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resistor types in audio repairs

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tab a

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I need to replace several --1/2 and 2W-- resistors in my 30 (?) year old Dynaco high powered amp. The amp has been professionally worked on a couple of times in the past, and some of these resistors show significant heat and trace repair around their mounts. I'd guess a couple have also been replaced with resistors that are physically considerably bigger than the originals.

1. Should I be concerned over what type of resistor to use: carbon, wire wound, metal oxide.....?

2. Would it be a good, or bad, idea to increase the power rating of those that show evidence of overheating in the past? (This is what I suspect has already been done to a couple of them.)

Thanks.
 

Optikon

New Member
tab a said:
I need to replace several --1/2 and 2W-- resistors in my 30 (?) year old Dynaco high powered amp. The amp has been professionally worked on a couple of times in the past, and some of these resistors show significant heat and trace repair around their mounts. I'd guess a couple have also been replaced with resistors that are physically considerably bigger than the originals.

1. Should I be concerned over what type of resistor to use: carbon, wire wound, metal oxide.....?

2. Would it be a good, or bad, idea to increase the power rating of those that show evidence of overheating in the past? (This is what I suspect has already been done to a couple of them.)

Thanks.

1) Yes, be concerned. Try and replace with same type if possible. There are suitable exceptions of course, but knowledge of what the circuit is doing would be required.

2) It would be good to fit larger power handling capability in the components if you have the physical space to do it. Performing such a feat will improve the long term reliability of your electronics.
 

tab a

New Member
I think the PC boards have pretty much all carbon composition resistors, and the 750 and 1k 2W ones I need to replace are also. Don't have any idea if they are the original type though. They're about 3/4 inch long by 5/16" in diameter, and seem large for the space allotted. Maybe more than the 2W spec'd?

Thanks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
As suggested, it's good practice to fit larger resistor where they are pained, and space them off the board as well. Types don't really matter, but generally don't fit wirewounds where they weren't original.
 

Dean Huster

Well-Known Member
I wouldn't think that noisy resistors would be much of a problem in the power amp stage vs. the preamps, filters, etc. Carbon comp resistors are getting more and more difficult to find these days (vs. carbon film) and anything in the 1- and 2-watt category in carbon comp will be impossible unless you find some NOS parts at an "antique radio" supplier (e.g., https://www.tubesandmore.com). You must always be wary of NOS carbon comps for their value may be very, very far from that marked. They do that. A lot.

Nigel's advice about spacing power resistors off the board is very important. A lot of kit builders (it was a Dynaco, wasn't it?) and even some commercial manufacturers didn't do that and the result was a charred PCB. I had a 5-watt wirewound that was in series with a capacitor used as an HF snubber at the output of a Peavey PA amp where the cap shorted. The 150-watt amp drove everything it had into that low-value resistor. The PCB was mounted "upside down" (component-side down) and that resistor got so hot that it desoldered itself and fell to the bottom of the amp. I'd imagine that during the last few seconds it was installed, one lead was getting its current through a molten pool of solder!

Dean
 

tab a

New Member
I replaced nearly all of the old carbon comp. resistors with modern non wirewound types. Interestingly--to me--five of the six 47ohm 5% resistors read almost identical 51.5 ohms, with the sixth being 47 and change. Not sure what to make of that, but they're gone now. :)

Wonder if this amp has another 30 years in it??

Thanks for all the info and advice!
 

tab a

New Member
For the most part I just changed the high power carbon composition resistors that either had clearly changed value, were cracked or blackened but still had correct value, or had 'twins' in the other channel that had problems.

Did check the electrolytics.

Thanks.
 

Dean Huster

Well-Known Member
And when electrolytics go bad, they can take prisoners as well! It's always the most important of all in the old tube-type (valve-type) amps to replace all of the paper-dielectric caps. Man, if some of those have high leakage or short, they can wreak havoc. For instance, a very common problem is a paper-dielectric coupling (blocking) cap within an audio stage either leaking heavily or shorting. The result is super-high plate current and a burned/open primary in the output transformer, a very expensive disaster!

Dean
 

tab a

New Member
<Don't check them, change them! - they are the parts which do age, and have specified lifetimes.>

Now you're making me paranoid, Nigel. ;)

Can you generalize about the lifespan of an electrolytic?

Thanks.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
tab a said:
<Don't check them, change them! - they are the parts which do age, and have specified lifetimes.>

Now you're making me paranoid, Nigel. ;)

Can you generalize about the lifespan of an electrolytic?

They are usually only specced for a few thousand hours (or ten's of thousands).

If you're restoring an old amplifier it's the first thing you do, and may even be the ONLY thing you do.

If it's a valve/tube amplifier, like Dean said, you would also change all the coupling capacitors from anodes to grids.
 
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