Component Tolerances

[TIMBO]

Hi guys, After doing some value checks on resistors (carbon comp)and caps(mostly Ecaps) on the old amps I have, some have drifted over the marked 10%. The main offenders are high value plate resistors (.5w)100k are measuring up to 110k and some are even higher. I know this is a problem with CC resistors.

If a preamp tube that has a 100k plate and 2.2k cathode resistor I would expect that one that has a 112k (marked 100k measured 112k)plate resistor and a 2.23k (measured) cathode resistor would perform differently (gain wise)
But if all resistors in an amp are around that +10% would there be much noticeable difference between this and an amp that has all resistors exactly on marked value.

Ecaps, most of the Ecaps are showing signs of distress (oozing and over marked value) I don't have any way of testing the state of the caps other than a visual and measuring value and as none of the old caps have a +- tolerance marked on the cap I toss them just in case.

This lead me to check out replacement cap cans and to my surprise the new caps have a -10 +30% tolerance but when a tube rectifier states that the reservoir cap is to be no greater than say 50uf this could then mean the cap value could rise to 65uf.
 OR are valve amps not that value specific. Thanks 

[AZJimC]

Most things in valve amps are okay at 10%, and even 20% from listed values. Yes, it can change performance a bit, but in most cases the math proves the differences to be minimal. I'd be surprised to see a tolerance of even 20% consistently between various tubes of the same type. Given that info, however, in the case of carbon comps, much drift caould also include slightly greater noise (hiss) or thermal induced drift. Not a bad idea to swap those out just to prevent future issues. That's just my thought on the matter. It's not so much the tolerance but reliability I'd be concern about.

[HotBluePlates]

After doing some value checks on resistors (carbon comp) ... on the old amps I have, some have drifted over the marked 10%. ... 100k are measuring up to 110k and some are even higher. ...

Not really a big deal.

If the amps are truly old and the 100kΩ resistors have a silver band, they probably never measured closer than 106kΩ (or above) or 94kΩ (or below). Composition resistors are made by mixing up the "composition," attaching leads and measuring the resulting resistors. The body gets painted with the color bands of the resistance it lands on. The resistors closest to the center value get the gold 5% band, and then 10% & 20% (no band) resistors get marked according to where the measured value lands.

Every good old amp I ever owned had resistors that drifted off spec. The few times I swapped with "right value" parts, the sound didn't improve.

... new caps have a -10 +30% tolerance ...

That's fabulously tight tolerance for an e-lytic cap. Some filter caps in your fave old amps were +100% -50%. The "good stuff" was +50% -30%.

... but when a tube rectifier states that the reservoir cap is to be no greater than say 50uf this could then mean the cap value could rise to 65uf.

Find a data sheet from the 60's or earlier that actually says that (I know I saw a JJ sheet that had a "max" value and 1 Philips sheet from the old days). Most don't really specify a maximum capacitance; they have a maximum current the cathode can pass steady-state and during brief sharp pulses. The impedance of the power transformer matters when figuring this stuff, as does any series resistance added. When you adjust for these, the capacitor size can go up because the series resistance limits the peak current possible.

When you hear "maximum capacitance of ____ rectifier", it's just people quoting someone else who mis-quoted a part of a data sheet they didn't understand. Oddly, none of the same folks seem to notice the handy tables showing the maximum permissible direct current draw, or the graphs with a.c. input volts with different types of power supplies (these already take into account the size of the input resistor or choke, as well as PT impedance).

Of course, the data sheet graphs tend to have limited utility these days, because there's generally 1 value of input-cap or input-choke, and PT impedance. RDH4 has a whole section on how to really calculate all this stuff if you want to mess with it (it's downloadable in the Library of Information).

[TIMBO]

Thanks HBP, The CC resistors are in the old Fi Sonic I am rebuilding and I have replaced all the Ecaps and as is, it sounds great. The CC's are not showing any signs of distress heat wise and are under the +20%, also most are a bugger to remove, so best left alone. The amp is pretty quiet hum/hiss wise.

The E cap in question is from the Hammond organ amplifier. It has a single 50uf/450v cap can that measure 51.1uf and is in a cardboard cylinder. It appears to be in good condition.This cap is not directly off the a tube rect but has a 64 ohm resistor between the cap and rect.

[6G6]

A few people have noticed old amps, mostly 60s Fenders can sound very different from one another,
even two of the same model with consecutive S/Ns.
When they started trying to find out why, the big difference was component value drift.
They don't all change by the same amount and the combined effect of what drifted how much, which way and where
accounted for the noticably different sounds.
The effect can be good or bad, partly depending on what you want it to sounds like.

[tubeswell]

The drift is part of your particular old amp's mojo and shouldn't be interfered with.