Resistors - values drifting

[FuzzOff]

How much drift is too much. Most of them are within 10%, but the 1500s on connecting pins 1 and 5 on my Super Reverb 6L6s are WAY off, reading 2400-2600. Just wondering/if I should get concerned, if the tube voltages are stable and it sounds good. Thx for any perspective.

[John Phillips]

How much drift is too much. Most of them are within 10%, but the 1500s on connecting pins 1 and 5 on my Super Reverb 6L6s are WAY off, reading 2400-2600. Just wondering/if I should get concerned, if the tube voltages are stable and it sounds good. Thx for any perspective.Carbon-comp resistors do drift, normally, as they get old and with heat stress. Many techs have differing ideas about whether to change them, or how bad the drift must be before they do.

Some change them if they go outside the marked 10% or 5% tolerance; I think this is completely unnecessary and don't do it unless it's much worse - I think the old drifted resistors are a part of what gives old amps their cool and often individual tone. I don't have a definite figure, but over 50% is getting to the point where it goes from 'drift' to 'sign of impending failure', so that's roughly where I draw the line, depending on where in the circuit it is.

BUT power tube grid stopper resistors are much more critical and one of the exceptions. Carbon-comp resistors don't like being subjected to a lot of heat anyway, and can get brittle and crack as well as drifting. If that happens, you lose the bias voltage to the power tube, and you'll blow it in a matter of a minute or so at most. I would change them if there's any drift outside the tolerance range, or even simply if they're original in an amp which has seen a lot of use.

So I would change those resistors immediately. I would use modern metal-film ones not carbon - they're much more stable under heat stress and there is no tone change from using them in that position.

I would also change the screen resistors (which are above them and it's easier to do both at the same time) for 2W or 3W metal-oxides or wirewounds instead of the original carbon-comps too - they don't blow tubes if they fail, but it does stop them working. That's why replacing a blown tube doesn't always get the amp working properly again - the resistor often burns out too (it sometimes literally burns and can cause major damage). Wirewounds will usually survive until the fuse goes instead, and are designed not to burn - but always make sure you have the correct fuse rating in the amp.

This is one of the standard jobs I do as part of any major overhaul of an old Fender BTW.

[67super]

This is one of the standard jobs I do as part of any major overhaul of an old Fender BTW.

John is right on as usual, the only other thing I do, especially on old Fenders, is replace the 100K plate resistors. So many old Fenders pop and cracle as a result of these going bad. It's an easy job while your in there.

[John Phillips]

John is right on as usual, the only other thing I do, especially on old Fenders, is replace the 100K plate resistors. So many old Fenders pop and cracle as a result of these going bad. It's an easy job while your in there.Ah, that's one of the ones I don't like to do unless they've gone way out of spec or started to crackle :).

There's mojo in them ol' plate resistors... ;)

[FuzzOff]

Thanks for the info guys. I have completed most of the overhaul, filter caps and all the mallory browns replaced, put a three-prong on, etc. Sounding incredible now and it doesn't shock me anymore when I touch other metal objects ;)

I do suspect a couple ceramic disc caps are giving me some trouble (original 43 years old now.)

[67super]

Ah, that's one of the ones I don't like to do unless they've gone way out of spec or started to crackle :).

There's mojo in them ol' plate resistors... ;)

I'm with you but it just seems so many of the old Fenders I see have the snap crackle pop thing going on. That said if the amp is quiet leaving them in is harmless.

[Wakarusa]

I'm with you but it just seems so many of the old Fenders I see have the snap crackle pop thing going on. That said if the amp is quiet leaving them in is harmless.

But the snap crackle pop thing is just as likely to be a bad ground, an old brittle solder joint, a coupling cap leaking some DC, or a bad tube :) I'm with John in keeping the plate resistors until they drift way out or become a source of unacceptable noise (after verifying that the noise is indeed the plate resistor).

[67super]

But the snap crackle pop thing is just as likely to be a bad ground, an old brittle solder joint, a coupling cap leaking some DC, or a bad tube :) I'm with John in keeping the plate resistors until they drift way out or become a source of unacceptable noise (after verifying that the noise is indeed the plate resistor).

I have found the plate resistors when bad seem to provide a fairly steady kind of rumbling and crackling that isn't affected by volume, tube tapping and the like. It's typacally the last thing I replace after bad tubes, solder joint problems and so on. Leaving them in is fine but if that's what it takes to get an amp quiet I do it. I do use carbon comps for replacing them though, (I know I'll probably get critisized for that) it's what was in there originally and will probably outlast me. New carbon comps have relatively wide tolerances and if they last for only 30-40 years then I guess someone else will have to replace them after I'm gone http://www.thegearpage.net/board/images/icons/icon12.gif BTW - power tube resistors I replace with 2 or 3 watt metal film for sure but I'm fine using carbon comps elswhere.

[Wakarusa]

I keep carbon comps on hand for those who want them, but the default for plate loads is 1W carbon films. Actually, that's pretty much the default for everything. I'll use metal films for high values (anything around 1M or more), but only if it's a critical value - i.e. it's got to be spot-on, or it's balanced with a like component. For power tube resistors the screens are always either metal-oxide or wire-wound and the grid stoppers are back to the 1W carbon films. The 1W part is more tolerant of the heat, and the tolerance just isn't that critical (a variation of several Kohm won't make any audible difference). I suppose you could argue it keeps metal films out of the signal path too, but I'm not sure how much stock I would put in that as a goal ;)

[John Phillips]

I suppose you could argue it keeps metal films out of the signal path too, but I'm not sure how much stock I would put in that as a goal ;)It depends where in the circuit. I wouldn't normally use them in a plate load position, but I like them for grid stoppers because they're more stable under heat stress, which is important when they're cooking away right over the tube in a typical Fender, and I doubt there is any tonal effect there - there's so little voltage across them. I do prefer to use carbon-comps for plate resistors in old amps, but I haven't done any direct A/B testing to confirm (or otherwise) if it's really necessary... I just get the impression that they do sound better, from experience of amps which have had wholesale replacement with other types.

The only type I would definitely keep out of the audio path are metal-oxide, but I happily admit that it's entirely due to what I've read and I've never tried them in an audio-path position.

I have found the plate resistors when bad seem to provide a fairly steady kind of rumbling and crackling that isn't affected by volume, tube tapping and the like. It's typacally the last thing I replace after bad tubes, solder joint problems and so on. Leaving them in is fine but if that's what it takes to get an amp quiet I do it. I do use carbon comps for replacing them though, (I know I'll probably get critisized for that) it's what was in there originally and will probably outlast me.I agree completely - what I thought you meant earlier was that you would routinely change the plate resistors, even if they aren't noisy.

I'm certainly not going to criticise you for replacing ones that have gone bad, or for using the right :) type for vintage amps. I don't believe in using original types in non-signal-path places where there are other more reliable alternatives though - it's not a black and white matter, the right part depends on where it is in the circuit.

I do actually put reliability above tone too, you may be surprised to know. Obviously if you can achieve both that's better still... but reliability is more important, to me.

[67super]

I agree completely - what I thought you meant earlier was that you would routinely change the plate resistors, even if they aren't noisy.



In re-reading my original responce it sounds like I always replace the plate resistors but in fact if the amp is quiet I don't touch them. I do find it fairly common though that they are noisey. I will admit that if one or 2 are noisey I replace them all.

I tend to be a minimalist with vintage Fenders. After the big caps, power tube resistors etc. I use the old addage if it aint broke don't fix it.