Electrolytic vs Polypropylene cathode


2007-06-26 7:30 pm
1. If using an Electrolytic or Polypropylene in a cathode bypass for a 12AX7, is there a difference in gain given everything is the same including the capacitor value? Example: If I use a 1u/100v Poly cap, vs a 1u/50V Electrolytic in the cathode bypass stage of a regular triode stage, will it have the same boost in gain?

2. If using an electrolytic capacitor for cathode bypass, what are characteristics to look for for best audio performance? Are Sprague premium caps any good for this application?



Paid Member
2009-02-11 12:58 am
If the capacitance of the cathode decoupling cap is the same, there will be no difference in the gain of the circuit.

Low equivalent series resistance (ESR) and low dielectric absorption (DA) are things to look for in inter-stage coupling caps and decoupling caps. I use polypropylene (polypro - PP - MKP) caps for this application. Teflon (PTFE) is another commonly used dielectric type for this.

Another option would be to add a reasonably sized polypro cap in parallel with your electrolytic cap. Say 1 uF polypro...

Tolerance is rarely an issue with cathode bypass capacitors, except for those cases when they form a significant part of establishing the LF roll-off. It is not a good idea to use an electrolytic for establishing the LF roll-off, so their wide tolerance should not be a problem in a well-designed crcuit.

Low ESR may help. The ESR ends up in series with the effective cathode impedance (roughly 1/gm), so you either want a linear ESR or one which is sufficiently small when compared with 1/gm. Note that this means that high transconductance valves, other things being equal, will be more fussy about bypass electrolytic quality. Other things are not always equal.