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.)
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:
-Microphones
-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.
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.