This will make the wah sensitive to the load that is placed on its output. It can also cause an adverse reaction with a low impedance input of the next stage, such as with a fuzzface type cicuit. The FF can break into oscillation with the frequency changing as the wah treadle is moved.

One suggestion to rectify the impedance mismatch is to add an output buffer:

This works fairly well but has one problem. The bias on the jfet is usually small, and depends on the gate-source characteristics of the individual fet, which limits how much input signal can be applied to the buffer before it begins to distort. Since the frequencies around the center of the wah’s bandpass are boosted, they will cause the jfet buffer to clip and a bit of distortion is added to the wah signal. While it may be possible to hand select a jfet that will perform well in the circuit, another solution is possible:

An extra resistor has been added to the jfet gate to set a higher source voltage. This allows a larger input signal before the onset of distortion resulting in a clean wah filtered output. Problem solved!

The improved wah buffer is identical to the jfet buffer on stripboard that was shown previously in this blog. The 10M (R1) resistor is not needed since the circuit is permanently wired to the wah and not switched. If you do not like stripboard, this circuit can be constructed using one of my Multi-Purpose pc boards.

Another option would be to use the leaky germanium transistor buffer. This is an especially good option since I’m offering free AC128 transistors that can be used for this purpose. The Ge buffer can also be built on stripboard or on the Multi-Purpose pc boards. Full details are included when you buy the pcb.

I’ve modified the instruction page for the pcb to show how to use it to build the Tillman. The instructions and parts list are on the site that is emailed to everyone that buys one of the boards. Ordering info:

AMZ MultiPurpose PC Board

The Rat is designed to have a large amount of gain and a significant treble boost. This is part of what gives it a characteristic sound, but also makes some players dislike the tone it produces.

The resistors R6 and R7 along with the capacitors C7 and C6, are the main determining factors for the low frequency rolloff.

The C6/R7 pair has a 60.5Hz cutoff.

We can see that the first easy mod is to remove C7, which will leave only the C6/R7 network determining the frequency response and gain. R6 is not in the circuit once C7 has been removed.

The capacitor is easy to find. Locate pin 2 of the opamp and follow the trace to the 47 ohm resistor. It will have bands of yellow-purple-black.  The 2.2uF will connect to it. Desolder the connections and remove the capacitor.  You could also remove the resistor.

Gain is now lower and the frequency response is flat across the band of guitar frequencies. This will give it a tone with much less treble boost, and somewhat like a Big Muff.

If the Rat has now lost too much bite for your rig with this mod, then it can be fine tuned by changing the value of C6. Remove the 4.7uF (C6) and replace it with a 0.47uF capacitor rated for 25v minimum (50v, 63v or any higher rating is acceptable too). This will give it a low pass corner of about 600Hz.

Make C6 smaller to roll off more bass – 0.22uF will have a 1292Hz corner. Use the AMZ Filter Calculator to work out a combination that you like.

Customize your Rat pedal for the best sound with your guitar and amp!

More custom mods for the Rat are available in the AMZ Muff-Rat eBook.

Update: You can apply these mods to the BYOC Mouse kit. Here is the parts cross reference:

• RAT : BYOC
• R7 = R4
• R6 = R5
• C6 = C5
• C7 = C6 (remove this one)

My new Epiphone Valve Jr. amp head

If you saw my previous post on the Epiphone Valve Junior 5w amp head, you know that I bought it to experiment with some mods and circuit changes. I’ve been looking over the schematic and drawing up some of the mods that I want to try. Here is the initial list of mods I want to make to this little beast.

• Add a tone control
• Reduce preamp gain
• Add a switch for a feedback loop
• Add a triode/pentode switch
• Add a conjunctive filter on the OT
• Add a standby switch

I’m not planning on changing output transformer in the first round of mods, though I have one on hand that would be suitable.

I want to reduce preamp gain because if I need overdrive from the amp, I will use a booster pedal to drive it hard enough to go into saturation

I might also end up changing the values of a few components.

There will be a page on my web site dedicated to the mods and the link will be added to this post when I get it online.

The Epi Jr. is a nice little amp for the dollars and I look forward to turning it into a custom unit drenched in good tone!

Their web site has plenty of free info so check it out… better yet, subscribe today and support DiY!

http://www.audioxpress.com/


An example of a recent project is the Hybrid Valve MOSFET SE Amp by Stephen Moore, published in the Oct and Nov 2007 issues. They are online for free download:

http://www.audioxpress.com/magsdirx/ax/addenda/media/moore2355.pdf

http://www.audioxpress.com/magsdirx/ax/addenda/media/moore2844.pdf

A Vox amp on a phone jack! It might be just the thing for practicing or jamming when the family is asleep.

Somehow I think the claims on its home page are just a bit exaggerated about the analog amplifier emulation, but it could be nice nonetheless. Check it out!

http://www.voxamps.co.uk/amplug/

My new Epiphone Valve Jr. amp head

I just bought a new tube amp head that is prime for some juicy mods! Priced no more than many effects pedals, the Epiphone Valve Jr. is an excellent buy. I got this one for $129 with free shipping.

The amp is well made and solid, not at all like I would expect for such a bargain priced item. The design is a basic 5w all-tube guitar amp that uses a 12AX7 and an EL84.

I had read that this amp had humming problems so when I first turned it on, I was amazed that it was stone quiet. I kept looking at the panel light to make sure it was working, and even checked the speaker cable to see if I had forgot to plug it in. Nope, this little beast is very quiet and hum-free. I later discovered that the tube heater filaments are now powered by DC instead of the AC used in the first versions of this amp.

A quick jam session with it driving a 2 x 15″ speaker cabinet proved it to be a capable performer. It’s not as loud as my own homemade EL84 amp but it has some nice sounds.

The best thing about this little amp is that it has plenty of room in the chassis for mods and additions to the basic circuit. A tone control would be a good feature and I can think of numerous other changes that would make it even better.

Buy one of these amp heads, even if only to add a line-out to use for overdriven tube sounds. I will have an article soon that shows how to add this feature along with all of the other mods I make to the Epi Jr.

While you are waiting for my mods to be posted, download the schematic to this amp and look it over.