Old harman Kardon integrated, need resistors but what wattage?
- Thread starter 240sx4u
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ConradH
Addicted Member
I just go by what was in there to begin with, but if you can look at the schematic and figure out what the maximum voltage across the resistor could be, you can calculate the wattage and then put in a safety factor of 1.5X or so. With higher values you can often use 1/4W resistors to replace larger carbons, but watch out for the maximum voltage ratings. Even if the power is OK, resistors also have maximum voltage limits that are lower than you might think. That's why I use big RN70D parts in some locations in tube amps.
240sx4u
Lunatic Member
Yeah I can't tell what the original wattage was and I don't have a schematic. Every resistor is the same physical size. I am considering just using 2W to be safe. I'm seeing some grid(?) resistors at 5W though. I assume that's not the case since those would likely be much larger.
ConradH
Addicted Member
How big is what's there, L & D? I've seen grid resistors in the 2-5 watt region, usually in guitar amps. I assume that's in case the tube fails or is overdriven, as I can't imagine that much current flowing in any normal operation with a hifi amp. One trick that takes care of some of them is just figure the wattage based on the maximum supply in the amp, and the value of the resistor. It will never dissipate any more than that!
mpholland
R.I.P. Michael Sheppard
I'm with this. The vast majority will probably be 1/2 watt. There should be only a few 1-2 watt. Anything larger will probably be wire wound.
jaymanaa
RIP 1961-2018
Most older schematics will state in the notes that all resistors are half watt unless noted. The only time you need bigger is for output tube cathode biasing, or in the first two decoupling stages of the power supply. Everything else, 1/2 will do fine. I like carbon film for signal and metal film for DC voltage, or anything going to ground.
I'm just not picky. Big wattage stuff in the supply usually gets wirewound, over 1w the metal film or MOX gets cheaper so thats what I use. CF is typically cheaper in 1/2w so I use that for those jobs. I shop by price for most of my components. The kind at the top of the list is usually what I buy, unless its the "micro" 1/2w that look like 1/4w. I do not buy those. They look silly.
They sound like half watt, carbon comps. Carbon film or metal film should be fine as replacements, the former often available in 5% tolerance (which is likely at least as good if not better than what was originally installed in the unit - I remember seeing a of of 10% carbon comp resistors back in the day) and the latter often in 1 or 2% tolerance. Tighter tolerance is better, given similarly priced options.
-Pat
-Pat
most amps I've been in were stuffed with 10% parts, with the odd 5% in special spots. Sometimes they even had 20% parts in places. I usually use 5% everywhere, and if its critical you can hand match. About the only spot thats needed is for say a bias test point. Out of a dozen 5% parts you can usually come up with 2 or 4 that are matched within 1% though. I don't even know if you can find 10 or 20% parts now. Probably would cost extra even if you could.
knockbill
Lunatic Member
Remember some of the old lytics tolerance that was +50 and -20 or something similar. Those always made me wonder about QC.
Exactly, and they lasted 50+ years!
jaymanaa
RIP 1961-2018
And so far, the only power supply filter cap I've ever seen shorted, was on an old Sui 1000, and I've seen a few filter caps.
Remember some of the old lytics tolerance that was +50 and -20 or something similar. Those always made me wonder about QC.
Actually, I think some of them were +80, -20. I'm not sure if it was QC as much as the manufacturing process for them at that time had a lot of variability, and as it typically wasn't terribly critical, there was no real need to generate scrap and instead they left the tolerances very wide. It is kind of shocking to see that kind of a range now, being used to the availability of surface mount resistors at 1% tolerance for prices of .003 cents in reel quantities.
I do seem to remember most of the carbon comps that came in the two packs (for $0.19) at Rat Shack back in the mid-late 70s being 10% tolerance ones. They then started to transition to 5% as time went on. I often saw 20% in the old TVs I used to dissect. These days I typically buy the 1 or 2% metal film ones when I need them.
-Pat
Consider how fussy modern electronics are compared to tube stuff. I'm sure I'm not the only one who has been inside a unit that was working fine, but filled with parts that were shot even by the standards of the day it was built in. With a lot of old gear, you could throw a pretty wide range of values into a circuit and it would still go, even if it wasn't quite optimal.
