Simple Effects Loop

Here is a simple, series, buffered effects loop. This is the loop that is currently installed in my J50 amplifier.

FX loop

The loop features both send and return level controls. Depending on where you place this loop in your amp, however, the return level control might not be necessary. I think I prefer a 12AU7 in this position, but you can use a 12AT7 or 12AX7 instead.

This simple loop is precisely that: simple. It’s not the best way to incorporate a loop into your amp. One of these days, I’ll get around to installing a real parallel loop in my amp. For details on the best all-tube parallel effects loop around, see Kevin O’Connor’s book, The Ultimate Tone, Vol. One.


Tech Info & Schematics

Good articles and technical info

One of the best forums for amps and effects

Take a peek at the guts of your favorite amp

Great projects and information, plus a very friendly and helpful forum. This is THE BEST place to begin if you’re new to DIY

A ton of amp schematics and modification information

This site features a tube data search engine and best of all, a Tone Stack Calculator.

Thriving online DIY community devoted to the Marshall 18-watt amplifier

Kevin O’Connor builds a full line of innovative amplifiers; he also writes the most authoritative books on the subject. The Ultimate Tone, Vol. I is by far the best and most comprehensive book on guitar amp design ever written

GUITARGEARY is one of the best blogs for guitar equipment reviews and HQ advice from pros.

Looking for tube amp schematics? Look no further –

Discussion forum devoted to high-gain DIY

Discussion forums for speakers, tubes, amps, and guitars. Also has links to many other popular forums

Parts Suppliers

Custom faceplates and logos for your amp

European source for DIY amp parts

A terrific source for NOS tubes, sockets, chassis, transformers, and countless other parts

Good source for teflon wire, misc. surplus

Nice cabs, great prices on Celestion, Eminence speakers

Chassis, kits, transformers, circuit boards

Buyer beware: I’ve heard dozens of reports of bad experiences with this business. Everything from slow and non-delivery, to people paying hundreds of dollars and getting neither a refund nor the parts they ordered. If you’ve had a good experience from Handwound Transformers, I’d be interested in hearing about it

Good selection of parts, excellent service. Highly recommended

Custom chassis: high quality, good prices

Mouser doesn’t stock tubes or guitar-specific items like tolex. But, they do have everything else you’ll need

Premium-quality output transformers

Great prices on Hammond transformers, also cloth-covered wire

Custom head, combo, and speaker cabs. Highly recommended

No tubes or transformers here, just a great selection of project and stompbox parts, including matched pairs of fuzz face transistors

Custom eyelet and turret boards

Speakers, chassis, transformers, amp parts, amp kits

Tube Retailers


The Ruetz Rat

The Ruetz Rat is a simple modification to the classic ProCo Rat distortion pedal that accomplishes two things. First, it lowers the overall amount of gain in the circuit, transforming the pedal into more of an overdrive rather than a distortion pedal. Second, it flattens out the frequency response of the circuit, resulting in a smoother tone and extended (but tight) low end.

The Ruetz Rat mod is as simple and easy-to-perform as can be. Anyone can do this mod: no electronics experience, soldering, or special tools required. Following the instructions and photographs below, all you need to perform the Ruetz Rat mod is a screwdriver (to take apart your Rat) and a pair of wire cutters or fingernail clippers (to perform the mod).

Don’t underestimate the power of this mod because of its ease and simplicity — the Ruetz Rat is a HUGE improvement over the stock circuit. Dozens of happy Ruetz Rat users have emailed me over the years to tell me how thrilled they are with their “new” Rat, and how usuable it is compared to stock. Try it: you’ll like it.

Ruetz Rat – Solderless Clip Mod

Some years ago, my uncle, Ron Wayton, asked me if I could mod his Rat pedal for a smoother tone and better low-end. At the time, I was unfamiliar with the Rat circuit, but (of course!) I accepted the challenge.

After locating a schematic and checking out the circuit, the first thing I tried turned out to be all that was necessary. Here is a close-up schematic detail of the relevant portion of the circuit:


Here you can see two resistor/capacitor pairs. As part of the feedback loop, this pair helps set the gain of the op-amp stage. But also, because of R/C interaction, not only does this pair affect the gain of the circuit, but also the frequency response.

The only mod I performed was snipping one of the leads of the 47 ohm resistor, effectively cutting that R/C pair out of the circuit. This leaves the 560 ohm/4.7uf pair to set the gain and frequency response of the op-amp. Here is a photo of the guts of my uncle’s pedal and the snip that I made:


There are several different versions of the standard Rat pedal — here’s an example of one with a different layout inside. I’ve circled the 47-ohm resistor:

Rat also

Here is a photo that someone kindly emailed me of yet another version of the Rat:

Here is a photo of a very recent Rat — notice the blue 3PDT footswitch:

Here is a very rough explanation of what this mod accomplishes. The value of the resistors sets the gain of the circuit: the lower the value of the resistor, the higher the gain. But notice that in the original circuit there are two resistors, of widely different value. This would suggest two different values of gain. But, of course, the circuit as a whole is going to exhibit only one overall gain value. This is where the caps come into play. The 2.2uf cap has a certain low-end cutoff frequency; above this frequency, the gain will be set by the 47 ohm resistor. But the 4.7uf cap will have an even lower cutoff frequency: the gain for these lower frequencies will be set by the 560 ohm resistor. But since the 560 ohm resistor is a much larger value than the 47 ohm, the gain for these lower frequencies will be much lower. The overall result is: the gain of the circuit is less for the bass response, and more for the midrange and treble response.

Cutting the 47 ohm/2.2uf pair out of the circuit means that the gain and frequency response of the circuit is determined solely by the 560 ohm/4.7uf pair. This lowers the overall gain of the circuit, but also smooths out the overall frequency respone of the circuit: the amount of gain is flat across low-end to high-end.

This simple mod suited my uncle perfectly: he loves it! Now, you might want to try this mod in the other direction: you might want to cut out the 560 ohm/4.7uf pair, for a higher overall gain and even tighter low-end. Or, install a switch to make one or both of these mods switchable for even more versatility.

Ruetz Rat — Trim pot mod

Here is a more challenging Ruetz Rat mod, for those of you who would like more versatility. Instead of clipping out the 47-ohm gain resistor, this mod involves replacing that resistor with a 1k trim pot. I’ve circled the trim pot in this photo:

This time around, Ron performed the mod himself. Here is what he had to say:

The new Rat mod was a huge success. I took my time and did it right. Took me longer to get the thing apart than it did to do the mod!! Certainly now with a fourth parameter there’s plenty of tweaking to be done but at first listen it was definitely worthwhile. With the pot wide open it sounds slightly hairier and warmer than with the resistor clipped. As you dial in the pot you get closer to stock Rat and can in fact REALLY hear when you get to the stock value. Almost like the pedal wakes up and is reborn to what it was (not necessarily a good thing) but just before that is the TONE!! As I said. There’s plenty to learn yet. The filter control and the trim pot are highly interactive so there are lot of tones to be discovered! The best part is that the trim pot is conveniently accessible just by opening the battery compartment door!! A 500-ohm pot would have been ideal but this 1K from Radio Shack works fine.

If you’re really ambitious: instead of using an internal trim pot, why not remove the “Filter” pot and replace it with a full-size 1k pot, so that the gain resistor is now externally adjustable?

Output Tube Biasing

This is the method I use to bias my amps. It’s not the only way to bias your output tubes, but I think it’s the safest and easiest method.

Essentially, you want to bias your amp such that the plate dissipation rating of the output tubes is not exceeded. Plate dissipation is measured in watts (but don’t confuse this plate dissipation rating with the audio output power of your amplifier, which is also measured in watts). By Ohm’s Law, power in watts is given by the following:

P = E * I

So, in order to determine the actual plate dissipation of a given output tube, you multiply voltage by current. In this case, E represents the actual plate voltage, and I represents the total current draw of the tube. But, although we can easily measure plate voltage, we need a way to determine how much current a tube is actually drawing.

In order to determine the actual current draw of an output tube, we can introduce a resistance in series with that tube. Because current is common in a series circuit, the current draw of the resistor will be identical to that of the tube. But we can easily determine the current draw of the resistor. By Ohm’s Law, current in amperes is given by:

I = E / R

In this case, E represents the voltage found across the resistor, and R represents the value in ohms of that resistor. We can easily measure the voltage across this resistor. And, if we make the value of this resistor 1 ohm, the total current becomes E divided by 1, or simply, E. Since the resistor is in series with the tube, the current drawn by the resistor is identical to that of the total current drawn by the tube.

Putting this all together: in order to determine the actual plate dissipation of an output tube, we need to solve P = E * I. E is simply the plate voltage, and now we can determine I by simply measuring the voltage across resistor R.

Here is an example. The maximum plate dissipation rating for the 6L6GC is 30 watts. Suppose I want to bias my amp such that my 6L6 output tubes run at a conservative 20 watts. Suppose also that the plate voltage of my amp is 400 volts. Recall that P = E * I. In this case, 20 = 400 * I, so we simply solve for I. A quick calculation gives 0.050. So, I simply adjust the bias of my amp such that the voltage across R reads “0.05” to reflect a total current of 50 mA.

In order to implement this method in your amplifier, you’ll need to install a 1 ohm, 1% resistor between ground and the cathode of each of your output tubes, as illustrated in the diagram below. Although I have heard of people using 1/2 watt resistors in this position, I recommend using a higher wattage device.

Bias current resistors

For reference, here are the maximum plate dissipation ratings for some popular output tubes:

  • 6L6GC / 5881 — 30 watts
  • 6BQ5 / EL84 — 12 watts
  • 6CA7 / EL34 — 25 watts
  • 6V6 — 14 watts
  • 6550 — 35 watts

For more information on biasing your output tubes, check out THIS article by Lord Valve.

DISCLAIMER: Vacuum tube equipment operates on very high voltages. 350-450 volts or more can be found inside a typical tube guitar amplifier. THESE ARE LETHAL VOLTAGES. Do not attempt to disassemble, repair, or modify any vacuum tube equipment unless you are qualified to do so. The purpose of this site is only to provide helpful information. The owner of this site is not responsible for any damage to persons or equipment resulting from misuse of this information.