# Trying to understand ESR

New here, self taught, been working with tube amps for a while. Currently working on my 64 Deluxe Reverb that I have restored to stock, with some of the original ceramic and film capacitors. Just received an ESR02 to test capacitors. I still cannot wrap my mind around exactly what ESR affects in these old circuits. The ceramics are used for coupling, electrolytic's for bypass and filtering. My question is what does the ESR affect. If I find say 1.5 ohms ESR on a component, what exactly does that affect? Obviously in a grounded cap, 1.5 to ground would not be too good, so how should I view ESR, the way it gives up it's capacitance, or passes voltage in the coupling, lowered voltage to the next? These parameters change the tone, feel, and response of the amp as I have tried substitution. Still a bit hard to comprehend. To me capacitance is the most difficult thing to understand, and yes I have read much about it.

ESR is effective series resistance. A cap with a 1.5$$\\Omega\$$ ESR acts like it's in series with a 1.5$$\\Omega\$$ resistor. In a tube amp, that probably means nothing (look at the values of all the other resistors in the thing -- if they're involved with the signal path then they probably range from 20k$$\\Omega\$$ to 2M$$\\Omega\$$).

Note that it's "effective" series resistance. It's what the capacitor acts like. And because it's in series with the capacitive element, it doesn't matter whether the reality is that some joker has put a resistor inside the capacitor body, or that there's 100 little resistors in series with 99 little caps -- the behavior of the part will be the same, or that the electrolytic goo that makes one plate of the capacitor has some resistance.

simulate this circuit – Schematic created using CircuitLab

• One thing it means is for a given ripple voltage, the cap with higher ESR will dissipate more power, heat itself up more, and, all else being equal, fail sooner. – The Photon Feb 23 '19 at 0:47
• Okay, so if the capacitance is there on spec within toleramce, higher esr than spec won;t affect it much at all. – BarryRobertson Feb 23 '19 at 1:13
• I am still baffled, thus I must view it as resistance within the capacitance - or a resistor on either side of the capacitor in series. Makes no sense. – BarryRobertson Feb 23 '19 at 1:29
• The key word in ESR is "effective". It describes how the thing acts, not what it is. – TimWescott Feb 23 '19 at 1:46
• Sheesh, I think these old ceramics are bad, will get back in there tomorrow. I always thought they last forever, maybe not, tested a few I took out of the vibrato circuit. – BarryRobertson Feb 23 '19 at 2:03

The effective series resistance essentially makes the capacitor less ready to accept or release charge

When a voltage change would normally cause an immediate high current in a capacitor, with elevated ESR, some of the voltage change initially hits the ESR so a lower current flows for a longer time.

If a capacitor is like a balloon, capacitor with high ESR is like a balloon that has a constriction in the neck making it harder to get air in or out