Friday, August 29, 2008

Fender Pro Junior Harp Amp Project - Sound Clip 1

Here is a short recording of the Pro Junior project in a transitional state -- the only thing missing is the new speaker. In this configuration the PJr has the 5751 preamp tube, the 12DW7 phase inverter tube, the new pair of EH EL84 power tubes, and the negative feedback loop has been disonnected. Check it out.

That is pretty remarkable. The Pro Junior has nice grind with the volume on TWO! Tone is on four. The original speaker is a little bright for my tastes, but this little amp is starting to have some real character. I can't wait to hear it with the Weber alnico speaker.

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UPDATE: Just for fun, here's a short clip of the PJr amp running through my 2x10 cab.

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…

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.

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Wednesday, August 20, 2008

Harp Amp Project: Fender Pro Junior

The other day I spotted a MIM (Made In Mexico) Fender Pro Junior at a very fair price on Craigslist, so I snapped it up. I owned a Blues Junior amp a few years ago but did not like it much. I may have given up on it too early. So now I’ll take a shot at its simpler little brother, the Pro Junior.

Where the Blues Junior is a Master Volume amp with a FAT button and all manner of bells and whistles, the Pro Junior is bereft of features. It has two chicken head knobs: volume and tone. That’s it. One channel; no reverb. The speaker in the Pjr is a 10-incher, two inches smaller than in the Bjr,

What the two amps share in common is basic circuitry: 15 watts, two EL84 power tubes, a solid state rectifier, 12AX7 phase inverter, and a 12AX7 preamp tube. They also share the EL84’s tendency to sound trebly and to chime when driven hard. That is not a good sound for blues harp.

Be that as it may, the Pro Junior is a pretty popular harp amp. I’ve heard passably good tone honking out of these little amps at blues jams. Some of my harp buddies swear that this or that speaker swap is the magic bullet that transforms the Pjr into a monster little harp amp.

This particular Pro Junior looks to be completely original, and it is in excellent shape, the tolex and grill unmarked. The tubes are all Fender/Groove Tubes/Sovtek and are almost certainly the tubes that came with the amp from the factory. The speaker is the original Fender Special Design. I played the amp and it sounds like… well, it sounds like a stock Pro Jr.

I’m looking for suggestions here. The basic plan: I’ll replace all the tubes. For the preamp tube I’ll start with my favorite, the NOS JAN Philips 5751. In the power section I’m thinking of EL84s known for a bigger bottom end and less sizzle, such as the Electo-Harmonix or the new production Mullards. When replacing a matched pair of power tubes you should always replace the phase inverter as well. I’ll try both a new 12AX7 and the 12DW7 I still have lying around from my Twin Reverb project.

I need recommendations on the new speaker. I don’t want to go with my favorite, the Weber 10A125-O, for two reasons: It is rather expensive and I already use it in my silver face Champ (it sounds amazing). I also don’t want Jensen “vintage” or MOD, or the Weber Sig. I’m thinking maybe a used speaker, or something different. What have you tried that sounded good?

I also want to limit the mods to those that can be accomplished without being (or hiring) an EE. I’ll try to disconnect any Negative Feedback Circuit in the Pjr, but I don’t really want to dive into the complexity and expense of slinging a soldering iron all over it, unless the mods are REALLY simple. My target is to radically improve the harp tone of the Pro Junior with, say, a hundred dollars worth of parts and with me -- a lay tinkerer -- doing all the work.

I welcome any suggestions either in the comments below or via email to me at

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Friday, August 15, 2008

Review: Danelectro Fab Echo pedal

First of all, this pedal sells for $14.99, about a tenth what you would expect to pay for a decent delay pedal. That, and a smattering of positive reviews from guitar players got my attention.

The Fab Echo is not long on features, of course. But it does one thing rather well… it has a nice warm slap-back that works well for harp.

Harp players argue constantly about which delay pedal is best: Analog or digital? Maxxon, Ibanez, Boss, or Digitech? If you are a Chicago-style blues player – if you lust after Little Walter’s tone – all those delay pedals have way more features than you will ever need. What sounds good for traditional amped blues harp is a touch of greasy slap-back echo. That’s all.

And that’s where the Danelectro Fab Echo comes in. It has two controls, Repeat and Mix. The repeat knob controls the number of echos, with a single echo at the minimum position. That is what we want for harp. The other knob controls the output mix of dry and wet signals, from all clean to all echo. Essentially, this is s a one-knob pedal for harp players.

What you pay all the extra money for in other delay pedals is the Delay Time control, which is conspicuously missing from the Dano Fab Echo. But if you only use one echo – a “slap-back” – then the lack of a Delay Time control is pretty much meaningless.

I recorded three very brief sound clips to give you a basic idea of how the Dano Fab Echo pedal sounds for blues harp. The first clip has the Repeats control at the minimum (one slap) and the Mix control at the max (all echo, no clean). This clearly illustrates the delay time built into the pedal, but you sure would not want to use it this way.

Clip 1

The next clip has a similar riff, with the only change being the Mix control is at 50 percent. The pedal sounds best to me at this setting

Clip 2

For reference, here is a very short clip with the pedal switched off.

Clip 3

How do you think the Dano pedal sounds? For its price and simplicity I think it sounds good, but lately I have preferred playing dry… no reverb or delay at all. But, Crikey! For chump change you can have a usable no-frills slap-back pedal in your bag of tricks. At the very least it makes a great back-up if your fancy delay pedal (most of whose features you never use) ever craps out.


This pedal is made of plastic, but it is hefty and feels solid. The jacks are soldered onto the circuit board but don’t feel loose or cheesy. Product support at Danelectro is known to be pretty much non-existent, but if it breaks, what the heck. Just buy another one.

This pedal is not noticeably noisy. The pilot LED is a brilliant blue. The controls are a little confusing at first because they face away from you as you look at the pedal. It has a 9V power connector so you can use a common power adaptor instead of hassling with batteries.

I ordered it on a Monday evening from Musicians Friend online, and it arrived at my house in Denver via USPS express mail on Thursday morning.

Test Notes:

I used a OneSpot adaptor to power the Dano pedal. The amp was a vintage
Masco ME-18 and 2x10 cab. The recorder was a Zoom H4. Instead of mic’ing the amp, I used the H&K Red Box Pro DI to avoid room effects. The harp mic was a Peavey H5 Cherry bomb with Shure CM element. The harp was a Hohner Marine Band Deluxe in the key of B-flat.