TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
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Post by TCW on Apr 13, 2022 22:32:53 GMT
I am having a problem with my amplifier on an AMi F-120. Its been getting more and more microphonic. I switch around the two 12AU7’s with no improvement. I noticed one of my 6N3C (replaces a 6L6) was glowing blue and the other had a hot spot on a plate (see picture). I switched them around and the blue glow and hot spot moved with the tubes (new as of July). With the amp on the bench the 6N3C in Valve 4 position has 412VDC on Pin #3 and 417VDC on Pin #4 and 34VDC on Pin #8. The same for the 6N3C in Valve 5 position. I am not as familiar with taking AC voltages. It says to perform AC measurements with a .010 V (AC?) input signal for 45 RPM units (see attached) . I don’t know where to input it too, the phono jack? The diagram (attached) says that the reading should be at 25VAC. It also requires the volume to me maxed and the equalizer control to be in position three. So I did all that except the .010 input and I got .179 VAC on Valve 4 and .233 VAC on Valve 5, no where near 25VAC required. I could really use some direction on how to correctly take the AC readings. Picture of the tubes  Picture of the amp diagram.  Expected and Recorded DC Voltages  |
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Post by jukenorman on Apr 14, 2022 8:56:18 GMT
Hi Tom,
I'm not familiar with the 6N3C but having googled it, it seems it is a 6L6GC equivalent. The DC voltages at the 6N3Cs look a bit on the high side to me at well over 400V? Has the factory DC voltage chart come from the manual - I'm confused as to why the voltage at V4 & 5 pin 3 in the chart is given as 410V, yet is shown as 380V on the schematic?
What work has been done to the amp. especially around V4 & 5, have the coupling capacitors and biasing capacitor been changed as a minimum? I don't think taking AC voltages is going to help you much in this instance; that is just a way to check that the input signal is being amplified as it should. Be careful working with these big voltages!
Norman.
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TCW
Full Member
Indianapolis, Indiana USA
Posts: 222
|
Post by TCW on Apr 14, 2022 12:30:00 GMT
Hi Tom, I'm confused as to why the voltage at V4 & 5 pin 3 in the chart is given as 410V, yet is shown as 380V on the schematic? What work has been done to the amp. especially around V4 & 5, have the coupling capacitors and biasing capacitor been changed as a minimum. Be careful working with these big voltages! Norman. Yes, fully recaped and all resistors replaced because not one was in tolerance. I copied the chart from the amp cover. - Tom |
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Post by jukenorman on Apr 14, 2022 14:38:28 GMT
Hi Tom,
I don't know! According to the AMi F diagrams, the amplifier was originally fitted with 6L6G's. I think you would be seriously pushing a 6L6G putting 417V through it. I cannot find a data sheet for your 6N3C. If it is actually a 6L6GC equivalent, you would most likely get away with 417V because I think the 6L6GC is the most robust and highly rated of the 6L6 range. What is your mains voltage? The diagrams shows 117VAC.
Norman.
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Post by Marcel Bogert on May 14, 2022 12:59:56 GMT
6N3C is not an equivalent for the GC more the G versions they do work at an higher voltage but quality of the tubes varies widely. I always test the tubes and match pairs if at all possible.
50 pieces will give you mayby 10 pairs if you are lucky.
Why not use new 6L6 tubes they will not break the bank.
But i agree with jukenorman not sure why you're voltage is above 400 volts
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