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Thread: Dropping resistors in b+ power stage

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    Dropping resistors in b+ power stage

    Hi all I'm building the single 6v6 version of the two stroke amp and the dropping resistor between the 16uf and 8uf filter caps is a 68k 1 watt do you think I would be better off substituting a 68k 2 watt in its it's place as the 1 watt carbon film I have looks tiny would a higher wattage resistor in its place have any detrimental effect other than physical size ?
    Sorry if it's a dumb question but I just want to make sure !!
    Thanks
    Scott

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    No problem whatsoever in going higher wattage and in no way will affect the sound or performance. Don't be fooled by the small size and appearance of modern components compared to the same spec (but physically much larger) older/vintage items.
    Justin Thomas and Jarvini like this.

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    Thanks Mick that's what i thought I'm sure they used to be 1/2 watt in older amps and are now upgraded to a higher wattage just wondered why in a newer design it was still only 1 watt when a 2 watt could be used?

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    Also out of the parts ive had to source and buy the 68k resistor seems to be the most awkward value to find ?

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    Old Timer Leo_Gnardo's Avatar
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    Quote Originally Posted by Jarvini View Post
    Also out of the parts ive had to source and buy the 68k resistor seems to be the most awkward value to find ?
    You can always string two 33K's in series, close enough for rock 'n roll. They're fairly common & easy to find.
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    Good idea just found it surprising as the 68k is used a lot on the input of a lot of amps I've looked at or maybe that's why as amp builders buy stocks of them and make them scarce?

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    Resistors are produced in massive numbers - probably billions annually per value. Amp builders count for nothing in the scheme of things.

    Bear in mind that components with leads are reducing in commercial availability, as most mass-produced items use surface-mount (SMD) parts. I've noticed that some suppliers are rationalizing values and types.

    In your amp the 68k dropper only supplies two halves of a 12AX7. Each triode section draws about 1mA, so 2mA flowing through the resistor. This calculates (R x I squared) as 0.27W dissipation, so 1W is fine.

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    Ah brilliant thanks Mick I think like you said I was just surprised at how small the resistor actually looked being carbon film as opposed to carbon comp and your calculations have put my mind at ease can't thank you enough
    Scott

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    Old Timer Tom Phillips's Avatar
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    Quote Originally Posted by Jarvini View Post
    Good idea just found it surprising as the 68k is used a lot on the input of a lot of amps I've looked at or maybe that's why as amp builders buy stocks of them and make them scarce?
    You didn't say where you looked but I just checked Mouser and they have 131 types of axial 68k resistors in stock. So...plenty of stock out there.

    Something to remember: Guitar amps use the same basic parts as many other electronic circuits. Except for special cosmetic parts and things like vacuum tubes and tube sockets the parts for guitar amps are still available from mainstream electronic supply companies. Therefore, it is not necessary to buy all the parts you need from specialized musical instrument parts supply sources.
    Last edited by Tom Phillips; 02-03-2018 at 05:39 AM. Reason: Added Info

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