MK III woes...

Thanks Bob. I built an st70 last winter, used a 2 prong plug and never hab this issue. A good nights rest and I'll give a try in the morning. Thanks again. By the way, my preamp is a 6 months old Rugue Metis that I use with the st70 and performs perfect. And yes the interconnects are in place as well as speaker load.

https://s1303.photobucket.com/user/grizzinca/media/image_zps1eeb8b8d.jpg.html?sort=3&o=1https://s1303.photobucket.com/user/grizzinca/media/image_zps2161cb99.jpg.html?sort=3&o=0https://s1303.photobucket.com/user/grizzinca/media/image_zps89f6c131.jpg.html?sort=3&o=2.
So I used the cheater plugs as explained and there was a slight change for the better, but still a noticeable hum. Switched power cord to its own outlet one one amp, that helped even more. Ran the amps for about 10 minutes and on both amps v2 became glowing red, so I shut both amps down. Doesn't appear I did any damage. Amps sounded ok. Also I disconnected the rca's from the preamp and shorted out the end and the hum stayed. Outputs biased ok. Steady. Trying to give as much info, along with more recent pics. The hum was present using different preamps. Didn't change.hope the pics are of help, could really use some expert eyes on this. Thanks, Mike

Hi,
Looking at the photos...I do not see the 680 ohm 1 watt resistor connected to the speaker output terminals between the Black & Yellow output transformer leads. Is this connected somewhere else or is this resistor located on the circuit board?

Kevin

In an earlier post Roy had asked about grounding. This post is in continuation of that thought.
The only ground reference to the power supply quad capacitor in your picture is through the twist mounting tab, the black wire and the socket of V1. The Red/Yel wire(CT of the X'frmr) connects to the chassis via the 3 lug terminal strip.
You are depending on a mechanical connection to complete the B+ circuit.
Have a look at the mechanical connections between the chassis and V1 mounting fasteners and the fasteners between the 3 lug terminal and the chassis. Tighten up all the fasteners. Oxidation between metal surfaces can be problematic when talking about current flow.
As a possible solution you can run a ground wire from the Red/Yel wire to the ground tab of the quad cap and another ground wire from the VTA board directly to the same ground tab of the quad cap. What you are doing is effectively bypassing any resistance in the grounding circuit(s).
Good luck and please bleed the quad cap before you get near them.

Thanks for your help. I'll 're-ground' as per your reply. Hopefully this will cure the hum. Would a bad ground cause output tubes (v2)in both amps to overheat? Again, thanks for the help.
Mike