The proper grounding scheme of a vacuum tube amplifier has been the subject of many debates, blogs, forums, and web pages. There are actually two important aspects related to proper grounding, hum reduction, and safety. The safety aspect is often overlooked, or ignored in the quest to eliminate hum. With careful design it is possible to satisfy both criteria simultaneously.
The important safety aspect that I must reinforce is the fact that EVERY CONDUCTIVE PART THAT THE USER CAN TOUCH MUST BE GROUNDED. This is usually fairly easy to do, and if done right will help reduce hum.
One of the highly debated aspects of the above statement is whether or not to ground the secondary of the output transformer. There are people who strongly believe that grounding the cold (black lead) side of the output transformer causes a degradation in sound quality. I find no technical reason for this belief. There are strong safety reasons why the secondary and the metal housing of the transformer should be grounded. It is possible for a short to develop between the primary winding and the secondary or the metal core of the transformer. A short of this type may not adversely affect the operation of the amplifier, however it could cause a high voltage (several hundred volts) to appear on the speaker leads. If a user touched the speaker wiring and a grounded object at the same time, he could be fried. This is very possible on a guitar amplifier. In my 40 years of working with tube amplifiers I have seen 3 shorted transformers, 2 were in guitar amps. The guitar amps blew the fuse or the rectifier tube because the transformers were properly grounded. The custom built high end stereo amp was working normally until I found the transformer problem. There was over 200 volts measured between the speaker jacks, and the input connectors on this amp. This is more common in vintage transformers because they used paper insulation, and paper absorbs moisture.
Further details will be added shortly.
In the meanwhile make sure that ALL accessible metal is connected to the power cord ground pin.