i last night tried pulling all but one pre-amp tube and the hum stayed the same. my linear power supply starts to climb in voltage quickly tho. my second filter capacitors are only rated to 500v and one pre had it up to 450v already
only slightly interesting thing i noticed is one channel has less hum then others. but the only wiring difference is im using a lm317 as a constant current source on the output tube on that channels heater, and im only using one 12at7 triode (6 total triode 'sides', only need 5, one doesnt get used) you can see it in my picture, look at the left most 9 pin socket
oh and btw thanks a lot for your time/effort in trying to guide me towards a solution. i highly appreciate it
only slightly interesting thing i noticed is one channel has less hum then others. but the only wiring difference is im using a lm317 as a constant current source on the output tube on that channels heater, and im only using one 12at7 triode (6 total triode 'sides', only need 5, one doesnt get used) you can see it in my picture, look at the left most 9 pin socket
oh and btw thanks a lot for your time/effort in trying to guide me towards a solution. i highly appreciate it
so its important to disconnect both sides of the filament at the same time? dont have any dual throw switches laying around so i guess this will have to wait a little bit to test.
if i pull the pre's my channels are absolutly silent, with the 12at7's still in there sockets. if i bypass the preamp audio path i can get audio out, with still no hum it just requires high mixer volume on my source to get good volume
Is there any difference between the B+ or heater connections for the 12at7's and the pre supply? ie are they the same supply connection point?
The reason I ask if the B+ is the same and the heaters are from a different supply try connecting one of the preamps from the power amp supplies!
The reason I ask if the B+ is the same and the heaters are from a different supply try connecting one of the preamps from the power amp supplies!
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the 12at7's get fed by each respective output transformer. refer to schematic on post 7
the 12at7 shares heater connections with the 12au7's
i actually put my multimeter into current measuring mode and bridged the input from source and just before or after coupling capacitor leading to the at7, i didnt expect it to work as well as it did
here, this is a slightly different drawing of the same circuit. this will help you understand perhaps. the b+ that runs through the 'wall' of the amp all run to the second filter capacitor
An externally hosted image should be here but it was not working when we last tested it.
this sounds like an likely scenerio. what is the best way to accomplish this?
its really pulling it through the output transformer. its not connected physically to the top of the pentode. it just branches off to both the 12at7 and the 807. branches are hard to see in that photo, here have another:
An externally hosted image should be here but it was not working when we last tested it.
that was a early picture of the build. ground buss was rebuilt among other things
Hi!
If your B+ is about 300V as indicated in the first scheamtic, connect a voltage divider 100k/33k from B+ to ground. 100k on the B+ side and 33k on the ground side. Connect a 47uF/100V in parallel to the 33k. Connect the - leg of the heater supply to the connection between the two resistors. 5W resistors will do.
Best regards
Thomas
If your B+ is about 300V as indicated in the first scheamtic, connect a voltage divider 100k/33k from B+ to ground. 100k on the B+ side and 33k on the ground side. Connect a 47uF/100V in parallel to the 33k. Connect the - leg of the heater supply to the connection between the two resistors. 5W resistors will do.
Best regards
Thomas
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