Tag Archives: tube

Finally, a functioning AX84 P1eX tube amp

Board installed in chassis

Board installed in chassis, transformer wires trimmed, heaters wired, switches wired.

Final components

The last post left off with me needing to order some missing components. I had genuinely screwed up the order, and wasn’t going to order 2 components online, so I ran over to Radio Shack during the week for a missing resistor, and a capacitor for the tone-stack. It took mere minutes to solder these in place.

I was worried that I had the wrong kind of capacitors in place for the 12AX7 cathode bypass caps. The schematic implied they should be polarised electrolytic, but the layout diagram specified both types. So it was a little ambiguous. Forum members on the Hoffman site confirmed that in that location it didn’t matter.

Wiring

By this point I was ready to install the circuit board into the amp and start wiring it up, but I wasn’t quite ready. The output and power transformer’s wires needed to be trimmed and wired in where appropriate. The guide recommends NOT wiring up the secondaries to the circuit just yet, this is all part of the power-up sequence. In-line with the AX84 build guide, I made sure I had the layout diagram and the schematic to hand at all times. This meant I could use my trusty green hi-lighter to mark progress as each connection was made.

This is where I made my first mistake. The layout diagram and the schematic wanted the black pair of wires from the power transformer primary to be connected to the mains switch. You can see, in the picture above, the white wire neatly capped and sitting there all smug in its incorrectness. My transformer, while being the correct model, had 3 wires (black, white and grey). I made a guess based on the diagram stuck to the side of the box the transformer came in, and went with black and grey. This is because Hammond made a “running change” to the primaries to accommodate the so-called modern line voltage of ~120V (as opposed to “traditional” 115V). Here’s the details on the Hammond site.

For those customers who are seeing too high a secondary voltage due to higher primary line voltages – use the White & Black wire and tape the gray wire…

Of course, I didn’t discover this until AFTER I had attempted to power it on. More on that later.

Wiring progress

Wiring progress. Preamp section and tone-stack.

Meanwhile, mistakes continued. In this picture you can see progress being made on wiring. All looks good, and I was trying so hard to be diligent and follow the layout (and schematic) but see if you can spot the mistake on the input jack. I didn’t until much later on. I put this down to never having worked with Cliff jacks (the shorting kind). More on that later too.

As I progressed through I realized something else. The layout diagram was in conflict with the schematic, or at least I had misread it. What looked like ground bus connections between turrets proved to actually be shield connections (the fat grey wires you can see in the picture to the left). So I had to remove some connections to fix that one. This was just down to me reading the layout incorrectly.

Initial power-on

Ready to try again, PT primaries corrected

Ready to try again, the PT primary wires have now been correctly wired with white and black.

Once I had wired up as much as I thought I could, (layout + hi-lighter = best practice), I proceeded to work my way through the Paul Ruby first power on sequence. All previous work had been done on my dining room table which gave me lots of space to spread out, but now I needed much more room and decent power as it really was time to fire this thing up. This of course meant I needed to use my one and only speaker cabinet, the 2×12 I built years ago. I moved everything down into my workshop in the basement.

So when it came to powering it up with no tubes or secondaries connected, it kept blowing fuses. I’d see a all too brief flash of pilot light and then nothing, so clearly it was drawing power, but too much. This was due to the PT primary issue mentioned above. I thought about it and decided to check the wiring again, which led to discovering the “running change”. The picture to the right shows the correct white and black wires, with the grey tied off and capped. Now I got a solid glowing pilot light, and good voltages at the PT secondary. Ok, cool. Turn it off.

At this point I got over (stupidly) confident, so I turned all the knobs to zero, plugged in the tubes and the speakers, and turned it on in standby for a couple of seconds. I did see the heaters glow, so I left it sat there for a minute or so. I got even more cocky, so I plugged in a guitar cord (I hadn’t been so bold as to have brought a guitar downstairs to my workshop), and flicked the standby switch into ‘play mode’. No fuses blew, and the tubes did not explode or melt.

With confidence surging, I plug everything in

With confidence surging, I plug everything in. That's an ECC83S and a 6V6, both from JJ via Eurotubes

Then I gently turn up the two volume knobs, just creeping each one up a little bit at a time, hoping for some sound. I suppose I was hoping for silence as this would imply to me that it was working. What I in fact got was crackling/hissing and even squealing when the knobs were turned right off. So after 20 seconds of this, I turned it off again in disappointment.

The time had come to stop. I’d done well that day to get this far, and I was definitely tired. So to stop myself from making any more mistakes I decided to call it a day.

The next day

I had a chance to poke at the amp some more the following day, so I went downstairs determined to get this damned thing working. I decided to re-check the power supply, and measure voltages at the tube pins. The schematic was good enough to specify what they might be, so it was easy to measure each one and write them down in pencil on the schematic. Good news, the voltages were all within 15% of the schematic. Cool, I did something right!

Input jack wiring

First noise

The amp all hooked up to my speaker cabinet and resting on a sophisticated cradle.

Hoffman forum members had suggested I check and recheck the jack wiring, both in and out. Their experience, and my sonic description, told them it was probably a grounded grid. And sure enough it was. The 1Meg load resistor was wired incorrectly, and also the shield on the wire going from the jack to the grid was soldered to the tip. Nice job Simon.

I made the corrections and prepared to power it up again. This time I had a guitar with me, so I plugged it in and left it leaning against the side of the cabinet on the bench. I hadn’t planned on playing it yet.

Ok, so I power it back on in standby….wait 25 seconds or so…and flick standby off.

Silence. Ok, good. Turn up the volume knobs a little…..more silence. Turn them both up about half-way….more silence, but a bit of hiss. I’m thinking this is odd, so I reached over to the guitar and strummed the open strings. CLAAAANNNNGGGGG! It made noise!!! Oh my god!!

So at this point I grab the guitar and strap it on. It’s a Tele-like guitar I built years ago, with a set of EMG pickups that allow for lots of flexibility. It has an 89-SA-85 with a 5 way switch, so I can dial in Strat and Les Paul tones easily, and there’s quite a range of signal strength coming out of the guitar so I can see how different amps behave. A great testing guitar.

A functioning "P1 eXtreme" amplifier

A functioning "P1 eXtreme" amplifier

Man this thing sounds good. At low volumes it’s very clean. Not too inspiring, but jazzy I suppose. Go about half way and it starts to come alive. The tonestack starts to have some influence on the signal so there’s different tones to be had. A very useful tonestack actually. I was expecting the same useless range that my 2204 has, but no.

And with everything on 10 it grinds and burns as well as my Marshall 2204 does. In fact thats a great way to describe this amp; when it’s cranked all the way, it sounds like my Marshall does in it’s sweet spot (which in itself takes a while to find, and is ridiculously loud). With the P1eX I just have to turn everything all the way up….what a dream! Induced feedback is present in copious amounts and is tweakable by simply adjusting one’s proximity to the speakers. The guitar feels incredibly “alive”..it’s the real thing. Don’t for a minute imagine that this amp is somehow a toy, or the very nature of it is a compromise. Far from it. This thing has tone…loads and loads of it. I’d be so bold as to say that I’d gig this amp now, in its current state. This is an exceptional blues amp. You could even do AC-DC with this. At low volumes, but with the preamp gain on about 80% it’s still got lots of sustain, but without the harsh fizz of 100%.

I’m clearly new to this amp so I will be playing it much more in the coming days. I have a variety of guitars that I can plug into it, so I hope to report later with some recordings to demonstrate.

What a day! 🙂

Lessons learned

  • It doesn’t matter how much you study the drawings beforehand; mistakes will be made. Plan to fail etc. I should know this from almost 15 years in software, but there ya go. My mistake with the input jack wiring was because I was rushing at the end of a long day of soldering.
  • Read all the instructions and build guides that you can find. The “official” guide is great, but others have gone through builds just like me and they are posted all over the internet.
  • Don’t be in such a hurry. I don’t know why but I kept wanting to “just get it done”. Why would I choose to hurry this experience away? The point of a journey is not to arrive, right?
  • When you’re a newbie like me, don’t guess. I could’ve destroyed a $50 power transformer by doing what I did with the PT primary wires. I was lucky.
  • I have a very patient wife and family. They put up with me taking over the entire dining room table, stinking up the place with solder flux fumes and leaving tiny pieces of copper all over the place, for two whole days. They didn’t complain once.
  • I have a great workshop, with lots of tools. It lets me do a lot of things I was previously not capable. I should appreciate that more.

Future plans

  • Make some recordings. Every time I turn it on and play, it surprises me. I must record it properly.
  • See if I can make it less noisy at high volumes. While it sounds good cranked, it’s noisy. I will revisit the grounding approach after checking against layouts and studying “the Merlin documents“.
  • Try different output tubes. I have some Groovetube EL34M tubes waiting to go into my 2204, so I can try one of them.
  • Build the cabinet to go with the amp. I can’t take it anywhere without that.
  • Take it to a local jam night and see what others think.
  • Modifications. Once I get to know its capabilities more I will know if I want to tweak it.

UPDATE! Here’s a post including some recordings of the amp.

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In an ideal world…

…this is what all tube amps should look like:

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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. 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 and 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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