Guyatone GA-600F Recording Model

I play a variety of music ranging from old Ventures and Shadows tune to a mild jazz. I play only with friends and family members and not professionally. I like to use the clean channel most of the time and sparingly for the distorted channel. However the distorted channel lacks gain for some heavy metallic rock sound so I used an overdrive stompbox whenever this channel is on to augment the drive.'Iv'e used this amp for many years but now it is showing the effect of time. The sound when it was new was very good but somehow, like every pure tube amps it needs some maintenance. I had already change all pre amp tubes which brought me good result. However switching from clean channel to the overdrive channel gives me some problem. I suspect a defective photo couppler needs replacement and I can't find this in the local electronic stores here in Manila. I would appreciate it if someone could tell me where to get it.