This one has me stumped.
Conn organ amp rebuild into a 6G6 Bassman circuit; you may have seen my thread about the dual rectifers. I am still running off of one rectifier because I got a little sidetracked by this interesting meter meltdown phenomenon.
Here's what happened - I finished the B+ circuit, five nodes, then built the bias circuit and got a good range of negative voltage for the power tube grids. I now tried it with power tubes in and although B+ was pretty high, 480VDC, I was able to dial down the bias to a conservative level.
The next night I tried it with preamp tubes and although I have what I suspect are some oscillation issues I am getting sound. Here's where it got interesting. With my GB meter (cheap, Lowes, overpriced at $19.95) I could measure all the B+ nodes just fine but when I touched it to the power tube plates, *POP* and the meter literally melted on the inside. Bad smell, deformed readout on the panel, the works.
Undaunted, I tried it next with my (cheaper, Harbor Freight, $2.99) meter I got the same *POP* and a glow from inside the meter, but it survived the momentary blast. Impressive, but what the hell? Just to make sure I wasn't losing my mind I tried it on all the B+s and got normal readings; it's only the 6L6 pin 3s that are causing this. Next I noticed that, when I tried to use that meter in series with the cathode to ground, it was not reading mA, so I must have fried that part of it. I made one more fatal volts attempt on the plates and the meter glowed inside and self destructed. Adios.
So...... what do you think is causing this? The only possibility I am coming up with are inaudible oscillations that are being amplified through the power tube and appearing on the plates...... but appearing as what? AC voltage? DC?
I have an old Philips solid state scope that I would like to learn how to use and to look at parts of the circuit, maybe I can use this as a test case and a reason to get into it. Naturally I don't want to fry it, too, so I imagine I would inject a signal, then compare it to whatever is on the power tube grids and work my way backwards towards the input.
Any advice appreciated, including any known online sources for a beginner using a scope on a guitar amp. I have searched but haven't found anything good.
Thanks,
RWood
Conn organ amp rebuild into a 6G6 Bassman circuit; you may have seen my thread about the dual rectifers. I am still running off of one rectifier because I got a little sidetracked by this interesting meter meltdown phenomenon.
Here's what happened - I finished the B+ circuit, five nodes, then built the bias circuit and got a good range of negative voltage for the power tube grids. I now tried it with power tubes in and although B+ was pretty high, 480VDC, I was able to dial down the bias to a conservative level.
The next night I tried it with preamp tubes and although I have what I suspect are some oscillation issues I am getting sound. Here's where it got interesting. With my GB meter (cheap, Lowes, overpriced at $19.95) I could measure all the B+ nodes just fine but when I touched it to the power tube plates, *POP* and the meter literally melted on the inside. Bad smell, deformed readout on the panel, the works.
Undaunted, I tried it next with my (cheaper, Harbor Freight, $2.99) meter I got the same *POP* and a glow from inside the meter, but it survived the momentary blast. Impressive, but what the hell? Just to make sure I wasn't losing my mind I tried it on all the B+s and got normal readings; it's only the 6L6 pin 3s that are causing this. Next I noticed that, when I tried to use that meter in series with the cathode to ground, it was not reading mA, so I must have fried that part of it. I made one more fatal volts attempt on the plates and the meter glowed inside and self destructed. Adios.
So...... what do you think is causing this? The only possibility I am coming up with are inaudible oscillations that are being amplified through the power tube and appearing on the plates...... but appearing as what? AC voltage? DC?
I have an old Philips solid state scope that I would like to learn how to use and to look at parts of the circuit, maybe I can use this as a test case and a reason to get into it. Naturally I don't want to fry it, too, so I imagine I would inject a signal, then compare it to whatever is on the power tube grids and work my way backwards towards the input.
Any advice appreciated, including any known online sources for a beginner using a scope on a guitar amp. I have searched but haven't found anything good.
Thanks,
RWood
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