112-65 RD First Problem in 21 years

Bought it new in 83. Never had a problem till now. I installed new Electro Harmonix Power tubes about 2 months ago. Worked great until last night. Non - Distrotion channel appears to no longer process gain which was great to increase volume. Never could play past 5 (gain)and 5 (volume). NOw at 10 it's not very loud. Is it the tubes? Distortion channel seem to work fine.
DS

If the distortion channel works it is not tubes.
maybe a chip.