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Dirty-South Blues Harp forum: wail on! >
Tube swapping
Tube swapping
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rbeetsme
649 posts
Feb 18, 2012
7:50 PM
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I'm not a tech, barely understand how amps work. But, like everyone here, I've swapped a few tubes over the years. As I understand it, trading out power tubes for lower gain tubes reduces the wattage of high power guitar amps to give the amp some warm tube distortion at lower levels. Swapping lower gain pre-amp tubes takes some of the bite out an amp. On older amps I have been told that changing tubes might require a trip to the tech to have the amp bias set. Many of the newer amps are self biasing and can be done by the user. As a layman I can only say that sometimes changing a tube can make an amp sing. Recently I acquired a Kendrick Champ style amp that sounded terrific, but a bit harsh in tone. This is my second Kendrick Champ, I remember swapping tubes in the first one and being disappointed by the result, too low volume. So I swapped back. That first Kendrick has been one of my favorites and always well received. Last week I changed the 12AX7 on the new Champ to a 12AT7. It just sounded a little dull, again, lower volume. I remembered swapping back on the first one so I returned the 12AX7 compared the 2 Champs. The first one just sounded a lot better, the new one is pretty nasty, a little too harsh. (the new one has a 10" speaker, the old one an 8', slightly different design too. I found a Sovtec 12AY7 and tried it in the new one, bingo, still good volume, still terrific Champ tone but not nearly as harsh sounding. I checked the first Champ. Well what do you know, a 12AY7 in that one too! I guess the purpose of this post is to let people know that alternatives exist and it doesn't hurt to experiment. Also, it would be nice if someone who actually knew something about tube amps could chime in, enlighten the rest of us about the do's and don'ts of amps.
Last Edited by on Feb 18, 2012 7:52 PM
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Greg Heumann
1484 posts
Feb 18, 2012
11:01 PM
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Swapping out tubes doesn't reduce an amp's power unless you can no longer drive the power tubes to saturation by turning up the volume control. And this won't happen. What it does do is reduce gain - that is, the SLOPE of the line that maps input level to output volume. Most amps are set up with WAY more gain than necessary to drive an amp to and way past the input level required to get the maximum power from the power tubes. In the real world it doesn't matter anyway, because feedback is the limiting factor. Amps are set up so guitar players can get Hendrix-style feedback - yet guitar strings and pickups don't couple to sound waves nearly as efficiently as microphones do. Moreover a typical harp mic has twice the output of a guitar pickup.
Reducing the input gain still, while still allowing you to drive the power amp to "saturation" - makes the feedback threshold less razor-edged and much more controllable and yes, does smooth out the amp's response.
Single pre-amp tube amps (small amps like Champ, a VHT or a Kalamazoo) tend to need no reduction (Kalamazoo) or a little (VHT). Amps with 3 tubes in the preamp circuit, like MANY Fender designs, tend to just LEAP to feedback when they have all 3 12AX7's. Replace V2 and V3 with lower gain tubes - I use 12AU7's - and you can STILL saturate the power tubes, you can still turn it up to the feedback point - which means you're getting all the useful power there is - but feedback comes on very smoothly and gradually and becomes very easy to control or avoid altogether, while getting much closer to it in real volume. So you can actually get a little louder without feedback because you don't have to leave such a large safety zone beneath it.
Don't confuse lower volume with needing a higher setting on the volume knob to get the same volume as before. You can still get more volume than you can use - but because the volume knob will be higher you get to run the power tubes harder and enjoy some of their saturation warmth.
You cannot hurt an amp by swapping preamp tubes with others in the same family - specifically the 12AX series. And a cheap chinese 12AX7 (like what comes in the VHT) sounds very harsh compared to a good 12AX7. But a slightly lower gain tube in that amp like a 5751 or 12AT7 makes a nice difference. So experiment away!
(Caveat - A 12AU7 may draw a little more current than a 12AX7. A really poorly/cheaply built amp with 1/4W resistors could burn up a resistor with that big a change. But 1/4W resistors didn't even exist when most of these amps were designed and built (they used 1/2W resistors), and I haven't seen them even in new ones. If they're anywhere they'll likely be on printed circuit boards and not in point to point wired amps. That doesn't mean there isn't one out there so caveat emptor.)
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/Greg
Last Edited by on Feb 18, 2012 11:06 PM
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