Kit Review: The Persuader Tube Drive Pedal

The Persuader is a tube-drive distortion pedal kit sold by Amplified Parts of Tempe, AZ. Amplified Parts is a relatively new entity that is part of Antique Electronic Supply, which has been operating for more than 25 years. The focus of Amplified Parts is on the musician who is not an electronic technician and who wants to create his/her own sound or tone by modifying their existing amplifier or guitar. Guitar Kit Builder received one these kits to review, and we gave it to Casey4s to build and try out.

Before we get into his review let's cover the basic circuit design as shown in the schematic below (click on the circuit for a larger pdf version).

The Persuader Schematic

CIRCUIT OPERATION

The main feature of The Persuader is a 12AX7 dual-triode tube operating in a starved-plate configuration. "Starved-plate" designs operate the tube at a very low plate voltage as a means of getting some tube-distortion sound at low cost. The low cost is due to the fact that no high-voltage power supply is needed, and very few parts are required. This is the case with The Persuader, which operates with a plate voltage of about six volts, which comes from the 9 volt battery or power supply. Starved-plate designs operate far outside the normal design parameters of traditional amplifiers so a traditional circuit analysis can be difficult.

The folks at Amplified Parts told us that the design was done primarily by ear. However we can still give some explanation as to what is happening in the circuit, with thanks to Robert Megantz, author Design and Construction of Tube Guitar Amplifiers, for his input on the circuit.

Referring to the schematic, the design begins with a traditional field effect transistor (FET) input stage, which serves as a voltage amplifier to drive the tube section.

The 5K potentiometer in the source lead of the FET is the Gain control and varies the amount of amplification in this stage. The output of this stage is fed to the grid of the first triode stage of the 12AX7 tube. The tube filaments are connected in series and powered from the 9 volt supply, which means the voltage is lower than specified for the 12AX7, so the cathode emissions will be reduced. It also means the battery will drain quickly. Both 12AX7 cathodes are grounded. The maximum plate current is very low, less than 0.1ma. Negative signal peaks applied to the input will reduce the current flow, and will eventually cut off the stage. Positive signal peaks may increase the plate current, and may also cause grid conduction, which will lower the plate current.

The second stage's grid is biased to the supply, so with no signal it will conduct as much current as possible, which is still not much, but slightly more than the first stage. Therefore only negative peaks applied to the grid, corresponding to positive peaks at the input of the first stage, will affect the output by reducing the plate current.

The distortion in starved-plate mode comes from two mechanisms. The first is that at very low voltage the plate voltage/current curve (see below) for the tube is not a straight line, and therefore introduces non-linearities.

12AX7 Plate Voltage vs. Plate Current

Secondly with such a low plate voltage it is very easy to drive the tube to its maximum plate supply, resulting in waveform clipping. The two tube stages together act somewhat like back-to-back clipping diodes.

Following the tube stage is a 250K ohm Volume Control feeding into the bypass switching foot switch.

Now we'll turn things over to Casey4s for his review of building the kit and giving it a try. We'll finish with a video demonstration of the pedal in use.

BUILDING THE KIT 

I had never built any electronics projects in kit form before and the last time I had built any guitar effect was circa 1976 from Craig Anderton's book "Electronic Projects for Musicians." So when Guitar Kit Builder asked me to build and evaluate the MOD Persuader (K-930) tube preamp pedal (Photo 1), I jumped at the chance.

Photo 1 - The Persuader Tube Drive Pedal Kit

The Persuader is a tube preamp pedal built around a 12AX7 dual-triode tube to go from clean, warm tone to smooth, real tube overdrive. It operates from a 9-volt power source and has a true-bypass switching. The kit includes a pre-drilled, die-cast aluminum enclosure with all parts and easy to follow instructions. When I first received the kit I noticed right away that it was well organized (Photo 2) with most of the small components in two numbered bags. I checked my parts against the inventory list and there were no problems there. The parts all seemed to be of good quality, even though the two jacks were imports. The enclosure is very high quality die cast Aluminum with all accurately pre drilled holes for everything. It is powder coated a vivacious violet color.

Photo 2 - Unpacking The Kit

The instructions I felt were excellent and took you step by step through the process and had well drawn illustrations that the printed instructions referenced. The instructions showed life size illustrations (Photo 3) of all the hardware and small electronic parts. It would be really difficult to confuse the parts or how to solder to them.

Photo 3 - Inventorying The Parts

There is also a section with some basic soldering tips (Photo 4) which should be helpful for persons who may have never worked with a terminal strip before. They also recommend "tinning" the stranded wire which is a good idea.

Photo 4 - Reviewing The Instructions

I read all of the instructions completely before I did anything and did my best to follow them implicitly to see how beginner friendly they are. I feel they certainly passed the test. The great thing about this type of electronics project is that it only requires the bare minimum of hand tools and a reliable soldering station or iron. This is what I call a coffee table project because it takes very little work space. You can build something like this on a kitchen table, coffee table, dorm room desk etc, So anyone who is so inclined can build a great little effect like this with minimum space and tools. I assembled this one on a small desk right (Photo 5) next to my PC.

Photo 5 - The Kit Can Be Assembled In A Small Workspac

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Because of the tight space I did some of the assembly procedures outside of the enclosure (Photo 6), like here I am soldering the leads to the Volume pot.

Photo 6 - Soldering Potentiometer Leads Outside The Case

I also pre assembled a big portion of the first terminal board, I just attached it to the outside of the enclosure (Photo 7) to hold it still for soldering.

Photo 7 - Temporary Mounting of Terminal Strip To Aid In Soldering

Here I have all three terminal strips in place (Photo 8) around the tube socket, I attached the battery leads too.

Photo 8 - Partial Assembly

This is another deviation in the instructions. I find that in really tight quarters like this tube socket between the two short terminal strips. I like to make "sub assembly's" when possible for tight situations. So I pre wired the tube socket (Photo 9) and slid it into place and attached the fastening hardware. it is really difficult in my opinion to solder to the tube terminals and not burn something else.

Photo 9 - Soldering Components To Tube Socket Before Installation in Chassis

When I deviate and pre-wire those pieces, I still follow the instructions but I get the connections and components already in place (Photo 10) for that portion of the instructions.

Photo 10 - Partial Assembly

I was challenged somewhat by the tight spaces (Photo 11) when working on effects. It is sometimes a battle between access to the point to be soldered and burning something else with the iron barrel.

Photo 11 - Nearly Completed After A Few Hours Work

It took me about 4 hours to assemble this really well made kit, but that included my assembly deviations. It is pretty tight inside the enclosure so I had to make sure I didn't have any components leads that might end up touching each other or the case. I checked my work (Photo 12) and then got ready to install the tube and battery. The battery install was a no brainer and there was plenty of room for the battery But it's too bad there is no universal holder for those batteries. I used a small square of two sided tape to hold the battery to the enclosure and prevent it from flopping around. As expected, this pedal goes through batteries quickly so I ended up testing with a power supply.

Photo 12 - Time To Check The Work

When I tried to install the supplied Chinese (White Box) 12AX7 (Photo 13), I found that the tube bottle is too large to fit inside the supplied tube shield. This seemed strange to me because I have handled a lot of tubes and shields over the years and this was new to me.

Photo 13 - Tube Installed But Shield Won't Fit

So I did a little investigating with digital calipers and there is indeed a wide variance in tube bottle diameter from what I have tested. Not just from manufacturer to manufacturer (Photo 14) but between bottles of different tubes by the same companies.

Photo 14 - Tubes Ready for Measuring

With a selection of tubes at hand (Photo 14) I measured at the base with digital calipers (Photo 15) and here's what I found: Chinese White Box - 12AX7 - 22.00 mm Sovtek - 7025 - 22.00 mm Sovtek - 12AX7WXT - 21.75 mm RCA NOS - 12AX7A - 20.70 mm Amperex NOS - 5751 - 20.50 mm Sylvania NOS - 12AX7 - 20.50 mm Groove Tubes (EI) - 12AX7 - 20.30 mm Amperex NOS - 12AX7 - 20.00 mm

Photo 15 - Measuring Bottle Diameter

I next measured the opening of the bottom piece of the supplied tube shield. This is a very good quality tube shield by the way, and I have used similar ones from AES or Amplified Parts myself, so the opening seems standard to me. It measured 21.75 mm, a quarter millimeter smaller than some of the tubes. Now I knew why there was a problem. When I measured the tube bottles the Chinese White Box was 22.00 mm. The only other tube that large was a Sovtek 7025, but their 12AX7WXT measures exactly 21.75 mm and fits perfectly in the socket so that is what I will use to test the kit. The variation in tube size is not necessarily a mistake on the part of the kit supplier, MOD Kits. To be fair, we have all seen the disclaimer that products may differ. Perhaps the Chinese supplier changed the delivered product without giving its customers a clue that the size has changed.

TESTING

Now I was ready to give the pedal a try. The instructions suggest putting the bottom cover on, but I like to have access during initial fire-up. I checked one last time to be sure that nothing would contact anything else, especially around the tube socket. I did in fact find that the input jack was not tight from assembly, which was my own mistake. That's why one last look is a good idea. Before placing the cover on or starting the project up I turn it upside down and make sure I shake out all the wire scraps and solder balls and other debris. I only have a couple of pretty aggressive amps to test with and I would prefer to test with a nice SS amplifier. So I don't think my current amp can give a fair overall representation of the Persuaders versatility.

Initially I put the Volume on Full and the gain at about 50% rotation. Upon stepping on the on button the Persuader came to life. It gave my amp an incredible burst of input amplification and gave a really cool controllable distortion and seemingly endless sustain. The distortion seemed smooth and not edgy like some preamps powered from op amps. I was impressed to say the least. There is no LED or other indication of whether the effect is on or off, so it can be a bit confusing to test. I went through two batteries in just a few minutes so that a 9V adapter is a good choice.

My overall evaluation of the kit experience with the MOD Persuader Kit is excellent. All of the documentation, to the credit of the folks at MOD, was well done. However this is a kit I feel might be better suited for a second or third time effects project and might be a bit challenging for some first timers. It is a point-to-point kit. I like the design but it's tight inside the enclosure and a printed-circuit kit might be a better choice for a first-timer. Some of the soldering on this might be a challenge to a fresh hand with a soldering iron.

All in all, it was a very fun build and I highly recommend this kit if you like the sound it delivers (see video below). If you haven't built an electronics kit project yet, just go do it. There are all kinds of kits for Musicians available. Work safely at all times...and have fun.

DEMONSTRATION OF THE PERSUADER

So, what does this pedal sound like? To give the pedal a tryout we took it to our local School of Rock (www.schoolofrock.com) and had one of their budding guitar-gods give it a workout. Check out the video below to hear the results.