05-30-2013, 12:41 PM
Jack
Make sure you DO know your 6A7 is good.
I had weird experience with them just a month ago (I needed to check the theory that my 6A7 was not oscillating properly below 6MHz, my FB band did not work there at all) where I bought an NOS tube from a guy (and he was an honest guy) and it tested about 600 instead of 1000, which is "?" territory. I was able to prove the theory (the oscillator worked as it should) but the tube was weak. That was #1.
Then I bought another NOS tube from another guy and it was near-dead, at 300. The guy admitted he did not test the tube as his tester does not test them, and sent me another RCA NOS one and that one was just fine (that's the one I keep in the radio). That was #2.
But before this guy had a chance to react I bought the third tube, also NOS and this time the guy claimed he tested it and it was like 150% or so (not sure what that means).
The tube came, and again, the pentode section was extremely weak. I suspect he tested only the oscillation section (in all three tubes it was indeed very strong).
So, out of four tubes and from three sellers only one was testing properly.
So make sure you buy from someone who tested it on a mutual conductance tester that tests both parts, the oscillator and the pentode, and both are close to new value or at least show strong "good". Ask the seller about how he tested them.
Or go to one of the sources folks here recommend (Bob Dobush at findatube.com).
Make sure you DO know your 6A7 is good.
I had weird experience with them just a month ago (I needed to check the theory that my 6A7 was not oscillating properly below 6MHz, my FB band did not work there at all) where I bought an NOS tube from a guy (and he was an honest guy) and it tested about 600 instead of 1000, which is "?" territory. I was able to prove the theory (the oscillator worked as it should) but the tube was weak. That was #1.
Then I bought another NOS tube from another guy and it was near-dead, at 300. The guy admitted he did not test the tube as his tester does not test them, and sent me another RCA NOS one and that one was just fine (that's the one I keep in the radio). That was #2.
But before this guy had a chance to react I bought the third tube, also NOS and this time the guy claimed he tested it and it was like 150% or so (not sure what that means).
The tube came, and again, the pentode section was extremely weak. I suspect he tested only the oscillation section (in all three tubes it was indeed very strong).
So, out of four tubes and from three sellers only one was testing properly.
So make sure you buy from someone who tested it on a mutual conductance tester that tests both parts, the oscillator and the pentode, and both are close to new value or at least show strong "good". Ask the seller about how he tested them.
Or go to one of the sources folks here recommend (Bob Dobush at findatube.com).
