Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kagliostro on February 01, 2017, 02:49:39 pm
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Today I was to the local electronic shop looking for a (round and relatively small) 300R 10W resistor
They have not so much components and had only a 330R ceramic resistor that I got also if it is a bit too long for my board
I got it also if it is 330R (waiting for a correct one) and I've think to put a 3.3k 2W resistor in parallel to have 300R as result
After that I remembered that may be I've some 10W 3.3k resistor that are of the correct lenght and if I put a 330R 2W resistor
I'll obtain the desired 300R
But I've a doubt, if I use a 10W 330R + a 3.3K 2W resistor the larger current follows the lower resistance path, so no dissipation problems
and if I use a 3.3K 10W + a 330R 2W resistor what will happen ? Is this an easy way to burn the 330R 2W resistor ??
Many Thanks
Franco
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and if I use a 3.3K 10W + a 330R 2W resistor what will happen ? Is this an easy way to burn the 330R 2W resistor ??
possible to leak much smoke.
can you get three 1K 5W? that with 3.3K 1W or 2W would work.
--pete
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Thanks DummyLoad
I must give a look to my stock to see what I've or use the 10W 330R + a 2W 3300R
this way I think I've no problem
Franco
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What is the purpose of the resistor. 300Ω may not be such a critical value. Perhaps any value between 200 and 400 will be acceptable?
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Ciao Steve
the resistor is for the cathode of the 1625 on the schematic you have seen (modified AA764 + 5C1)
Franco
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In that case I definitely would not zero in on 300 as the magic number.
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Grazie Steve
In that case I definitely would not zero in on 300 as the magic number
(http://www.diyitalia.eu/images/smilies/Trilly-18-39.gif) (http://www.diyitalia.eu/images/smilies/Trilly-18-39.gif) (http://www.diyitalia.eu/images/smilies/Trilly-18-39.gif) May be I must think to a more flexible solution ..... :rolleyes: :rolleyes: :rolleyes:
Franco
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... if I use a 10W 330R + a 3.3K 2W resistor the larger current follows the lower resistance path, so no dissipation problems
and if I use a 3.3K 10W + a 330R 2W resistor what will happen ? Is this an easy way to burn the 330R 2W resistor ?? ...
Let's solve the problem and find out.
How much voltage do you expect across the resistors? Pulling a number out of the air, I'll guess 15v.
Resistors in parallel see the same voltage.
15v / 3.3kΩ = 4.5mA
15v * 4.5mA = 68mW
15v / 330Ω = 45.5mA
15v * 45.5mA = 0.68W
Do you expect more than 50mA of idle current (plate & screen)? If no, it looks like 2w is plenty for either resistor and 10w is overkill (but might be cooler for a nearby cap).
Ofcourse you could have calculated as 15v2 / Resistance, and found power directly. But knowing the implied current might be helpful in some cases.
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was wondering the same thing... usually we double up on wattage or even 3X if there's big swing in current demand.
--pete
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Many Thanks Bill & Pete
I think there will be ~18V across the resistor(s), it is the cathode resistor of a 1625 tube in an SE amp
here schematic
(http://i.imgur.com/W3Sxrbv.jpg)
(There is a typo, bass cap is .1 NOT .01)
and ... yes, 10W was planned as to be a safe situation, in an SE amp I don't think there will be a large swing in current demand
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I've this nice (and smaller 3.3K resistors that fits better the board)
respect to the 330R ceramic resistor I got yesterday
(http://i.imgur.com/UtGUbhS.jpg)
But when Steve told
In that case I definitely would not zero in on 300 as the magic number.
I started wondering a different solution (more flexible)
a 220R 10W resistor + this russian 220R ceramic pot and this give me a range from 220R till 440R to perfectly tailor the bias
(http://i.imgur.com/JzrPwQ7.jpg)
(http://i.imgur.com/PT0TFGA.jpg)
but I'm still wondering about what to do (there are also other solutions, one is to find a lot of correct sized resistor in various values :icon_biggrin: )
Many ideas but well confused :laugh: :laugh:
Franco
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But when Steve told
In that case I definitely would not zero in on 300 as the magic number.
I started wondering a different solution (more flexible)
a 220R 10W resistor + this russian 220R ceramic pot and this give me a range from 220R till 440R to perfectly tailor the bias
You probably won't know the "ideal resistance" until you actually build the amp. You could use the wirewound pot, but I suspect once you get it close to a happy value of resistance, you probably won't change it much ever again. In that case, it seems a waste to use the pot. But if you already have the pot, then cost doesn't really matter unless you have some other project where it would be critical to have.
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that's nothing more than a 807 SE.
https://frank.pocnet.net/sheets/155/8/807.pdf
probably going to have around 14 to 15V Vk. to keep Pdiss around 22.5W @ 335V, you'll need about 75mA Ik. so 14V-15V/75mA ~~180 to 200 ohms.
--pete
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Thanks Pete
I've this extract of the 807 datasheet
(http://i.imgur.com/7IfwTBp.jpg)
Seems that (with 350V B+) plate + screens are around 59mA
I don't know the exact value of Rk (I think they refer to a fixed bias with that -18V)
but seems that also a less W rated resistor can be used (may be 7W)
I'll prefer to use a bigger one but I'll see what I can find
Franco
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if V bias fixed is shown to be at -18V with 350V, a good approximation is that you need 18ish Volts on the cathodes with auto bias. so 18V/58mA ~~310R. use a 330R. Va-k is actually 350 - 18V = 332V but still close enough to 350V to make the approximation valid. more or less. you're cutting hairs where you don't need to. fire it up with a 330R, measure Vk and make the determination if Pdiss is where you want it. in the spec sheet it's about 20W. on a footnote, during the initial power up, monitor your screen V closely; the closer to 250V it actually is, the closer the approximation is.
--pete
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Unrelated detail. Bass cap in the tonestack is .01?
--Craig
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Thanks Pete
Those are important info for me, I'm not very skilled and all this info give me help
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Many Thanks Craig
is a typo, correct value is .1
Grazie
Franco