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OrthodoxBluesJun 24, 2009 7:15 AM GMT
I know that many of you swap tubes in guitar amps to optimize them for harmonica. Since I also play guitar, I've never changed any of my amps from guitar-friendly settings. I think I've been assuming that the tube swaps were just to decrease feedback, but are there other advantages in sound quality?
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Less gain in the preamp section means that the volume knob needs to be turned up higher to get the same volume. Probably not so much less feedback, but more sweep in the volume knob before you go from the point with no good tube distortion to feedback. It mostly makes it easier to find the sweet spot in the amp. The lower gain preamp tubes also seem to be a little less edgy, something most harp players don't want but most guitar players do. Still, some guitar players use lower gain pre-amp tubes, this is particularly true where your amp goes to way to loud at like 1 or 1.5 on the volume knob. Probably bigger changes in tone in harp amp are acheived by altering the bias on the main and power amp tubes.
Feedback comes not only from absolute volume, but also from GAIN. 2 12AX7s yield 10,000X gain. 2 12AU's lowers that to 400X. Lowering the gain reduces feedback at the SAME volume, AND allows you to drive everything further into the clipping zone, which in tube amps produces desirable second order harmonics - also known as "tube distortion."
Some people claim that using a preamp tube with an unbalancec triode (12ADW7) in the phase inverter slot (preamp tube closest to the power tubes) fattens the tone on certain amps when they are used for harmonica.
The Bassman being talked about is the reissue, and I have the real thing, and on the reissues, the first thing to do is get rid of the 12AX7 in the 1st position and replace it with what was used on the original, which was a 12AY7/6072. The original reissues used a solid state rectifier and the real ones used a 5AR4/GZ34 rectifier, and the power tubes in the originals were Tung Sol 5881's, and in the 50's those were a better version of a 6L6, and the main difference between a real 5881 and a 6L6 was that 6L6's were more trebly and the 5881's had more middle. If you change the power tubes and/or rectifier tubes, make sure the amp gets properly rebiased first or you can damage the amp.
12AX7 tubes today are really 12AX7A/6025, which in order to quiet them down, inreased the treble. A real NOS GE12AX7, is a tad noiser, but has better mids, is not cheap, but they sound better and last longer, and that's what I have in the 2nd position on my real Bassman.
I've replaced the JJ 12AX7 preamp tube in Epi Valve Junior with some no name 12AT7. I can turn up the volume about 10% more and I can deal with feedback easier. Easy mod, hearable results.
I appreciate everyone's comments. I'm learning a lot. I was assuming that what was good for the harmonica might be bad for the guitar, but since I play at low volume, it might be a win-win to do some experimenting with tubes.
Does an amp need to be rebiased if you go for lower gain preamp tubes?
Interesting comments here. Let me preface by saying that I am talking specifically about bassman (5f6a) type layout, but comments apply to many other amps because they take their layout from that.
By all means, mess around with different preamp tubes, cause they could help you with guitar sound as well as harp sound. The lower gain tubes, 12au for example, will probably sound crappy with guitar and you simply won't get enough volume out of them.
All tubes need to be biased. Essentially that is the amount of power you feed them to do their job. That being said, you can change the preamp tubes without rebiasing them using any 12a?7 type tube. If you are working with an amp tech to get your amp to the right place, rebiasing preamp tubes is one thing you can do to darken up the tone (or get it edgy and burn out your tubes quicker).
If you change power tubes then you should re-bias because they have a wider range of types and tolerances, and some are less tolerant of higher bias voltages (will burn out quickly if biased too high). The old tungsol (NOS) orignally for the bassman are very spendy, but the re-issue tugsol equivalents sound pretty darn good. Both have lower bias voltages they can tolerate than some more modern tubes.
Now onto gain versus volume controls and volume that comes from the amp out the speakers. The only thing a volume number means on the volume knob is how much the signal is being cut out from wide open (12 on the bassman). Just because you can turn the knob up higher with lower gain tubes does not mean you are getting more volume.
The volume pot in the bassman cuts signal after the first preamp tube, so your first tube always is operating at full signal, but since it is first, it does not get much signal in the first place, it bosts it but is not likely to clip or distort unless you really hit it with a loud sound (big signal). The tone controls come after the second tube and thye cut out signal in specific tone ranges between the second and the third preamp tubes. These are more useful for getting greater volume with less feedback because certain frequencies are more prone to feedback, so you can somewhat cut those and this allows you to let more signal into the preamp section with the volume control before you get feedback. In general the mid-ranges are turned down in the bassman style amp playing harp, and the treble knob is used to get cut, but leads to a harsher sound. This would be a poor choice of tone settings for guitar.
In general, lower gain tubes do not increase the total volume you can get before feedback, they do decrease the total possible volume you can get out of the pre-amp section, and this allows you to fine tune the section before you hit that feedback point. Just because you can turn the volume knob higher does not mean the amp is louder.
Different tubes of the same gain have different tones, and if you can afford to mess around with them, you can improve an amps tone. I was forutunate to get a whole box of 12A? tubes, some of them very nice ones (rca blackplate nos), but the nice ones did not always sound better than the current cheapies. This ends up being infinite and lots of trial and error.
I recently posted some pictures when I bias'd my Sonny Jr. Cruncher. You can view them by clicking on Deacon Blues.
Good point Joe... I was being a bit non-technicial.. though the amp techs to refer to "starving" a tube if you give it too little voltage by putting too much resistance in the bias circuit. Still, the lower voltage to the preamp and power tubes can make an amp more harp friendly, but not so much for guitars. A number of commercial amps for guitars put too much bias voltage onto tubes (drive them at their upper limit), which runs tube life short, but sounds louder and edgier in the store. There seem to be tradeoffs and difficult to get the best of both worlds in amps, yet many prefer a stock Bassman circuit (bbq bob) for harp or guitar with different effects pedals up front, and the harp probably would not use the "bright" channel.