Tuesday, September 30, 2008

Pro Jr Problem: The Top Middle Screw on the Back Panel

I’d read several places online that the top middle screw on the back panel of the Pro Junior amp caused the amp to be noisy, so removing the screw and leaving it out was the way to go. I thought it was just one of those weird online rumors and ignored it. My Pro Jr. sounded fine.

But after I’d taken the amp apart and put it back together several times to make modifications, I noticed during testing that the tone control did not turn as freely as the volume control. It was bound up somehow. Here’s what I found:

The picture above is looking upward at the back of the amp with the back panel removed. The screw hole you see is the top middle screw for the back panel. Below that you can see a rectangular piece of metal; presumably to shield the volume and tone pots from heat or RF signals. The pots are right above it, with the other amp components below.

The metal shield is attached to the circuit board using silicone sealant, and it will move pretty easily. What I discovered it that it can easily get bent upward toward the bottom of the pots when handling the chassis, particularly when you are coaxing the chassis into or out of the cabinet. (It is a tight fit.)

If the metal shield is deflected upwards its free end will be above the top middle screw on the back panel. As you drive in the screw it presses the shield up against the tone pot, causing it to bind up and possibly short.

Problem solved. Just make sure that metal shield is parallel to the control panel just above it and out of the way of the screw.

(BTW, the violet wire at the lower right of the photo is the Negative Feedback Circuit. Follow this wire to the speaker tap, unsolder it and tie it off to give your Pro Junior a slightly coarser harp tone.)

Tuesday, September 9, 2008

Fender Pro Junior Harp Amp Project – Speakers

The original Fender Special Design speaker (made by Eminence) that comes in the Fender Pro Junior amp is very efficient and bright. Together with the high-gain 12AX7 tube in the preamp socket, the stock Pro Junior can be a feedback monster for harp players. A warmer-sounding alternative was needed for this project, but I wanted to keep the cost low. That meant I had to forget about the great Weber VST speakers and used vintage Jensens.

I’d tried the Jensen reissue P10R and C10Q speakers in other projects and didn’t like them. I’d also tried the Weber Signature 10 ceramic with smooth cone and loved it, so I decided to try its alnico twin in the Pro Junior.

Several players have said they saw a drop in volume compared to the original speaker when switching to the Sig10 alnico, and they are right, sort of. Ted Weber describes this speaker as “compressed at high volumes” and he ain’t kidding. This speaker has a nice blat for harp but in the Pro Junior it squeezes the dynamics down a bit too much for my tastes. This speaker is a touch bright (not nearly as bright as the original speaker), making the Pro Junior sound much better but still a little nasal; a little boxy. The compression made the amp slightly monotonous. I could see it in the wave forms when I recorded.

So, I swapped it out for the Weber Sig 10 ceramic I already had on hand. The difference was immediate. The ceramic speaker is a little warmer and less compressed. The amp responds better to mic cupping; it is less boxy, more open and lively; more articulate. To my ear, the ceramic sounds much better.

You be the judge. Here are clips of each speaker in the Pro Junior, recorded under identical conditions.

Weber Sig10 alnico, smooth cone
Weber Sig10 ceramic, smooth cone
NEW: Shure SM57 mic, Weber Sig10 ceramic, smooth cone

The smooth cone in both these speakers promotes earlier break-up and a warmer tone, ideal for blues harp. The amp is not stock: I have replaced all the tubes with harp-friendly options:

NOS JAN Philips 5751 preamp tube (lower the gain slightly)
JJ 12DW7 phase inverter tube (unbalanced wave form for a fuller tone)
Electro-Harmonix EL84 power tubes (warmer versions of the EL84)
I also disconnected the negative feedback circuit. (a little more “dirt” in the tone)

I like the way the Pro Junior sounds now, although I need to take it out and run it around the block in gigs or blues jams. So far, it sounds great for a low-buck project. Total for the used amp and new parts was under $300.

UPDATE: That great-sounding ceramic Signature 10 speaker from Weber is only $30.00, ten dollars less than the alnico version. The cost of this project is only $280.

: The Fender Pro Junior amp was set at volume on 4, tone on 4. The amp was mic'ed with an AT2020 condensor mic about 10 inches in front of the amp, slightly off axis. Harp mic had a Shure CM element; harp was Hohner Marine Band Deluxe in B-flat. No effects of any kind were used on the amp or in the recording or editing. The backing track was from a Boss DR-3 drum machine. No animals were harmed during the production of this test, but my dog looked at me kinda funny...

Friday, September 5, 2008

Review: Adam Gussow’s “Amping the Harp“

Frequent visitor to this blog and master harp instructor Adam Gussow has produced an hour-long video devoted exclusively to getting good blues harp tone out of an amplifier, Amping the Harp. This is must-see Amp Tone 101, it costs a measly five bucks, and it belongs in every blues harp player's reference library.

Adam knows what he’s talking about. His tone is the real deal, developed over many years as the blues harp half of Satan & Adam, starting out on the streets of Harlem. The video features his five harp amps and what he likes about each one. The magic of this video is the “Aha” moment you feel when you hear the amps begin to “sing.” Adam is a skilled teacher (Professor of Blues History at Ole Miss) and an immensely talented player who makes complex concepts simpler – even the black art of blues harp amp tone.

The package includes a reprint of an article he wrote for The American Harmonica Newletter in 1993, “Adam’s 10-point Guide; How to Amplify Harmonica for that Great Sound.” The topics include:

-Tube vs. Solid State
-Speaker Configuration
-Volume and Tone Controls
-One Amp vs. Two
-To Elevate or Not to Elevate
-Amp Placement
-Miking Your Amp
-Reverb Units, etc.
-Putting it All Together.

The production values on the video are not high… It is Adam with a small digital camcorder. He must have just had six cups of coffee because he is his usual manic self and he takes a “pause for the cause” several times. Still, the video is worth many times the tiny cost.

There are a few places I disagree with Adam. He makes the blanket statement that NOS (New Old Stock) tubes are better. Maybe, but there are vendors out there who will sweet-talk you into parting with a ton of money for tubes they swear will turn you into Big Walter Horton, but sound no better than some inexpensive new production tubes. Caveat Emptor.

Also, Adam insists on using his Mouse amp (a small, bright, portable solid state amp) in tandem with his great vintage amps. He calls the Mouse his “tweeter.” It gives his tone a quality not everybody likes. (I think it sounds great, but it ain’t my favorite Chicago sound.)

Adam does not go into the minutia of tube swapping and other electronic mods, but his advice and examples are solid gold. Any harp player who watches this vid will have a better understanding of harp amp basics.

Go to Adam's
store on his website, Modern Blues Harmonica. Scroll down until you see “Amping the Harp." Click on the link to take you to TradeBit to complete the transaction. I paid using PayPal and the transaction was quick and flawless.

The Blues Harp Amps Blog gives Adam Gussow’s “Amping the Harp” video a V on the I-IV-V scale. Very highly recommended.