humming gibson ga19rvt

[chris shaw]

guys
I own a fabulous sounding gibson ga19rvt-- with one problem-- it has a significant background hum. I
it comes on as it warms up even with volume set to zero. I doesnt go up a lot as the volume is increased.
I know gibsons of this era (early sixties?) are notorious for this
any tips on how to remove/ reduce it (simple please - I am a beginner but have built before )
thanks for your time and expertise
chris

[dkevin]

Humming Hawk

Hi Chris,
The hum, as you describe it, could be a result of failing (or failed electrolytic caps in the power supply. Check the schematic (readily available online) and replace them as necessary.
I have seen two other instances of similar hums in Gibson amps and they were real head scratchers. The first was a GA-25RVT that continued to hum after new electrolytics were installed. In that case, the output tube's (two 7591's) cathode bypass cap had failed. When I spliced in a fresh one, the amp lost the hum and sounded great! I continued to replace all the cathode bypass caps one by one and with each install, the amp gained volume and clarity.
The second case was a GA-5 Skylark tweed. I had wanted to keep the Sprague bumblebee interstage caps (2 each .02/600v) and had replaced the electrolytics and cathode bypass caps but still the hum persisted. I checked everything else (grounds-failed components-resistors) but all signs pointed to the interstage caps. When I replaced them, the amp quieted down and behaved itself.
I borrowed a Gibson Hawk from a friend back in the 70's and it began to exhibit the same symptoms. I wish I could have repaired it then- Such sweet reverb and tremelo!