Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oddvar on May 14, 2016, 02:16:26 am
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Hi. My Weber 5f6a build is breaking up a bit to soon. Any advic e on how to clean the sound up a bit? I use Scumbacks 55hz 16 ohm speakers.
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Cheapest, easiest, least intrusive: replace the first or second (or both) preamp tube(s) with a 12A_7 having lower gain factor.
The usual approach is to replace a 12AX7 with a 5751. Or a 12AY7. Or a 12AT7. Or a 12AU7. With two tubes and 5 choices, that's 2E5 or 32 possible variations. Probably....a 12AU7 will not cut the mustard, but then again I have no idea what you might like.
In pictures and a nice little video piece: http://www.thetubestore.com/Resources/Guitar-Amp-Info/Gain-Factor (http://www.thetubestore.com/Resources/Guitar-Amp-Info/Gain-Factor)
All of these tubes have the same basing diagram, use the same amount of heater current and thus are "plug-compatible". Nothing will blow up. You either like the result or you don't.
In the video you will note that going from a 12AX7/7025 (mu = 100) to a 5751 (mu = 70%) has almost no perceivable effect. You would think it would, but there are several factors at work that are difficult to separate on a pure, absolute level.
One is, he is using his volume control at 3-1/2. If it were me, I think I would have done that video with the volume control on "10". (or 11)
Two, no circuit in an actual real-world amp uses 100% of a tube's specified gain...otherwise the circuit would stop working when the tube aged to 90% of its spec'ed value, which is a perfectly normal condition after a fair amount of use and at 90% or even 80% or even 70%, a tube should work pretty well in just about anything but the most sensitive, picky piece of test equipment. In a guitar amp? Should not care. But the point is that a Fender preamp does not "stop working" if its OEM-spec'ed 12AX7 goes all the way down to 70% of its spec'ed gain. You almost see nothing on the scope with the 70% 5751 until he swaps in a 12AT7 or 12AY7 which are 60% and 45% of 12AX7 gain, respectively.
So the gain of a particular stage is (certainly a large part) tube gain but also the gain imparted by certain component (resistor & capacitor) values surrounding the tube; and their ratios. No need to get that technical, really. Unless you wish to change them, and some people do. A tube gain stage has not only gain, but it has frequency response as well. Many folks on this forum get a lot of mileage from changing the bypass cap sitting under the cathode of some gain stages. A very simple mod, but not as easy as changing out a tube.
That does not mean you will not hear a difference, though! You may, you may not.