12AX7 Tube Swaps, Hot Mics, and Other Myths

I am constantly getting questions like this: “I have a small tube guitar amp and I really want to get that Chicago harp tone and breakup. What tube should I use in place of the 12AX7 in the preamp section.” I see this question on harp forums and on Harp-L. It is probably the most common gear-related question asked by new harp players.

The answers they often get are incomplete at best and ludicrous at worst. I’ll try to lay out a clear and simple explanation of the issues here.

First, the short answer to the question is “None.” A preamp tube swap won’t get you “Chicago tone and breakup,” at least not as I define that sound. It might get you going in the right direction, but it’s only a start. And if you follow much of the advice that springs from anonymous commenters on the Internet your tone could get ugly.

The common theory is that a preamp tube with a lower gain potential will give a player more headroom, allowing him to drive the amp into distortion before the amp is loud enough to start feeding back and howling like a banshee.

There are several problems with this: In small amps (which are usually the subject of these questions) using a lower-gain preamp tube will rob so much volume that it will become unusable except as a bedroom practice amp. With only 5 watts to begin with, any reduction in gain is a big sacrifice.

The tubes with less gain than the 12AX7 – typically the 5751, 12AT7, 12AY7, and 12AU7 – are not all compatible with the circuits designed for the 12AX7. Tubes with lower gain also usually have lower impedance. This affects the components downstream of the preamp section, which are the coupling capacitors in the tone stack. The lower impedance will shift the frequency of the tone stack upward, making the amp more shrill and less warm.

The 12AT7 tube is a terrible tone generator. It was meant to be a driver (such as a phase inverter) not an audio tube. It will work well enough to get you through a gig, but it is not the tone you seek.

The 12AY7 has 44 percent of the gain of a 12AX7, so in a small amp it takes a big cut. It also suffers from an impedance mismatch. The 12AU7 has a fraction of the gain of a 12AX7 and is not compatible with these circuits. I don’t advise using it.

That leaves the 5751, a more rugged version of the 12AX7 with about 70 percent of the gain. Its impedance is a close match. This is the tube I recommend.

A common notion among harp players is this: They want a HOT microphone that will drive the preamp tube into crazy distortion. But this has its own issues that are related to our discussion of preamp tubes.

Preamp tubes are biased just like power tubes, but they sit in the circuit before the volume control. In other words, preamp tubes are always working full blast. The only way you can turn them down is with the volume control on your microphone. If you hit the preamp tubes with a big signal from a hot mic you can overwhelm the tube into square waves, which sound unmusical and ugly. Here’s the rule: If the signal into the preamp tube exceeds its bias voltage, the tube stops working. Preamp tubes are biased at 1 volt or a little more. My red bullet mic with a hot Shure CM element puts out about 1.5 volts on a big tightly cupped riff.

Here is the secret you need to know: turning down the volume control on your hot microphone has the EXACT same affect on your tone as swapping in a lower-gain preamp tube, except you don’t have the impedance mismatch issues with the tone stack.

So, what does all this mumbo jumbo mean?

-To get slightly more headroom, swap in a 5751 tube in place of your 12AX7 preamp tube. This is the tube positioned closest to where you plug in your microphone. Leave the other tubes alone for now. You can buy a very nice-sounding NOS JAN Philips 5751 from Tube Depot in Memphis (or at their website) for about $20.

-Don’t crank your hot Shure CM/CR mic to the max and then complain about feedback! Turn it down! If your mic does not have a volume control you can buy a wonderful in-line device from Greg Heumann at www.blowsmeaway.com. He has sold hundreds of these to all players from beginners to full-on pros.

-You still will not have the tone you seek. Your next step will be a speaker swap. For 8- or 10-inch speakers I recommend trying Weber Signature series. For 12-inch speakers I recommend Eminence Patriot series.

-Why go to all this trouble? Save you money and buy a HarpGear HG2 from Brian Purdy. No tweaks required.

OR, for a true custom harp amp voiced to your taste, contact a tone guru like Bruce Collins at Mission Amps in Denver.

The bottom line is this: There ain’t a tube swap that will get you the tone you seek. Amps are complex systems, and guitar amps don’t always make good candidates for harp projects. It is a common myth that you can substitute a couple of tubes and get good tone, but it is a myth nonetheless.

[Lets see how long it is before someone chimes in that to get good tone it takes PRACTICE. I left that part out because it is painfully obvious.]

UPDATE (02/12/10): Another thought on hot microphones. There seems to be a cultural myth among blues players that the hotter your bullet mic is, the better your tone will be. It ain't necessarily so. I often noticed that my red bullet mic -- which is extraordinarily hot -- would sound crappy with some amps. I always blamed the amp. But I discovered that the mic was beating the input tube into really ugly harmonic distortion. You can see it on a scope.

I also discovered that if I turned my hot mic down a bit I could crank up the amp more, getting a louder better tone before feedback. That is the same goal we week when swapping input tubes, but this only works if your microphone is a bullet with high output, greater than 1 volt on loud passages. Some CM/CR mic fit that description.

UPDATE (12/13/10): In the comments Big Ernie Fuller asked if using the lower-gain preamp tubes will actually do any damage to the amp. Bruce Collins from Mission Amps touched on it in his comment but I wanted to write a little more about that here on the front page.

The short answer is “No.” It is conceivable, on some cheap amps with printed circuit boards and weak components, using a tube with lower impedance could burn out the plate load resistor. But on any point-to-point wired amp it is unlikely.

But remember, a lower impedance tube draws more power from the B+ rail, which is the pool of power to supply all the components. Your lower gain tube in place of a 12AX7 is like a bucket with a hole in the bottom. It leaves less power for the other components, so they are now running out of spec.

You CAN sub these other tubes for a 12AX7 preamp tube, but it is unlikely to get you the “Chicago tone and breakup” you are looking for. I suggest taking it easy on the preamp tube swaps and concentrating instead on other parts of the amp system – such as the speaker or your mic – to get where you want to go.