Trying to understand ESR
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 &ndash; Schematic created using CircuitLab