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In Reply to: RE: 12ax7 for ARC SP16 posted by btbluesky on October 03, 2011 at 23:34:10
Maybe, just let the system break-in, before rolling tubes. Surprisingly, rolling various 12A(_)7's in the gain stages sometimes doesn't really change the gain all that much. Maybe, the overdrive, esp in guitar amps. And perhaps the headroom. But not the overall loudness--- as that is often set by parameters (voltage, bias) around the tube and not just the tube itself.If you are dead set on rolling, the input stage usually has the most effect on the gain and overall tone. Tubes connected to the RCA inputs.
Edits: 10/04/11Follow Ups:
This classic circuit uses a lot of feedback, so changing to, say, 12at7 or even 12au7 will not change the gain as much as you may expect. The plate resistances of these tubes are also much less than the 12ax7, throwing off the operating point considerably. I recommend staying with 12ax7 types. The Gold Lion reissues have gotten some good comments, as have several others.
My experience with rolling tubes in an ARC pre proved to me that I could do almost nothing to change the "gain from hell" using various 12--7 tubes. Indeed the first position in the three tube chain had the most noticable imoact on tone the other ttube seemed to have akmost no real impact. I happened to like the original OEM old stock tubes which were made for ARC by BEL of India. I also found that using 12AU7s did something which seemed to result in an awful lot of woofer travel and didn't sound any better but scared me.
5751s are usually my favorite 12AX7 type and worked out OK but the real "High Definition " signature ARC sonic was best heard using 12AX7s. I stopped using it in favor of a 6SN7 pre and left more than decent sounding Shuaguang 12AX7C9s and regular new production Tung Sol 12AX7s in it.
Recently found a 12AX7 that one would not think would be all that special, BUT, in fact, it's one of the best performers I've herad.Found a stash of Japan 12AX7 (14 mm) plates that are marked Raytheon Military with a 64 date code. These are amanzing, very low microphonics, very low noise, balanced sections, and one of the most neutural tonal balances I've ever herad. They look like a Mullard clone, but they could also be a Raytheon clone, as I've one pair from 63 marked Raytheon USA that perform the same.
Also found a stash of Japan 12AU7's that are either marked Daystrom or Baldwin. They are 17 mm plates, and look to be a Mullard clone. Same type of perfromance, low noise, microphonics, matched sections, etc. These are also great perfromers.
The Japanese tubes from the 60's seem to be as good as the US/Euro tubes they copy. Thanks to what appears to be high quality control, there is a high probability they will perform well.
Lastly, noticed the Japan tubes from the 60's flashing looks like the US/Euro tubes, in that they silver vs. carmel colored.
"What this country needs is a good 5 watt amplifier!" (Paul Klipsch)
Edits: 10/08/11
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