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In Reply to: RE: OK- its an older 12-tube model posted by Ralph on September 25, 2015 at 14:10:27
Older 12 tube but brought up to 3.1 status. I first switched the cables at the preamp side to see if it was the preamp but got the same lackluster performance from the same channel and then switched cables at the amp side to see if it was a bad cable but it was the same lesser sound from that amp side. I will try the speaker cables next to make sure I don't have a dirty connection or that the cleaning lady did not knock something loose from the Zero autoformers.
Are you OK with checking voltages as you did in the M-60?
You don't have to remove the bottom cover. Just take a power tube out of the socket from the left bank and one from the right bank. Then locate the index of the socket and count 5 pins clockwise; that is one of the grids of the power tube. What is the voltage? You can measure it with respect to the chassis.
We need the grid voltage on the left and right banks.
Just to make sure it was not the tubes, I swapped all the tubes from the poorly functioning amp to the one working well and vice versa. Still got the 2v offset reading on the left and still got a pinned to zero reading on the right. Sound wise still have the powerful right as compared to the left.
Pulled the tubes and measured the grid voltages on grid 2 to chasis and got 410 on both rows. Tried readings comparing anodes to cathodes and grids to both on standby and fully on and despite small or drifting values as the amps warm up there is little difference between them and the readings are almost the same as the functioning amp. I only tested the two end tube sockets of each amp and maybe it would be different if I tested all the sockets.
Just to be clear, I pulled all the tubes including all 6as7 and all 6sn7 before measuring. Do I need any tubes to be in place to do the measurements? Also, you say the two banks are six left and six right, do you mean that drawing a line down the center from front to back, the two rows of six are not divided into each row, but instead comprise two rows of three tubes on the left and two rows three tubes on the right? This makes a difference for me as to how I balance the power of the two banks against each other.
I did not originally measure the other tube sockets other than the end two on the right and now measured the two sockets on the left and got the same 410v.
The 410V reading is incorrect as there were no tubes.
I reinserted tubes to start measuring bias and thought again about the fuses. Visually they all looked intact but when I used the continuity tester one of the two 5A slow blow fuses on the front of the amp was not making the short and the ohmeter showed infinite resistance. Strangely when I measured grid voltage on all the tubes at pin 4 they all showed 184-5 on the left and 181-2 on the right, even without the left fuse in circuit. With the fuse out I depressed the voltage offset switch and even without that fuse it was pinned at zero. I put a new fuse in to its place and listened and it sounds good. I think that pulling and reinserting the fuses or the tubes must have cleaned some crud off of something to make the difference. Otherwise the tubes that I thought had to be balanced one row of six to the other row I now know have reinserted to be balanced left to right. Now offset is at zero and sound is balanced between the two amps. No explanation readily available but sound is back.
As an aside to Ralph, are there any different roles played by the five 6sn7 tubes that would let me know which needs to be least microphonic, which needs the best plate to plate match or if any of the tubes need to match one another. On my amp, looking from the front, the rightmost tube is next to the xlr jack and the leftmost tube is next to the offset meter.
Thanks again for the advice.
Glad you got it sorted! FWIW we no longer use those fuses on the front panel by the power switch, and disable them when we see them. They lead to confusing issues like this one :)
The 6SN7s closest to the input connection will have the most effect.
The last 6SN7 in the row should be either a -GTA or -GTB version.
Thanks! Do those fuses serve no function? Would it hurt to put a larger value fuse in to avoid burnouts?
Thanks for the info on the 6sn7's - I take it there is no particular need for tube matching or plate to plate matching except for perhaps the first tube?
Yes, we no longer use those fuses. So we bypass the fuse holder and remove them.
The input tube does seem to work better with matched sections.
I think you need to leave the input and driver stage tubes (6SN7s) in place when measuring the output stage grid voltages. "410" is not plausible for grid voltage wrt ground.
And on pin 1 and 4 of every socket was 410 compared to chasis ground. Not sure if the 6sn7 sockets should be tested or where to look next.
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