Are Motor Run Caps Of Any Use On Guitar Amps?

[Cliff Schecht]

Rp, it's easy enough to calculate the time constant of the RC filters in the bias supply and see if it's longer than maybe 20 seconds (t=1/RC). I think this is about what the typical Fender values get you. Easier than this is to open up your Duncan PSU Designer tool and put the RC values in there.

If you're taking longer than the plate warmup time to even get to a moderate value of bias (like 1 minute to get to 20% of the bias value) then you're going to hurt something but otherwise the bias usually get's close enough to be safe within the time it takes for the plates to get voltage (although SS rectifiers are quick so the standby is smart with these if you overdid it on the bias supply capacitance).

[Structo]

Generally if we have a standby switch that is after the first filter caps but before the power tube plate supply, won't the bias voltage be up by the time we throw the standby switch?

I normally give it about 30 seconds before I take the amp off of standby, so by then the bias supply should be stable, right?

I'm referring to my Dumble ODS 100w clone for example.

It has a 100uF/ 100v bias cap.

Being on the larger size for a bias cap, won't this provide better regulation than a smaller cap?

[diagrammatiks]

So you have 1Hz and 0.5Hz ripple in your bias circuit?
You don't need filter 3db points here, you need to know AC impedance relations at 60, 120,180,240Hz
50uF is roughly 53 Ohm at 60Hz, 26 at 120Hz etc. How much ripple attenuation you will get depends on both current draw (or resistive load parallel to the cap) and impedance of the voltage source charging the cap.
You need to figure out what, if any, influence ESR has on ripple filter performance.

that circuit will have ripple no matter what as a consequence of the rectification process.

the influence of esr on ripple filter performance is well documented by far more qualified people then me.

[niftynev]

Jaycar Electronics in Australia have non polarised electros in the 100 volt range of up to 400uF. They are 16 x 37 mm. You might be able to get them by mail. I believe they're mainly for speaker crossovers, so I'm not sure on their ESR.

I built the Silicon Chip Magazine 'Low Ohms & ESR Meter' for about 80 bucks, & it's an extremely useful piece of test equipment! Great for repairs! It sits right next to my old 1982 vintage Fluke multimeter!

Hope that helps.