The Phase Inverter Tube

The least understood tube in your vintage harp amp is the phase inverter. What the heck does it do?

In amps with two or more power tubes the phase inverter (PI) splits the music – after the preamp gain and tone stack EQ – into two out-of-phase signals. Each of your pairs of power tubes gets one of the signals to each tube, so that one tube pushes the sound and the other pulls it. If you have a master volume amp there is another gain stage after the phase inverter, but that is a different topic.

Amps with only one power tube don’t need a phase inverter. One of the main differences you hear between single ended amps and Class AB amps (with multiple power tubes) is the phase inverter. Stick with me here while I explain.

The common theory you hear spouted over and over by harp players is that we want to reduce the preamp gain in our amps so that we get to that sweet spot of power tube distortion before we hit the feedback threshold. Heck, I used the believe that myself, but I've come to learn that it means a lot more in small single ended amps.

In Class AB harp amps the power tubes are almost never driven hard enough to distort much. What you hear is the phase inverter breaking up! The power tubes just amplify that and pass it along. It can be a very sweet tone with a nice organic breakup.

The phase inverter circuit is not a gain stage, so the difference in amplitude you hear from a 12AX7, say, to a 12AU7 is not as great in the PI socket as it would be in the preamp socket. The goal in the choice of PI tube is no so much to quiet the amp down as it is to get a nice warm dynamic distortion when you hit the amp hard.

In general, my advice is to leave the phase inverter tube alone. Many amp makers now use all 12AX7 tubes in the preamp section (first and second gain stages and PI tube) to keep things simple and cheap. Older Fender amps used a 12AT7 as the PI. (Note: Never use a 12AT7 tube in any circuit for which it was not specifically designed. The AT7 is a terrible tone generator and very different from other tubes in the 12A?7 family.)

But, you CAN improve the tone of your amp by plugging in a 5751 in the PI socket. As you may know, this is my favorite input tube for harp amps. It is an industrial grade 12AX7 with slightly less gain and a warmer tone. Going all the way to a low-gain 12AU7 will give you more clean headroom, which is not really what you want in the phase inverter.

Or, you can throw out everything you just read and try it yourself. The phase inverter does not have to be biased or anything like that, so this is pretty much plug ‘n’ play. The PI tube is usually the small tube closest to the bigger power tubes. In a Fender Blues Junior, for example, it is the third small tube. Start counting at the tube closest to where you plug in your mic.

I’d encourage you to try the 12DW7 trick as well: very interesting results.

Remember to turn your amp off when swapping tubes and BE CAREFUL! Getting zapped can ruin your day. But understanding the role of the phase inverter tube in your vintage-style harp amp can help you get where you want to be tone-wise.

NOTE: Coming up soon I will have a video tone comparison of three different input tubes in the VHT Special 6 amp: The stock 12AX7, an NOS JAN Philips 5751, and an NOS 5965 tube.

Feedback and the Rectifiers

[Hmmmm. That would be a great name for a band.]

A reader emailed me to ask if it is true that changing to a copper plug-in solid state rectifier “will get you more power without feedback.”

The short answer is, No.

Let’s review feedback for a moment: We all know it well. When your amp starts to howl the very first thing you do is turn down either your amp or your mic. In other words, you reduce the amplitude of your entire system: harp, mic, and amp.

Also, we all know that if we crank up the treble on our amp it is more likely to start howling feedback, so we usually have the treble set relatively low.

Swapping your tube rectifier for a copper recto of the same value will result in a slightly increased B+ voltage and a reduction in vintage sag tone, which gives the perception of a tighter, more present sound. Does it make your amp truly louder? Maybe very slightly louder, but it would be hard to hear.

Will it reduce feedback? How could it? All the remedies we know of for feedback involve reduced amplitude and/or EQ filters. The rectifier does neither. The claim is bogus.

Update: Here is a good article about rectifiers, solid state vs. tube.

Review: Dan Treanor – My Blues Diary

Dan Treanor’s latest album is a tour de force; a musical roadmap of Dan’s blues journey from 1997 to the present. Yet each cut shares a stylistic and thematic key with the others. This is truly the blues diary of the last 14 years from a man steeped in blues music, and blues harp in particular.

The talent on the album is impressive. Darryl Lee sings most of the cuts with an understated passion and brief moments of sobbing power. It is a unique and effective blues voice. People I know on the album include session man Rich Reno – a great slide guitarist – and my bandmate Charles Billiris. Outstanding bluesman Randall Dubis also makes an appearance on ‘Blue Bossa,’ the 6th track. The entire roster is impressive.

But this is Dan Treanor’s baby and you can tell he loves it. Dan is known mostly for his brilliant harp playing and it is on good display here. His teasing, lilting harp lines on Sonny Boy’s “Nine Below Zero” are beautiful. His tone on the Chicago-style “Love Me With A Feeling” is worthy of Maxwell Street. Dan is at his best in minor blues with a Latin beat, and there are a couple of those gems on the CD. Lonesome Railroad Track, 3 O’Clock in the Morning, Stoop Down Baby; they all kill. My band performed the song “Cut With Dynamite” at the Colorado Blues Society IBC Finals with Dan sitting in. All thirteen tracks are keepers.

The album is well produced. The sound is warm and personal across all the cuts. Vocals are vibrant and detailed. I thought the opening to Mean Woman Blues was a bit heavy, but that is mild criticism.

Mostly this album is about Dan Treanor’s love for blues harmonica. His licks are actually infrequent, but always beautifully soulful. I’ve already heard phrases I plan to steal.

I give this album a IV+ on the I-IV-V scale of blues harp excellence. Highly recommended. Contact Dan Treanor to find out how to get the CD: daniel2102@comcast.net

The Mission Chicago 32-20 Harp Amp

Bruce Collins at Mission Amps tells me the Chicago 32-20 amp is selling extremely well. Players are buying it for the warm crunchy tone and great features you can’t find on any other amp. You can switch the amp from fixed bias to cathode bias which changes the texture of the tone. You can use the DEEP switch to fatten your sound, like the loudness switch on old Hi-Fi gear. Another switch disables the cathode bypass capacitor. All these adjustments allow you to tailor the basic circuit design of your amp to your tonal tastes.

Players are also choosing the Chicago 32-20 amp because of its tremendous value. At only $999 it is hundreds less than other amps in its class. Every Chicago 32-20 amp is hand-built by a master amp maker who is known worldwide for tone. Bruce uses only New Old Stock (NOS) military-grade paper in oil tone capacitors and solders every part himself.

When the amp was being designed, special attention was paid to the rectifier. The copper solid state rectifiers were tried but discarded in favor of real tube rectifiers. The copper solid state rectos deliver slightly more B+ voltage with less sag, which is why they give the impression of “tightening up” the sound, or making an amp sound more “crisp.” But that is at the expense of real tube sag and the warmth it provides. It would have compromised the true vintage tone of the amp. The rectifier tube is one of the tone generating components in a vintage tube harp amp, and solid state just doesn’t cut it.

Mission Harp Amps

Comparison: Kalamazoo amp vs. VHT Special 6

Once again, here is the video of Nic Clark playing the modified VHT Special 6 amp:


Here are three links to Youtube videos of the Kalamazoo amp:

Kalamazoo model 1 blues harp amp

custom kalamazoo harp amp model 1

Kalamazoo amp and BluexLab harp mic

The Kalamazoo amp suffers from a design that gives it that raspy harsh tone: It uses a single EL84 power tube, while the VHT Special 6 amp uses the warmer sounding 6V6 power tube. The 6V6 was the tube used in many classic harp amps from the glory days of tone… Amps such as the Fender Champ and Princeton, vintage Gibson amps, the National/Supro amps, the classic Premier amps like the Twin 8, and cool old PA amps like the Newcomb E-10B.

I cannot think of any classic harp amps that used a single EL84 tube. The EL84 has a characteristic tone that I call a “crackly mush.” The tube has a very low negative bias voltage, so the tube is driven to square wave distortion very easily. I wrote about the details of this problem here: EL84 Harp Amps.

The 6V6 tube has a much warmer tone and smoother distortion, which is why it was the choice of so many of the great blues harp players of the 50s.

The Kalamazoo amp has been popular as a low cost practice amp for harp players. One particular vendor rehabs these old amps and sells them for $269 for a “beat up Model 1,” and up to $379 for a “really clean Model 2.” No matter which one you buy, you still get and old amp made of particle board that often disintegrates when exposed to moisture.

For less than $200 you can buy a brand new VHT Special 6 whose cabinet is made entirely of finger jointed plywood. Its circuit is soldered point-to-point on an eyelet board, and it is guaranteed for 5 years (90 days on speaker and tubes). The amp circuit is essentially a vintage Fender Blackface Champ, with a 10-inch speaker in a larger cabinet.

When bench tested the VHT amp produces an honest 6 watts of power right at the point it begins to clip. It is very loud for a small amp. The Kalamazoo amps produce about 3 watts at the point they begin to clip. Some vendors claim the amps put out 8 or 10 watts, but that is an exaggeration and does not comport with the generally accepted methods of measuring amp power for the purpose of comparison.

Right out of the box the VHT is a much better harp amp than the more expensive Kalamazoo. Its tone is warmer and more dynamic, and much bigger and less boxy. For about the cost of the “beat up” Kalamazoo amp you could purchase a brand new upgraded VHT amp that is among the sweetest I have ever heard.

UPDATE: From Wikipedia - "When overdriven, the EL84 power tubes in these amplifiers produce a distinctive chiming, articulate, treble-heavy sound..." Yep, that's what I hear when harp is played through the Kalamazoo and other EL84 amps.

Nic Clark playing the Modified VHT Special 6 amp

Great tone, eh? And it sounds huge. The amp was on 3 (9 o'clock on the volume control) and everybody was like, "Holy crap, that thing is loud." This was during a break at Ronnie Shellist's blues harp workshop last Sunday.