Power & Standby switch off
between 2 & 8 i'm getting now 105ohms
between 7 & 8 i get 105.2ohms
between 2 & 7 i get 0.3ohms
Yup, just checked and they look burnt!Maybe we are on to something here.
In post 54 you indicated 51Ω, and now it is 105Ω. To me that indicates an intermittent solder joint on R80 or R81 (the 100Ω resistors in the heater circuit). Go back over those visually. Also hook up your ohmmeter across each one and then chopstick it. See if the resistance bounces around between 50Ω and 100Ω. We are looking for ~50Ω, if you can't get it, then one of those resistors R80 or R81 has blown open.
Good job!Maybe we are on to something here.
In post 54 you indicated 51Ω, and now it is 105Ω. To me that indicates an intermittent solder joint on R80 or R81 (the 100Ω resistors in the heater circuit). Go back over those visually. Also hook up your ohmmeter across each one and then chopstick it. See if the resistance bounces around between 50Ω and 100Ω. We are looking for ~50Ω, if you can't get it, then one of those resistors R80 or R81 has blown open
I've seen one like that. It was either a short in the power tube or a loose screw making it's way to the tube pins. Took out a 6L6 and those 100 Ohm.
I snipped one of the legs of both R80 & R81 resistors, not so easy task without taking out the pcb.When everything is operating the way it should and say you turn the amp on without tubes, those resistors conduct current constantly. If the unloaded heater voltage rises to 7VAC (in the worst case scenario), they will conduct about 35mA RMS continuously burning off about 1/8W per resistor. 1/2W resistors should be fine for long term use. 1W are even better. Alas, I do not think undersized resistors are the problem here.
If one of the resistors were to fail open, then the other resistor would stop conducting too. One would expect then that only one of these resistors could burn up. The fact that both of yours are burned show to me that you have a short somewhere in the system. Current is flowing into that heater winding from another source, passing through one or both of those resistors, and going into ground.
Others can chime in on the possible sources of a short here. I have already suggested taking a good look at the sockets and you have taken some resistance measurements there. If you want, you can remove those 100Ω resistors and then go back through all the sockets and measure resistance from the heater pins back to the other pins. Without those 100Ω resistors, all pins should measure OL back to the heater circuit. I think you know this, but the noval sockets are heated on pins 4,5, and 9 while the octal sockets are heated on pins 2 and 7. You should also get very close to 0Ω between the heater pins themselves. If you can check the PCB where the sockets are soldered onto the board, even better. I have read this method of construction is notorious for unreliable service over time.
The other possible short is within the PT itself where a breakdown of insulation could allow current to flow between windings when it shouldn't. I like this hypothesis because it could also explain the drop in B+ voltage you documented. This is a little easer to troubleshoot. Since you have the 100Ω resistors temporarily removed, there should be no shared connection through ground with the HT winding. Measure resistance from anywhere in the heater circuit back to the HT secondary winding. It should be OL. (Does your have the heater circuit fuse or not? If it does, check it and make sure it's intact before proceeding.)
View attachment 984402
So those are the possible culprits that I can think of. Many smart minded folks here might dream up other possibilities worth checking into before replacing those resistors and turning the amp on again.
Good luck, let us know what you find.