EL84 Harp Amps

It has happened twice recently: I run into someone I’ve never met who says, “You’re the guy who hates EL84 amps!”

Well… “Hate” is kind of a strong word. Let’s just say I prefer the tone of other amps.

The EL84 is a power tube used in some amps popular for harp. I am unaware of any custom maker of harp specific amps who uses EL84 tubes in any of his products. However, there are lots of EL84 guitar amps that are adapted for use with blues harp. These include the Fender Blues Junior, Fender Pro Junior, Peavey Custom 20 (and Custom 30), Epiphone Valve Jr, and the vintage Kalamazoo amps, among others.

Here’s the problem: Some harp players like the EL84 tubes because they are easily driven to distortion with the high output of bullet mic meant for blues harp. The output of a bullet mic is MUCH higher than a typical electric guitar pickup. The distortion that results is annoying. I describe it as a crackly mush in the upper mids. I hear it to some degree in all EL84 amps.

Here’s what causes the problem: The EL84 tube is subject to a condition called Grid Blocking when it is driven with a hot signal. The tube has a low level Negative Bias Voltage that can easily be overwhelmed by an input signal larger that it expects. This causes the tube to stop working as intended and become a diode; an On-Off device. The resulting signal is a square wave; an ugly, non-musical tone that guitar players call “farting out.” Amps with a single EL84 power tube do this the worst. The effect is lessened when there are multiple tubes.

Some harp players are happy with this distortion. When new players acquire small amps they immediately start looking for “That Chicago Tone and Breakup” and any distortion sounds better than icepick clean tone.

It’s not that I hate EL84 amps; it is that I MUCH prefer the tone of amps with 6V6 tubes. These amps include the Fender Champ (and all the 5F1 clones), Princeton and Deluxe, vintage Gibson amps, and old PA amps such as the Newcomb E-10b, among others. A good 1970s Fender Champ can be found on eBay for around $300, and modifications to its old-school point-to-point circuits are simple.

The 6V6 tube has a more robust Negative Bias Voltage that is not so easily driven to square wave distortion. Its overdriven tone is much warmer than the EL84. The crackly mush in the upper mids is gone, replaced by a darker hair-on-the-notes quality. There is a soft tearing of the notes on attack and decay, not the harsh fuzz-tone of an EL84 fighting for its life.

I’ve heard some EL84 amps that sound pretty good, and I’ve written about them. But for the most part, they make me wish the player were using a different amp. They are the ultimate one-trick-pony, and the trick gets old in a hurry.

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.

"Oh yeah? Well, you play like CRAP!"

LOL. It never fails. Every time I get in to an online debate with someone about anything that has to do with harmonica (and never has anything to do with anybody’s playing ability), the other guy in the debate will get frustrated and attack my playing, spewing what he thinks is the ultimate put-down. It reminds me of that hilarious scene in the movie “The Sandlot” where a bunch of adolescent little leaguers throw down their best insult: "You play like a GIRL!"

Yesterday I had an email exchange that went like this:

Hater- "You suck. You are an intermediate player!"

Me- "Uh, yep… I’ve always claimed to be an intermediate player."

Hater- "Well…. I was wrong. You are a BEGINNER!"

Sheesh!

As I’ve written before, I am a PROUDLY intermediate harp player. I have NEVER been one of those pretentious twits who claim to be better than anybody else. Insulting somebody’s playing to make cheap points in an argument is reprehensible. It is a failure of imagination, no different than those adolescent boys in the movie.

In the last 12 months I’ve played 70 paying gigs, including one last night. That is not a big number, but it’s about what I want. (You can check the schedule in my band website to see where I've played.) This blog gets more than 500 hits per day, almost all of them from people searching Google for information about harp amps and gear. Am I the best player around or the biggest authority on all things harmonica? Heck no! But I am happy with my place in the blues harp community.

For you haters with a festering urge to insult my playing, I say, "grow up!" You’re acting like a 12-year old little leaguer.

My Front & Center Harp Microphone

After reviewing the Front and Center Mics a few weeks ago I finally ordered one for myself. It arrived today.

Holy Shit! It sounds better than I remember.

First, the feedback rejection: When I compare it directly with my (formerly) favorite gig mic -- a Greg Heumann-modified Peavey Cherry Bomb bullet with Shure CM element -- the F&C mic has the same perceived volume at the same amp settings. BUT... when you crank it up the F&C mic does not start feeding back until you get to stupid levels. Seriously, the Peavey mic starts ringing at 6, while the Front & Center mic didn't ring until 10.

(I used my Mission 5F2H custom harp amp for this test. The volume goes to 12. I didn't use any effects on the amp or on the recording. I did the test in my office at home.)

Next, the tone: My Peavey bullet is a very good sounding harp mic. The problem it has --and which it shares with all Shure CM/CR microphones -- is that it is a feedback magnet and it can be muddy. In A/B tests at the same perceived volume the F&C mic sounds crisper without being brighter. It's more defined. The notes seem to jump out of the amp and spin around the room.

Here is a clip of me noodling around with the mic this afternoon right after it arrived. Here is a clip of Ronnie Shellist playing his Front And Center Mic and Bassman amp when we did the review. As you can see from the photos, the mic is beautifully finished. The wood is Cocobolo, and I selected a semi-gloss finish.

In this photo you can see the on/off button I asked Scott to install on the mic. I can tell by the feel of the button if it is on or off. I use volume controls mostly as on/off switches anyway, so I didn't want one.

These microphones sell for only $165.00, which seems ridiculously low. The performance is truly astonishing. If you are having feedback issues, don't spend a fortune on an expensive, complicated device that robs your tone. Try one of these first.

The Blues Harp Amps Blog gives its highest recommendation to Front & Center Microphones. This is the best blues harp microphone I have ever played.

FULL DISCLOSURE: I received a discount on my purchase of the Front & Center microphone.

Cool Amps

At Ronnie Shellist's blues harp workshop last Sunday I got to play a couple of very cool amps. Ronnie's Harmony 420 is a 2x6L6 amp with a big ole 15-inch Jensen speaker. Kind of like a 1959 Fender narrow panel Pro, but two thousand dollars less in current price! This amp sounded Excellent, with a deep smooth tone. Not a lot of breakup, but FAT, ya know what I'm sayin'? Ronnie mentioned that this amp *MIGHT* be for sale. I already made a bid on it.

This is a Blues Box amp. I've been seeing them on eBay for years and always wanted to try one. As soon as Ronnie heard it, he said, "Chicken grease!" The amp has a cool sizzling quality to the tone, lots of crunch and grit. LOTS of character. The guts of the amp are a rebuilt Bogen tube PA amp from the '60s. A very fun amp, but it might be a one-trick pony... which is not necessarily a bad thing.

This is my two of my amps makin' friends with the Harmony 420.

Masco Update

This is renowned Denver harp player AC Blue blowin' through my Masco ME-18P amp and Epi cab. The place was packed and the joint was jumpin' at the Sunday Blues Jam my band hosts at Ziggies.

All the players raved about the tone of the Masco, including Ronnie Shellist, Nick Clark, and AC. That amp has finaly reached its potential, I think. It sounded fantastic. The amp has two old Coke bottle 6L6 tubes and puts out exactly 20 watts just as it starts to clip.

The Masco was on for several hours. The only glitch was when it stopped working for a minute, but the best I can tell is that the fault was the mic cable or microphone used by the guy who was playing it at the time. When we changed up players the amp came back to life and worked flawlessly for the rest of the night. Not bad for an amp born in 1953.

Ronnie Shellist

Nick Clark (playing a Front and Center microphone)