Tag Archives: marshall

Building a Z.Vex Box of Rock on vero-board

0405_Marshall_Amp_630x420Why?

As most guitarists will admit, we’re always looking for a better sound. Some look for new and perhaps innovative sounds. Others, such as myself, want to sound like the players we admire. One classic tone I had never really toyed with is the sound one gets when you crank a JTM45 Marshall. In my opinion it is in my Top 5 sounds. The likes of Clapton, Hendrix, Young (Angus), and many others, started their careers with it. There’s no need for me to recount the origins of the circuit as that’s well documented.

So, you might ask “why not just buy a JTM45?”. Well, one in good condition from the 60’s might set you back 5-10 thousand <local currency units>. A new one, such as those available at Ceriatone go for about $1000….a ‘real’ Marshall? more like $1800.

How

One solution is to approximate the tone in a pedal. And yes, I’ve built way too many overdrive/distortion pedals already, but not one like this. The approach here is to cascade two or more gain stages to ape the design of the JTM45. This has been done may times in plenty of pedals, but a popular one is the Z.Vex Box of Rock. I first heard of this device when it was mentioned by Davy Knowles, who is an excellent blues guitarist. Searching briefly online and I found that the design of the pedal built upon the Super Hard On booster; one of which I built last year. This struck me as a great way to go as I understood that circuit.

Schematic

ZVex Box of Rock schematic

ZVex Box of Rock schematic

Even if you can’t read schematics, you can probably see a pattern repeating itself. There are 4 BS170 transistors arranged as gain stages. They’re chained together and setup in such a way to simulate the characteristics of a tube amplifier. There’s a 4th one (at the bottom right of the schematic) which is the boost section. That will have its own footswitch, so it can be turned on when you “need a bit more”.

I went for a slightly different EQ section. As it stands the BoR just has a simple Muff-style filter. As in, a low-pass filter and a high-pass filter ‘mixed’ together by the tone knob. There’s a great variant that adds a ‘mid scoop’ control, as specified beautifully here by AMZ. It was simple enough that I decided to add it myself.

Layout

I got the veroboard layout from my usual place: Guitar FX Layouts. This guy always does a great job, and is always around to help or explain things. Fantastic!

ZVex Box of Rock - Complete

ZVex Box of Rock – Complete

Build

The build in-progressThe build started off ok, but proved tricky. I quickly realised that I’d done something very wrong as it just sounded very odd. I remembered experiences from previous builds and went back with a fresh printout of the layout, and ticked off each cut, link and solder spot, and made sure the component values were correct. It turned out I had got some cuts in the wrong place. Literally as simple as that. I was careful to use sockets for the BS170s; this meant I was able to leave them out until the last minute so as to not expose them to static shock risk.

Enclosure

Spray and bake

Spray and bake – the black paint hardening under the lights

I went for some custom graphics again. As is typical I couldn’t really think of anything particularly innovative, and ended up calling it “Bed Rock”. This stands to mean two things: 1) This tone is the bed-rock of modern music 2) You can get that tone at low volumes so you can ‘rock’ in your ‘bedroom’. I know, cheesy.

Most of all I wanted to created something with a splash of colour. So many pedals look so very boring. I did the usual flow of finishing the enclosure with enamel based spray paint. This time it was a cheap can of black from Ace Hardware. I was actually going to try something new and bake the enclosure in a toaster oven, but I realised that I was already doing a kind of slow bake with my work-lights. And as I wasn’t in a hurry, I stuck to my usual routine.

For the decal itself, I used a combo of Photoshop and Illustrator to create an image. It was printed on my trusty HP Office ink-jet printer, onto white-backed decal paper; purchased from Small Bear Electronics.

One pleasant discovery was that you can get high-quality fonts for free at sites like FontSpace. Here’s the one I used for the main wording, All Ages.

Almost there!

Almost there!

The finished 'Bed Rock'

The finished ‘Bed Rock’

Demo

I think it came out well. With a Les Paul, it’s ridiculously easy to get that classic AC/DC rhythm tone with everything at 12 o’clock, and with the boost section kicked in you’ve got just that little bit more sustain available for a solo. Even a Strat sounds good. On mild gain settings it really makes single-coils come alive. I ought to do a recording of that too, I suppose. Here’s a demo file of the device.

My setup is the same as usual: Steinberger GM-7SA plugged into an AX84 tube amp, running clean into a 2×12 open-backed cab, with Celestion G12-75T speakers, miced by a Shure SM57 being recorded by an Apple Mac.

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The quest for tone

Further to the Tub-screamer pedal project (I’m waiting for a few final parts), I found a great article on the Geofex site all about the original Tubescreamer internals. Here’s a quote I found amusing:

The best thing to do, I believe, if you’re hunting the Tube Screamer Holy Grail is to stick in an 8 pin socket on the board, noting the orientation of the original chip carefully, and then just plug in various dual opamps until you find one that conjures up images of your own personal guitar deity.

In this world of tube amplifiers and guitars/effects there are snake-oil salesman around every corner trying to get us to spend hundreds (if not thousands) of dollars on all kinds of gadgets that’ll bring us closer to “that tone”. They recognise that at the heart of it we (guitarists in general) are star-struck wannabes that have a very simple desire to be “as good as that guy”. And “that guy” may be Clapton or Vaughan or whomever. In quiet moments when we’re listening to our favourite music we think “if only I could sound like that”. I don’t care who you are, you do it. I bet even Clapton does when he’s listening to Buddy Guy, or Freddie King.

For me it’s Eric’s tone on the studio version of “Spoonful”. It’s clearly some kind of Gibson, probably a Les Paul. It’s also certain to be a 100 watt Marshall cranked pretty high so it’s “alive” and on the brink of feedback. In my view that’s the sweet spot for any rig. In the mix there’s an ocean of reverb on the track which adds to the haunting sound she creates. You can hear him playing with a variety of pick strength…sometimes he’e gentle, other times he’s really digging in. Never does his guitar sound harsh; it’s always creamy smooth and very, very musical.Ā Now for me to get that tone I simply use my 50w 2204 Marshall and either my mid-70’s Les Paul or my Tokai copy. That’s all I need to fulfill that particular dream; especially as I realise I’d never truly be able to recreate that sound as I’m not Eric.

However, if I were more susceptible to the snake-oil salesman I could do the following:

So the question is, how much closer to that tone will I get by spending > $200k? Maybe 1% closer. I can get 95% of it with my so-called cheap rig. The other 5% is all about Eric.

Back to Tubescreamers. The aura surrounding them is due to people like Stevie Ray Vaughan. He used a TS-808 in his rig, so naturally everyone wants one. Conversely, nobody wants the TS-9 or any kind of reissue as folklore would have it that they “aren’t the same”. The fact is, the pretty much are the same. The Geofex article linked above proves that beyond doubt. So why is it that people will pay $350 for a handwired TS-808 from Ibanez? Why is it that sellers on eBay think they can get $600 for an old one? It’s all snake-oil.

I’m going to build my $37 clone from the circuitboard I got at DIY Effects, and the parts in got from Small Bear Electronics/Pedal Parts Plus, and be 95% there. šŸ™‚

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Replacing/discharging capacitors?

While searching for stuff about my Marshall 2204, I happened upon a fantastic source of information; The Hoffman Amplifier forum, hosted by EL34 World. I saw that there was an ‘introduce yourself section’ so I posted this: Hello from Chicagoland

A forum member was kind enough to point me in the right direction (the Hoffman Amp store) after suggesting I replace the ‘electrolytic capacitors’ rather than replace the output tubes.

So from Hoffman, I’ve ordered 3 50-50 500v ‘can’ capacitors. The logic being, the Daly capacitors on the amp are original and therefore very old. They might be the source of all the noise/hum when the amp is even slightly cranked (in the sweet spot, so to speak). I’ve also ordered parts to make a capacitor discharge probe. It seems there’s no consensus on how to do this. Techniques vary from takingĀ a screw driver and shorting the caps to the chassis. This can’t beĀ particularlyĀ safe, although it’d be dramatic. Other methods involve essentially the same thing, but using a resistor in series to slow the rate of discharge. But nobody can agree on the value of such a resistor. I have found these variants:

  • 100k Ohm, 5W
  • 470 Ohm – 2k Ohm, 2 Watt

I have ordered a 5k, 5W wirewound resistor. And I already have some clips and heat shrink with which to make the tool.

I might eventually make one of these though: Capacitor Discharge indicator. It implies that “finishing it off with the ol’ screwdriver” is still a good idea once voltage drops below lethal levels.

UPDATE!

My good friend Tom Reid has some insights on this:

My standard discharge tool is a very large Craftsman screwdriver. It worked
well for picture tubes over the years. The added resistor to a discharge probe creates a current through to discharge procedure. For best results, discharge through a standard incandescent light bulb, as the filament is both resistive, and inductive so as to absorb the shock.

UPDATE AGAIN!!

Finally I found a page where someone quantifies the value of the resistor one might use:

It’s smart, then, to discharge the unit deliberately, through a resistor equal to about one ohm per volt of charge.

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The existing tubes

I thought I would have a look to see what kind of tubes I already have in my 2204.

The EL34 output tubes are labelled “PM Power Tubes”. Mine were bought back in 1994 and are seemingly still available here.

The pre-amp tubes are ECC83 (apparently the British model number for 12AX7) branded as Zaerix. This thread discusses their origin a little bit.

As far as knowing which tubes to buy to replace them? Hard to tell, to be honest. From what I’ve read, the tubes I have ought to sound somewhat mediocre, but they don’t My amp sounds great. I found an interesting discussion here outlining different brands and the author’s experiences with them.

So my dilemma is this: Mr Eurotube (who really knows what he’s talking about) does not rate Groovetube EL34-M tubes at all. These are the pair I have ordered (and in fact they arrived at home today). But Mr Mitchell, the author of the book I have been using won’t use any other brand (well, perhaps Mesa Boogie tubes). What may be going on here is that this information is 20 yrs old and since then companies like J.J. (available through Eurotubes) have sprung up and produced much much higher quality tubes.

I’d love to hear Mr Mitchell’s more recent opinions on tubes. I wonder where he is these days?

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My 2204 – Initial Exploration

This post talks about how I did an initial exploration of my Marshall JMP 50 Watt ‘Master Volume’ (model 2204). As a continuation of my previous post, here’s an update:

  • The multimeter arrived! It’s incredibly basic (it’s even a manual ranging device, which means I must be careful) but very well built.
  • I couldn’t get hold of a 1k Ohm wire-wound resistor to make my own capacitor discharge doohicky, but unless I was going to be desoldering components I left that for later.

So my goal was to use the voltage chart contained on page 178 of the Tom Mitchell book to check my own amp. I was hoping that my amps values would be “within 10%” of the published values. To do this I needed to measure voltages “at the pins” of the various tubes in the amplifier. That meant I needed access to those pins in a safe manner. The last thing I wanted to do was injure/kill myself so I followed accepted procedure by making sure I only ever used one hand to place the red probe onto the pins. The black probe was clipped onto the chassis (so I didn’t even have to use my other hand to hold the black probe, which was behind my back, and therefore unlikely to touch anything).

2204 and Multimeter

The 2204 chassis on it's back ready for probing.

I unscrewed the back panel of the amplifier, and then removed the 4 screws that secured the chassis to the cabinet. When the last screw came out, the chassis kind of plopped forward due to the sheer weight of the transformers. Probably 90% of the weight of this thing is those transformers. I then carefully picked up the chassis from inside the cabinet and placed it upside down on top of my 2×12 cabinet (which was itself on it’s side). This gave me easy access to both sides of the chassis, and non of the tubes were touching the supporting surface, and therefore not being strained mechanically. It occurred to me that I really should build a support frame if I was going to do this on a regular basis; something that would allow me to flip the chassis about easily without having to risk touching components inside.

The rabbit's nest of wiring

The inside of my 2204 wasn't exactly pretty

There was more risk involved as the inside of this particular amplifier wasn’t exactly pretty. After looking at the insides of other people’s custom builds, my 2204 looked like it was thrown together correctly but with little regard for “craftsmanship” or “functional beauty”. I was hoping for this kind of attention to detail, but was disappointed to find a rabbit’s nest of wire and seemingly shoddy soldering. Firstly I made sure the black probe was safely clipped to the chassis, and that anything and everything was out of harms way (including some of my pets which were messing around on the floor. I made sure they were out of the room). Secondly I attached the speakers, as it’d be very bad to power up the amp without them; the speakers are “part of the circuit”. I then turned on my multimeter and dialed it to the highest DC voltage setting.

2204 and the multimeter

You can see the black probe clipped to the chassis, on the left. The multimeter set to it's highest DC voltage setting. The Mitchell book and the 2204 chart ready on the right.

The assumption here is that all the voltages measured are “in relation to ground”. It’s not like testing individual components where you might measure the voltage drop across a resistor, or test for continuity. It seems all voltages appearing on schematics or charts like the ones in the Mitchell book, are all about their relationship to ground, hence clipping the black probe to the chassis and simply touching the red probe to the appropriate pins.

So, now I was ready to measure something! I thought I’d start with the amplifier on standby as I knew from reading that all this did was provide power to the pilot light (in this case, inside the rocker switch) and power to the heaters (filament) inside each tube. I could see that each tube was indeed glowing and the pilot light was on, so with my red probe I headed for pin 9 on the first ECC83 with excitement. Nothing. Nothing at all. Zero.Ā I tried pins 4and 5 , which turns out are soldered together. Nothing on those either. So, first lesson of the day….the book goes on to describe the voltages expected on the heaters of a 12AX7ECC83…3 volts. Ok, so where’s my 3 volts? Well, Mitchell is very clear to point out that the voltage on heaters is AC, not DC. That requires a different multimeter setting. Once set correctly, I got a nice steady 3 V AC on all my tube’s heater pins.

Right, so back to the other pins.This was where it got weird. I was getting great numbers as I made my way around the ECC83s. It all made sense until I got to the EL34 tubes. According to the Mitchell chart I was to expect a voltage on pin 6. But it was clear from looking at the pins that nothing had EVER been soldered to those pins. I did some poking around and found that pin 6 on an EL34 is officially “no connection”. Mitchell’s own diagram in the book agrees with this page.

I was able to measure DC voltages in my 2204 that were within 10% of the ones in the book, which I took to be successful. I also took this to mean that nothing was fundamentally wrong with the way voltages were in the amplifier, so my odd metallic resonance may be coming from something mechanical. I decided to take a break so I turned off the amplifier and unplugged it to have a think and perhaps read some more.

2204 in "tube tapping" mode

With the amp in more normal repose,with a guitar attached to reproduce the metallic resonance.

One particular test in the Mitchell book describes the act of tapping the tubes with an insulating object (such as his much touted Sharpie). So after getting the amp the right way up and turning it back on, I plugged in a guitar and fiddled with it to get it loud enough to reproduce the errant noise. I then found a nylon pen of suitable length and proceeded to gently tap the output tubes. Sure enough, one of the EL34 tubes made a very distinctive “thonk” when I tapped it. The other was completely silent. I believe I have found the problem and I think it simply comes under the term “bad tube”. My plan today is to buy a matched pair of EL34 tubes. Mitchell unconditionally recommends Groove Tubes or Mesa Boogie tubes. I’m going to shop around for a decent price obviously, but will stick to these recommendations for this first effort. That’ll be another post as this one’s way too long already.

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An introduction

I am starting this blog to help document my experiences as I learn about how tube amplifiers work.

Back in the mid eighties I was bought my first electric guitar, which was a Les Paul SG copy made by Hondo. I also was given a Fender Champ. All I knew at the time was that it was really small, and had no reverb. To me that was somewhat boring, so within months I traded it in for a transistor based amp branded with John Hornby Skewes. It was brown and heavy, but it had reverb!

A few years later I saw the light and bought an old Marshall 50W (the 2204 model) for about 80 GBP ($150 in today’s money I suppose). I had to have spells cast on it to get it to work, but for what amounted to $200 I had a gem of an amp. I purchased an Orange 4×12 from out of the paper, and was soon up and running. Henceforth I discovered “that tone”…you know the one…Eric Clapton..Peter Green…Angus Young. Not much in the way of distortion by today’s standards,but soooooo much sustain andĀ sensitivity.

Since that day, tube amps have always been a mystery to me. I know how a tube works…not hard. But how it all fits together is somewhat confusing to one raised on digital technology. I’m old enough to have owned and played vinyl, but I’m also young enough to know that all that analogue crap is just a horrible compromise.

So, this is my effort to come to grips with basic electronics, with these goals in mind:

  • Repair my old Marshall. (It is now noisy and resonates)
  • Build a tube amp, from existing designs.
  • Modify said tube amp.
  • Design new a magical tube amp.
  • Profit!!!
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