Friday, August 22, 2008

Update on the Pro Jr. Project

I’ve received several emails with good suggestions for the tube and speaker swaps in the Fender Pro Jr harp amp project.:

-12AY7 in V1 (suggested several times)
-12AU7 in V1
-12AT7 in V2
-12DW7 in V2

(V1 is the preamp socket; V2 is the phase inverter)

-Jensen C10Q reissue speaker

-Weber 10 Sig alnico straight cone speaker
-Weber 10A150 speaker

I emailed Ted Weber and he suggested a 10F150-O, but that costs 90 bucks and I’m trying to keep this project down to a moderate level. I’m still looking around for an interesting used speaker. If nothing turns up I’m leaning toward the Weber Sig ceramic with straight cone. I use that speaker in my 2x10 cab and I know it sounds great with harp.

I ordered a NOS JAN Philips 5751 preamp tube from Tube Depot in Memphis. On a whim I visited my friend Al in his tube heaven junk store and found an old used Sylvania 12AY7 with some corrosion on the pins. I’ll try and clean it up and see how it sounds.

In the article about my Fender silverface Champ project I dug into the issue of the Negative Feedback Circuit in Fender amps and how it affects harp tone. The NFB circuit in the Pro Jr. is a violet wire that extends from the middle of the circuit board to the speaker jack. I’ll unsolder or clip the wire and tape it off. Some players install a switch or pot in the circuit to adjust the NFB, but with the Champ project I discovered that zero NFB is best for my tastes.

I had heard that the green filament heater wires in the Pro Jr are not twisted and are routed along other wires that could easily pick of noise from them. In my Pro Jr. they aren't twisted, but they are dressed pretty well, crossing other wires at a 90 degree angle. The amps does not seem noisy so I'll leave well enough alone.

The Pro Jr. has a bleed circuit that drains the amp of any power within a few seconds of shutdown, which is supposed to keep you from getting shocked. Still, I short the no. 1 pin of the V1 tube socket to ground for extra caution before working on the guts of any tube amp. I suggest you do the same.

Jump to Fender Pro Junior Harp Amp Project – Speakers


Mike Fugazzi said...

I am super interested in this project! I once had a PJ, but could only use it with the Kinder AFB+. Know I just have an Allen Old Flame (works out really well...especially with the Kinder) and a late SF Vibro Champ.
One of these days I got to get a 10" speaker in there. I also plan on trying the 5v4 like you suggested. That audio clip rocked.

Ideally, I'd like a mini-Old Flame...meaning the Black Face sound around 15w. However the Princeton Reverbs are expensive. The VC is close to being gig worthy (miked) but I think a 10" and 2-3 times the watts is the way to go.
Sonically, what does the Jr compare to???

Rick Davis said...

Mike, thanks for checking in. I'm pretty excited about the Pro Jr project too. I'll have an update later this week.

What does the Pro Jr. compare to sonically? It is the little brother of the Fender Blues Junior, so they certainly share some sonic characteristics.

The PJr and BJr use the EL84 power tube, which certainly has a signature sound. Think of the Vox AC30 or the small 18-20 watt Marshall amps. The Mesa Boogie 20/20 amp (which I owned for a few years), and the Peavey Classic 30 and 50 are all EL84 amps.

The EL84 is very punchy and tends toward a brighter sound with singing overtones in the upper register. That is why you don't see many popular harp amps in this list. The 6V6 is much more suited to warm-sounding blues harp amps.

But the EL84 is not without its virtues for harp. When properly tweaked, some EL84 amps really sing with blues harp; a little brighter, a little more open than the 6V6 or 6L6 amps.