Rickresource Rickenbacker Forum

First a little history about my M-14-A amplifier,
I have owned this amplifier since 1967 as it was a gift from my parents along with a 1956 Fender Musicmaster six string Guitar. They bought both from the original owner that played in a country western band.
The amplifier has had the original tubes in it all these years, well original as far as what was in it since 1967 when I received it. The Power & Rectifier are Sylvanias 6V6GT Power and 5Y3GT Rectifier. The pre amp section has 3 ECC83/12AX7 Amprex Bugle Boy Tubes made in Holland.
Recently I have been doing a little work to the amp bringing it out of retirement. I have replaced the old dried & brittle power cord and the tremlo switch cable. Also replaced 4 caps and removed the original tubes for safe keeping replacing them with GT-5Y3GT GT-6V6GT-S duet and 3 GT-12AX7-M a Mullard copy from what I understand.
This little amplifier sounds amazing and blows away my friends 64 & 65 Fender Deluxe Blackface Reverbs as far as sound and tone quality.
There is one problem I am having with it, when I first plug it in and turn it on after a few seconds I hear some on & off scratchy sounds even with both volume pots off. And after a few minutes the amp cleans up and is quiet as a mouse. But after about 1 hour the sounds return but now just a tiny bit louder.
I have tried to isolate the problem by waiting until it makes the noises after 1 hour and shoot a little “freeze spray” in selected sections of the board thinking it is a heat related issue but with no avail.
I have not gone as far as replacing any of the ceramic disks but I have heard of putting heat shrink tubing on them to insulate them.
Also, I believe the AC line filter cap cans are originals.
Any info on where this scratchy sounds could be coming from would be appreciated.
Thank you,
Brian

My first move would be to replace (temporarily, to isolate the noise) the 6V6 Sylvanias as they're notoriously noisy when aged. If the noise persists, it's probably the phase inverter's plate load resistors that have deteriorated over time. That's where I'd start anyway. Best of luck. The M-14-A is one of my all time favs!

Possibly tube noise, but also cap charge/discharge noise. Without a doubt I would replace the filter cap cans, as there is no way they are still good. This is a reliability / performance / safety issue.

???

But he wrote that he has ALREADY replaced the old tubes and caps...
:::scratchin' head:::

Andrew, he said "Also, I believe the AC line filter cap cans are originals."

These have been a problem with almost every old amp I've had- I agree with Seth that this is the most likely cause of your problem.