Wednesday, February 10, 2010

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 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.


Paul "Kingley" Routledge said...

I do agree with a lot of what you have said Rick. I do have a different view point on some of the things you mention though.

would always use a 12AY7 in the pre-amp of most harp amps (not all though). This is simply because I have found over the years that a good 12AY7 (NOS RCA for example) gives a deeper thicker tone and adds a lot of warmth to the overall tone, especially in Fender amps.

I totally disagree that turning down the volume on the mic with a CR/CM element gives the same result as a tube swap. That is just so far removed from my own personal experience.

Also If someone is using a crystal element for example then the mic is not working at it's optimum output to generate the capabilities of it's full tonal spectrum, if the volume is turned down on the mic.

A 12AY7 can live perfectly happily in a 12AX7 preamp slot with no problems to the circuitry. Which I think is worth mentioning. .

It's interesting that you recommend that people buy a HarpGear HG2 (a fantastic amp by the way). That amp is based on a 5F2H circuit and uses a 12AY7 in the preamp, as does the HarpGear Rock Bottom.

I would always advise people to try a 12AY7 if they have feedback problems, along with other things such as tone settings and mic cupping techniques.

Rick Davis said...

Paul, thanks for visiting my blog and posting a thoughtful comment.

The 12AY7 tube in a custom harp-specific amp does not have the liabilities it may have in a guitar amp, such as a Fender. The HarpGear Rock Bottom's circuit has been designed to accomodate the 12AY7 tube. The problems may occur when this tube is used in a circuit designed to support a 12AX7.

In a small amp (the subject of this article), a 56% reduction in gain is significant and unneeded. If the goal of the tube swap is to reduce feedback, turning down the volume on your hot bullet mic will indeed give you a better result.

Swapping in a 12AY7 will not give the player "Chicago tone and breakup." You know that. Using expensive NOS tubes won't do it either. Getting an amp voiced correctly for harp -- especially a guitar amp -- is a bigger job that requires patience and a good ear. There are no instant plug-n-play fixes.

Again, thanks for visiting my blog.

Joe's Blues Blog said...

To get good tone, you've got to practice.

Ev630 said...

It's important to observe, that the first preamp slot in Bassmans, Bandmasters, Pros, Supers and Deluxes SHOULD be a 12AY7. That's how they were manufactured, shipped and used in the 50s - and using a Bassman Reissue with a 12AX7 in the first slot is a recipe for harp feedback.

As regards those old tweeds - I wouldn't change a damn thing, tubewise. Just make sure you have killer NOS tubes. After that it's down to the mic you use, your technique with the mic and whether or not you know how to dial in an amp.

Rick Davis said...

Drew, the 12AY7 tube is GREAT in circuits for which is is designed. This article is about swapping tubes into guitar amp circuits specifically designed for the 12AX7 tube. There is a common misconception that all the 12A tubes are interchangeable. They're not, and there are tonal consequences -- however slight they may be -- for using them in place of a 12AX7 without also making other changes to the amp.

BTW, the 12AU7 -- which I don't recommend using in amps at all -- was a great tube in some old vintage HiFi amps. They used two of these low-gain tubes in series for a lush tone.

Rick Davis said...

Joe, yer killin' me here....

Joe's Blues Blog said...

Somebody has to point out the obvious. Is the HG2 a 5F2H or a tweed Princeton?

Paul "Kingley" Routledge said...

Well the HG2 is basically a Princeton with a few tweaks. My mistake I should have typed 5F2A not H.

Big Ernie Fuller said...

Interesting post. I find many harp players will recommend tube swapping, not so much for tone change, but just to cool down the feedback monster. My CM bullets are hot, and while it's not a problem with my Champ, stepping up to my Deluxe or reissue Bassman I need lower gain preamp tubes, as even using a volume control at the mic will make these amps scream at relatively low amp volume settings.

I agree with you that tone is not improved with a mere preamp tube swap. But does using lower gain tubes, such as the 12AY7 or even the 12AU7 in an amp designed for the 12AX7 cause any problems to the amp circuits?

Dave Eisner said...

I really like this post, Rick. In addition to what you said, the circuitry must be a true biased circuit.

When Fender sold the company to CBS (i.e. the start of the Silverface era), CBS changed a number of values of the smaller capacitors in that were originally Blackface amps to reduce production cost.

What a lot of people don't know is that CBS Fender advertised these amps as having a true biased circuit, when actually they were not. The slight changes in capacitor value does not allow for a true biased circuit. For example, my '71 Twin Reverb has a sticker on the inside panel with tube values and it actually says "AB763 Circuit," when it actually should read "AA763" or "AA270."

Now, it doesn't really matter because I swapped out the capacitors and changed them to their original values as they were in the '65 TR, so I have that circuit. I can take a screwdriver to the pot while playing and turning to changed the power going into the tube. It's pretty cool, but like you said, when you bias them too high the sound waves square off and it sounds pretty awful.

Bruce Mission Amps said...

Something to keep in mind is that just because the tube base and the pin out is the same for any given group of tubes, (specifically, base #9A, the very tubes this thread seems to be about) doesn't mean you can simply swap them all around with absolutely no consequence.
Yes, of course some consequences very subtle and others are more severe!!
Almost all triode tubes discussed here (dual or otherwise) are set up to operate in single ended Class A mode. That means they are drawing MAXIMUM plate/cathode current at idle.
A good example of a glitch I've seen is when a customer is experimenting with preamp tubes and installs a tube such as a 12AU7, which has a very low impedance and draws much more quiescent idle current then the 12AX7 it replaced, ... and all in an cheaply made amp circuit, designed to just barely support the original 12AX7 in the first place.
Most of the time these amps are built around a very very cheap price point and use inexpensive parts with low power rated supporting resistors on a clock radio style printed circuit board.
The original 12AX7 could be running it's supporting plate or cathode resistors right at it's 1/8w to 1/4w rating and when trying to support a tube such as a 12AU7 and it's subsequent higher idle current, those low wattage parts can and do fail. Sometimes they get so hot the come unsoldered from the PCB!
Another side effect, which also effects the amp tone as much as the "tone" of the tube, is that the extra current these other tubes demand, create additional voltage drops across the resistors in the B+ rail nodes. The nodes are used to deliver the proper B+ to the original tube as the designer had planned.
Now those "delivered" plate load voltage nodes are all different too.

Bruce Mission Amps said...

In regards to Dave Eisner's comments;
What does a "true biased circuit" mean?
Also, what do you mean by:
"I swapped out the capacitors and changed them to their original values as they were in the '65 TR, so I have that circuit. I can take a screwdriver to the pot while playing and turning to changed the power going into the tube."
What circuit and do you mean? What capacitors?
And, do you mean change the bias voltage pot setting to alter the idle current of the power tubes... or what? Capacitors won't change the bias circuit so I am just a bit confused by what you are descibing.

Dave Eisner said...


The way it was explained to me is specific to the Twin Reverb, but to my understanding is true for any early Silverface Fender amp that had a Blackface predecessor (and maybe this is wrong but I've read it in other places too when researching the early Silverface Twin Reverb).

When the Silverface Era began, CBS changed the values of various capacitors and resistors in the amp to create a cheaper production cost. In lessening the values, some wires had to be re-routed (I guess to redistribute power?), which in turn created an unbiased circuit (AA-270) that was advertised as a biased circuit (AB-763).

Now, the changes that were made were very minimal but ultimately changed the tone and overall sound quality of the amp. Historically, the Silverface TR is shot down and said to be crap, but what people don't know is that you can very easily "Blackface" the circuitry, to be identical to the original AB-763, and very sought-after, Blackface TR, which is what I did to my amp (1971 TR in near mint condition).

As you'll notice in the AB-763 schematic layout (which is the biased circuit), there is a pot with what looks like a flat-head screw in the middle that is connected to the transformer. This would be the biasing pot. You can change the voltage going into the tubes by turning this clockwise and counter-clockwise to your liking. In the AA-270 schematic, this pot is there, but had virtually no effect or use within the amp. You couldn't change the voltage going into the tubes.

So, to create a "true biased circuit" (as opposed to the advertised "biased circuit" that was actually unbiased) in the early 70's TR's, the capacitors and resistors need to be changed back to their original values, and one or two wires need to be re-routed. This allows you to use the pot as it was originally made for, and to have better and a more custom tone in the Twin Reverb.

Maybe I'm wrong, I'm not an expert. But this is how it was explained to me, and I've also read this in articles about the early 70's Twins. Did this help?


Bruce Mission Amps said...

Ah OK... other then simple black face component values used vs silver face component values, I think what you are describing is mostly about changing the power tube's biasing circuit from a "bias balancing" circuit to a "bias level" setting circuit.

The notion that "to create a "true biased circuit" (as opposed to the advertised "biased circuit" that was actually unbiased) in the early 70's TR's,"... is actually incorrect as all tubes including power tubes have to be biased in order for them to work.
The biasing circuit in the stock AA-270 amp sends a differential bias voltage from that pot to each power tube in order to "balance" the idle current of the tube as a set.
However, there is no easy way to adjust the bias current level without changing the fixed resistor in the circuit.
If that fixed resistor was also a pot, you'd have the best of both worlds, Power tube current balance and power tube idle current level!!

Anonymous said...

Guys, I need tube help. I have a 66 aa165 bassman head in a fender clone type cab w a weber 15a150(jdnewell cab 1x15). My ech removed the ss rectifier and installed a tube 5u4gb rectifier for sag. I play blues guitar and harps in bands. I had this done to have a true 2 channel amp w sag w independrnt settings,tone wise. He also insalled a bias sw. for cathode bis to fixed bias. Heres the issues: first I dont understad the simple diff of the bias switch's purpose, other tha moore or less power?. Two: My band is not loud, I can only use the volumes at about 3! Unless I turn up the vol on the guitar and bullet mics. I decided to pull the 12ay7s and ax7's out of v1,v2,v3. I can now turn up the amp volume more, but according to all these posts, I need to knoow am I hurting the amp itself? Please, I dont understad much techtalk, other than the tube swap ideas all over the internet, and the diff. between the 5_ _ series tube rectifiers. (I also use a Clark Beaufort tweed deluxe, and a magnatone w a 10" weber 10a125 spkr.) Please advise and thakks drew davis,ny dmaj135@yahoo

Buzz said...

Thanks for your posts here. I just replaced the 12AX7 in my Blues Deluxe Twee reissue with a 5751 I picked up online. I still have a nice tone with more volume, playing with a Shaker crystal mic. I also use a Shaker Retro Rocket that I like for a different style and a Shure 520DX.

I still cannot turn this amp up beyond 4 without feedback but I think I'll be fine with that. Planning to try it out at a jam tonight so we'll see.

Anyway, just wanted to say thanks to you and the other contributors here and leave some feedback (of the good kind) here.


tominrexmont said...

here"s my problem i have a amp made in china and want to use more commonly available tubes the preamp are 6n2 tube.which i want to go to 12ax7,i know a rewire of it will have to be done. then the power tubes are 6p1 . and i want to go to el84 ,does anyone have a simple plan that this can be done with ?thanks in advance .

pigeon pete said...

[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.]

you got that right It is all about balance, I use a crystal mic when playing blues, and adjust the amp and mic to get the best sound, then mic the amp, but for Rock and roll I use a Fireball dynamic mic with a wireless sender (so we can Rock n Roll). With the fireball I need more volume on the mic and more volume on the amp to get a comparable but smoother sound. The mic, amp and speaker all combine to create the sound, [lets see how long etc, } and then capure that sound as faithfully as possible with micing up. I used to turn the amp up till I could hear it on stage, which destroyed the tone, and I used to use a line-out which bypassed the speaker. No point in installing an expensive speaker upgrade and then bypassing it. Pete.