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'61 supra, loud hum, Vp drops

10/5/2000 9:07 PM
Tom B
'61 supra, loud hum, Vp drops
Hi all!  
First, what a great site!  
I have a '61 supra that I just recaped and retubed. The new replacement 6973 output tubes are Pro/Comm :( (NewS did not state the mfgr. and can't be too choosy on odd ball stuff). Amp came with only one output tube.  
When power is applied and the heaters begin to warm up (tube starts to conduct) the amp hums louder and louder while dragging the plate voltage lower and lower. I also get a quick cathode bias measurement (across a 250ohm || 33uF) of only 1V before I shut the amp down.  
A note came with the 6973's that stated the tube may not work in all amps *screen grid was not connected to pin 8, if wires are connected to pin 8 move all wires from pin 8 to pin 1*. I looked up the tube basing which shows both pin 1 and 8 connect to this grid, so I jumpered 1 to 8, there were no other connections wired to pin 1.  
The 5Y3 rectifier and other tubes are good, Supplies  
appear good (without the 6973's plugged in). The OT appears good.  
I will start to up the cathode bias resistor value and go from there, will also substitue the original 6973 that was in the amp.  
Anyone use these tubes in this circuit or have any more insight as to possible causes, suggestions,...etc.?  
Tom B