Bottlehead Forum
Bottlehead Kits => Legacy Kit Products => Foreplay III => Topic started by: John Roman on October 03, 2011, 10:59:41 AM
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I'm changing my extended fp 3 to use 6sn7's as output tubes, keeping the 12au7 as shunt. The existing heater resistor is composed of two parts, one is a .62 ohm/5w and a piggybacked 1.2 ohm/3w. My configuration calls for a .15 ohm/5w. Do I leave the piggybacked resistor out as well?
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John, I was going to leave this one alone since I dont have the extended and Im not one of the gurus. But I believe that the .62 ohm resistor was paralleled with the 1.2 ohm as part of the extended upgrade (about .41 ohm total). The Stock FPIII uses a 1.2 ohm. The stock FPIII with 6SN7's uses a .22 ohm. Deducing from Docs sticky post on top of this forum on resistors for different tubes in FPIII, as well as going by Pauls post to you regarding a .15 ohm resistor for extended FPIII conversion to 6SN7's ... I think that your total resistance desired is .15 ohms. But it would be completely understandable if you want to wait on confirmation from a more highly evolved electronic mind :-)
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Thanks Desmond,
I think you're right, that the .15 is correct. That's what I was thinking also but I tend to be cautious. That's the only thing left to do then I'll power it up and check some readings. I'm tempted but will probably wait.....
Perhaps you don't give yourself enough credit. Thanks for the help.
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I usually dont give myself enough credit for good reason :-)
I read Pauls reply in the other thread. I was pretty confident on this one but not knowing 100% for sure, I wouldnt feel right not tempering my reply. If I were doing it, I would have dropped the .15 ohm in and been comfortable with it. But, then it's on me and my stuff if Im wrong, not someone elses.
Really though, if I was 100% sure about something I would just say so. It's not often the case.
And I'm very cautious myself. Some of my questions are just dumb, and 99% of me knows that they are dumb but that 1% of me still has to ask them to be 100% sure.
Anyway, glad you're on the home stretch. It'll work fine.
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OK, too much confusion so it's time to post these numbers again.
First, of course, you all know that we don't "support" modifications to the products. Nobody does, because nobody can. We DO encourage those who wish to try things, and help out where we can - it's a great hobby!
Under the "helping out" label, I have posted the appropriate resistor value to get 6.3 volts DC from a PT-3 when the input line voltage is 120vRMS. Here are the numbers, current in column 1, resistor in column 2, notes in column 3:
0.3A 1.37ohm
0.6A 0.61ohm (stock FP has 0.62)
0.9A 0.35ohm (stock Extended has 0.62 paralleled with 1.2 equals 0.41ohm)
1.2A 0.22 ohm (stock FP with 6SN7s would use this)
1.5A 0.15ohm (this is what John has)
1.8A 0.10ohm (SEX amp, or eXtended Foreplay with three 6SN7s)
2.1A 0.06ohms (a SEX amp with 6EM7s - though I would just use the 0.1 ohm, it's not that different.)
The formula, which is just a curve fit, is approximately as follows:
V = 7.10
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I have a question on this resistors power rating.
I have an extended Foreplay 3 that I plan to replace the two 12au7's with 6sn7's. The appropriate value is 0.15 ohms for the 1.5A current draw of the filaments. I don't see where the 5W rating for this resistor came from. For a 0.15 ohm resistor with 1.5A flowing through it I calculate the the power dissipation at 337.5 mW. A 5W rating or even a 3W rating would seem like overkill. Am I missing something? Is the higher current rating to compensate for start up transients? Can you get away with a 3W or lower rated resistor?
Thanks
Debra
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The resistor is before the capacitor, so the current in the resistor occurs in short bursts of high current. That gives an RMS value much higher, usually between 1.5 and 2 times the average current. I get 1.35 watts at 3 amps RMS - power goes with current squared. I prefer a safety factor of 3 minimum for resistors - most resistors max out at something like 150-200 degrees C, while capacitors and other internal components are in trouble well below 100C.
Another engineering concern is that metal film resistors get expensive above 1 watt, and the cheapest resistors of more than 1 watt rating are usually the 5-watt cement resistors. They have a bad reputation sonically, which I believe is due to their metal-oxide construction in the higher resistance values (typically above 1K but varies with manufacturer). However, in the lower values these resistors are always wirewound - the gold standard IMHO for quiet, stable resistance.