I recently built a practice amp using surplus stuff I got cheap. I was using a plug in transformer and it had lots of hum so I built a filter using a 100 ohm resistor and a 1000 mf electrolytic cap. it seemed to work good at first. I came back after a while and played some more and I noticed it made a tremolo effect when I played a loud chord and the power light blinked. I was wondering if the cap is breaking down. its rated to 50v but its old and been stored in a hot area. should I buy a new cap and try again. it is a SKB footnote guitar amp I got cheap and put in my on cabinet. I takes a 9vdc center negative supply.

It seems like you might be operating a circuit that requires a dual-polarity DC supply on a single DC supply. Or, maybe one of the circuit-common/"ground" nodes is not joined with the others.

The link below is to details for obtaining a dual-polarity DC supply from a single-polarity DC supply

Single to Dual DC Power Example

I had the wrong cable for my speaker