrive, the screen's current draw will increase; so the voltage drop across a large screen resisitor will also be larger. That's the wattage rating of concern.
KOC recomends 3W for 2.2K screen resistors on EL84's. Marshall uses 5W 1.5K screen resistors on EL34's (I think).
BTW: large screen resistors might alter the tone of the amp. E.g., I tried 2.2K screen resistors in my EL84 amp and hated the resulting overdrive tone.
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So suppose we didn't have historical examples to refer to, how would we go about calculating a proper power rating for a screen resistor?
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You may start with design examples in the datasheets, then calculate typical dissipation.
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Hi All,
Shouldnt you also add a little extra in w. rating for safty from heat build up, since the screen resistor is located on the output socket? For the very small price differance from a 3w. to a 5w. resistor, I think its well worth it.
Remember -If- that screen resistor gives up the ghost :huh: output tube goes with it ? then maybe the OT ? than you wont have to calculate, you will :cry:
Brad :angel
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So suppose we didn't have historical examples to refer to ...
FYL is steering you straight by suggesting looking at the data sheets, particularly if you look at the numbers for max signal (often for 2 tubes; remeber to divide the value in half).
What's a worst case? How about a single tube's screen pulling 30mA (could be much more in certain cases)?
30mA * 1k = 30v, and 30v * .03A = 0.9w. Double that and you get a 2w resistor safely derated; of course, if your resistor is 470 ohms, you get half the voltage drop and 1/4 the power dissipated. Which tells ya that 1w resistors are fine (and often seen in old amps).
Shouldnt you also add a little extra in w. rating for safty from heat build up, since the screen resistor is located on the output socket?
You can, and the cost of a 3w resistor is not too much. Or you could decide that it will take a couple-decades to overheat, and that when it does the value will go up. So maybe you don't care so much.
Remember -If- that screen resistor gives up the ghost output tube goes with it ? then maybe the OT ?
Recall what function the screen provides: in a tetrode, pentode or beam power tube, the screen has more influence over plate current than the plate. As long as the screen voltage stays relatively high, the plate voltage can swing very high or very low with very little impact on actual plate current (this is good for making big output power).
If the screen resistor opens due to too much heat dissipated, then screen voltage should drop to zero. Plate current should be well below its normal idle value (though perhaps not quite zero). The amp makes little, if any, clean power but doesn't die, and there's no further damage to the amp.
For the screen resistor to damage the tube or OT, it would have to short to something:
1. Short to ground - screen voltage drops to 0v, tube current lower, possible power supply damage (burnt series resistor or maybe choke).
2. Short to plate - screen and plate see near-same voltage all the time; functionally equivalent to triode operation. Lower output power, no damage.
3. Short to filament - high voltage on filament likely to damage the tube, maybe (*very* unlikely) the PT filament winding. Screen resistor has no effect either way.
4. Short to control grid - high positive voltage on the control grid makes the tube pass huge current, can damage the cathode, control grid, screen and/or plate; if fixed-bias, the tube likely dies, may damage OT and/or power supply components.
So as a quick guesstimate, the screen resistor would need outside help to damage anything if it were to fail, and that outside help (likely an unintentional short) would probably kill anything that would likely die regardless of whether the screen resistor were present.
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HBP,
Thanks for clearing up what I mucked up. :rolleyes:
Sorry guys. "And the :lipsrsealed: award goes to" :sad:
Thanks, Brad
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No problem, but please don't think you mucked anything up. You are very accurately restating things that are often heard (I'd bet I can name the author you read in 1 guess).
But tube screens get very little press, and are often little-understood until you read some of the old-school tube books.