Resistor tolerances

[miltown]

I had a question about carbon comp. resistors. I have resistors that are suppose to be 68k ohms and it reads 86k ohm and another that reads 77k ohms. Obviously out of tolerance when each has a silver stripe. So how much will that effect amp performance. These 68k are the input jacks on a Fender Bandmaster. I have scratchy guitar volume pots when plugged in on either channel normal/virbrato with any guitar. No DC is measured on the 1/4" cord. Also 100k's on the plates are reading 111k to 120k with silver stripes. So is this not so big of a deal .

[Enzo]

I imagine they are pretty old, aren't they.

WOuld a 77k and an 86k affect performance much at the input? No, not by their values anyway. The typical Fender dual jack input with the two 68k resistors is there to provide the 6db pad on the lesser jack. When you plug into the low gain jack, the two resistors are in series across the signal, and the two form a voltage divider, sending 1/2 the signal voltage on to the amp circuit. 68k/(68k+68k) x the signal voltage.

If your resistors are 77k and 86k, depending upon which one went to ground, instead of a ratio of 0.5, their ratio would be either 0.47 or 0.52 (that is either 77k or 86k over the total of 163k). The difference between 0.47, 0.50, 0.52 is not enough to even think about.

And 100k plate resistors up to 120k won't hurt it. That is within 20%, which was what Leo Fender used on his amps for years.

However. The very fact these resistors have aged that much tells me they are not in the best shape. They will be more likely to be noisy, and potentially even intermittant. If the amp is working to your satisfaction, then don;t mount a total resistor replacement effort unless you just want a project. But if the amp gets noisy, I'd suspect the resistors first.

If this amp makes guitars scratchy, then there very likely is DC there, whether you are mesuring any or not. WHat sort of meter are you using? Such DC comes from a grid-leak voltage building up, and that really is not a matter of resistors having shifted in value.

[nosaj]

Not that big a deal is similar in having to air up your tires everyday to go to work because they are old an leaky. One day those tires are gonna be a serious issue. Right now you have warning signs proceed at your own peril.

[Chuck H]

Just to add to what Enzo was saying... If your amp makes guitar volume pots scratchy there is DC at the input jack. This is most likely not a resistor issue. It's most likely a capacitor issue. I've also seen bad preamp tubes cause it. If the amp has always done it, it may be a design flaw in the circuit.

[jrfrond]

I WILL agree that increase in resistor value, which is normal, especially for carbon comps, is a sign that they are aging. Eventually, said resistors MIGHT crap out.

Then again, they might not.

Tolerances are for NEW components. As components work and age, their value changes, sometimes radically, as you've noticed. Part of the vintage amp mojo is the aging components. Caps dip in value, resistors increase. Whether or not you like the resulting sound is subjective. However, IF you have an amp like this and decide that pre-emptive resistor replacement is the way you want to go, and you like the tone, the you MUST measure all resistors removed from the circuit and replace with resistors of similar value and type. Hence, if your 100K plate resistors are now 120K, the REPLACE it with 120K. However, one of the issues with carbon comp resistors is the lack of readily-available non-ISO ("in-between") values.

Did you know that one way of revitalizing carbon comps is to resolder them? It's true. Carbon comps tend to be hygroscopic and absorb surrounding moisture, but applying heat via baking or soldering tends to boil-off moisture contamination.

I don't recommend a sweeping resistor replacement any vintage amp, unless you want it to sound like it did when it was new.

[miltown]

I'm using a DMM for measurements. So how do I check grid leak voltage buildup ? Also the vibrato thumps like Yo Mtv Raps.. This is a Fender Bandmaster, AB763 is the schematic that I'm using. The signal pretty much goes inputs through the pad and to the gird on both the normal channel and the vibrato channel and both are scratchy. Could it be a coupling cap down the line in the other half of the tube. The tubes are about a year old I have not swapped them out yet except on the vibrato to check the thump.

[Enzo]

VOltage on the input is between the input tube grid and the guitar. ANything elsewhere will not be involved.

FInd the trem tube, take the wires from its socket to the eyelet board and bunch them together. Now look at the socket next to it. Move its wires as far from the trem tube wires as possible. Just shove them over. ANy help?

[miltown]

o I found the scratchy guitar. It seems the ground that grounds out the input jack on the vibrato channel is not making a good contact. Tightened it up and filed the contact andthe scratch is gone. I have bunched up the wires to the vibrato tube and moved the others and still noise. I have noticed that when I move the wires there is no change in thump. That would include tube wires and wires at input and volume and tone stack, speed and intensity.

[miltown]

I just put new input jacks on vibrato channel and the new inputs have lessened the scratching and thumping effect . Thumping still there but alot lower in volume. I still notice when I measure from grid on either channel input stage to ground on guitar cable there is .023 vdc. Is that an acceptable level?

[km6xz]

The easy access to precision calculations, to 6 or more decimal places has gotten everyone used to such resolution but in engineering reality anything more than 3 digits is wishful thinking. We have to build and measure things to find out how far off from theoretical the results are, not matter how sophisticated your SPICE model is. The case of the 68k resistor drifting up to 86k is not unusual as others have stated, for old gear. Radios, phonos and TVs were full of 20% resistors and they all worked, for the most part.
Will parts drift change the sound, yes, but so does everything else, from material of the drapes to age of resistors. But is it bad? Not unless it sounds bad to you, but it might sound good to someone else. There are too many moving variables to definitively peg the sound of an amp, but many people try and sort of come off as foolish. Hi-end hi-fi has gotten extreme in its nuttiness but tube guitar amp hobbyists and those who prey on them are getting almost as out of touch with the reality of sound, perception and electronics.
If you like the sound now, leave it alone until it breaks. If it is getting noisy, find out why and fix just that. Plate resistors of the carbon comp type change a lot and in drifting they can develop noise due to the less uniform packing of the carbon, iron mixture inside over time. If you have a well insulated pair of pliers, try gently squeezing the body of the plate resistor which listening with the gain up high with the input shorted. A noisy resistor will certainly change the sound when compressed a little. It will also pin-point carbon comp resistors which are not yet noisy but will be soon. If the change in sound/noise is smoothly related to the the pressure as you squeeze, it is a good sign that the resistor is fine and will be for a while. You can do the same with a true differential input scope plug-in to measure the difference in drop across the plate resistor with no signal present but the listening test and pliers is a very sensitive test. The only difference is that the scope can show noise products outside of the audio spectrum that will broaden to be audible later.