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In Reply to: RE: All the 6SN7s must be in place and as many of the power tubes as possible. posted by Ralph on September 29, 2015 at 09:28:56
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.
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