Wednesday, August 27, 2008

Pro Junior Harp Amp Project: Progress Report


I’ve finally settled on the main components for the Fender Pro Junior harp amp project. I tried to keep the cost low so I wouldn’t violate the low buck nature of the original amp. Thanks for all your suggestions.

Speaker – Weber Signature 10 alnico, straight cone – $40.00

This took a lot of thought and research. I looked around for used premium speakers and found a Kendrick Blackframe, but the deal fell through. True vintage speakers were just too much to spend on this project. I dislike the Jensen alnico import speakers for harp, and the C10Q by itself would be not much different than the stock Eminence.

After trying many speakers and consulting with speaker vendors and amp builders I decided on the Weber Sig 15-watt alnico. I use its ceramic twin in a different amp and like it a lot. The straight, un-ribbed cone gives the speaker a warm crunchy tone, with early break-up and a punchy sound. This should be killer in the PJr project.

Preamp Tube – NOS JAN Philips 5751 -- $20.00

Harp players often swap out stock preamp tubes for lower gain tubes, to reduce feedback and increase power tube distortion at lower volumes. My favorite tube for this when using small amps has been the 5751, which has a gain factor of 70 percent when compared to the stock 12AX7. Harp players often use the 12AY7, but its gain factor of only 45 percent sometimes sucks too much power from amps that are already volume-challenged. I have a used Sylvania 12AY7 I’ll also try in the Pro Junior.

Power Tubes – Electro-Harmonix EL84 -- $20.00

The EL84 family of power tubes is known for a bright sound with chiming overtones. That is great for guitar but not so good for blues harp. The EH EL84 tubes claim to have a warmer tone with less accent on the highs. (And they’re cheap.) The power tubes in the Pro Junior amp look to be originals, so replacement is a good idea anyway.

Phase Inverter Tube – JJ 12DW7 -- $10.00

I wrote extensively about the feedback-fighting properties of this tube here. But in this project I’m not so much interested in that as I am in the tone the 12DW7 can bring to the Pro Junior. This tube is unbalanced, causing the two power tubes to work at unequal levels. The result is a slightly more ragged edge to the harp tones, with a fuller sound. I’m also thinking of trying mismatched power tubes.

Total for parts -- $90.00

All the tubes were purchased from Tube Depot in Memphis. The speaker was ordered directly from Weber. With shipping costs included the total may go slightly over my $100 limit. If you work eBay and Craigslist really hard you can probably assemble the components for less.

The only other modification was to disconnect the negative feedback circuit in the amp. That amounted to un-soldering the violet wire that runs from the center of the circuit board to the speaker jack. I taped it off and secured it out of the way with a zip tie. I also sprayed the tube sockets with some Deoxit.

So, we’ve added 90 bucks worth of stuff to a little $200 used amp in search of better harp tone. How does it sound? So far, it sounds outstanding; a gigantic improvement over the sterile, guitar-ish tone of the stock Pro Junior. I’ll post sound clips next week after the new speaker arrives and I have a chance to sort it all out. Stay tuned…

2 comments:

Joe said...

It's going to be interesting to see what kind of volume you get out of that speaker. My experience was that the tone was really nice, but the volume was nowhere near as loud as the original speaker.

Rick Davis said...

Hey Joe-

Yep, I'm aware some players have seen a reduction in volume with this exact swap. I'm willing to give up a little volume for better tone, so we'll see what happens. I may end up using the ceramic version of the Sig 10 that I already own.

I talked about this with several people and decided the brightness of the stock speaker might give a perception of loudness. But ya never know 'til ya plug in and crank it up. As I said, we'll see what happens.