Tag Archives: P1eX

AX84 – P1 Extreme – grid stoppers?

The problem

While playing my P1 Extreme over the last weekend I noticed an odd sound when letting a note sustain. Even when run clean, the amp would produce a fuzzy/distorted version of the note and then within a second or so the whole amp would cut out.

Investigation

It didn’t seem to matter which guitar I used, or which note I played. This has meant the problem has been hard to reproduce. Still trying though.

I asked members of the Hoffman forum and they thought it was caused by oscillation, and suggested the following:

  • “Chop-stick” the wires inside the amp. i.e. use a non-conducting device to reposition wires in relation to each other. The proximity of low level signal wires (guitar input) to high voltage wires (power supply) can lead to problems.
  • Make sure the grid-stoppers are located as close to the tube socket pins as possible.

Upon reading the forum’s suggestions I realised that I knew nothing of what grid-stoppers were meant for. Also, looking at the amp’s schematic I saw that the input stage tube, V1, doesn’t even have a grid-stopper. Here’s an excellent discussion on why this might be, in the case of an AX84 amp. Secondly, the power tube’s grid stopper is located on the main board, with a wire linking it to the tube socket. This would appear to be an opportunity for improvement.

Here’s what I’m going to do:

  1. Digest this article about grid-stoppers on the Aiken Amps site.
  2. Get myself a chopstick and carefully poke around the amp while trying to reproduce the problem. “Lead-dress” might be the issue, and is relatively simple to fix.
  3. Move the existing power tube grid-stopper to be soldered directly to the tube socket.
  4. Add a grid-stopper (10k – 65k value resistor) to the V1 grid.

Work (not) done

I read the article about grid-stoppers and also some more specific AX84 project stuff. These articles were discussing why the P1eX doesn’t have grid stoppers on the pre-amp stages, and why the grid stopper for the output stage is on the main board, and not soldered directly to the pin on the tube socket. One poster reasoned that the pre-amp stage grid stopper was “missing” due to the fact that the original designer didn’t have RF problems in his house. THere is a note about this on the schematic. Who knows?

As for the output stage? I didn’t move the resistor. I didn’t make any modification to the output stage at all as I could not reproduce the problem. What I think I saw/heard was oscillation caused by resonance from the speaker cab. It was literally vibrating itself into oscillation.

However I did replace some of the cheap/crap/oversized Monster shielded cable that I had originally used. Now it has some decent shielded cable bought from Hoffman Amps. It’s listed here asMini shielded cable RG174″ I got about 5 feet, so that’ll last me a while. This also meant I was able to rewire the input jack properly and take advantage of the Cliff Jack’s switching feature. When a guitar cable is unplugged, the tip is now shorted to ground. It makes for a silent amp when nothing is plugged in.

I also added a shield for the pre-amp tube.

Outcome

So my P1eX sounds great again. My son and I had time for a jam yesterday, so I was able to use it for a couple of hours without any issues at all. These days I use a a 6L6 for the output tube, that I bought from Hoffman at the same time as the wire/shields. It doesn’t sound all that much different to the EL34, but I haven’t trully cranked it which is where I may hear differences.

Also I noticed the power transformer (PT) was warmer than I had noticed with a 6V6 output tube. After some brief reading I decided to not worry about it at all as it’s not so hot that I cannot keep my hand firmly on it with no problems.

Update!

I recently made a few little changes:

  • Switched back to a 6V6, and changed the output tube cathode resistor to a single 680 Ohm.
  • Replaced the last of the crappy Monster cable.
  • Moved the grid-resistor to the tube pin.

This led to the amp not producing any sound at all, which was worrying. I decided to check all the voltages, record them and ask a question in the Hoffman forum. You’ll find the thread, here: Odd voltages in P1 eXtreme.

The outcome was that I found a dry solder joint right where the cathode resistor/bypass-cap attach to ground. Once I removed old solder, and re-did these turrets all the voltages returned to within spec.

Dry joint in cathode ground

Dry joint in cathode ground. I removed and re-did thes joints inside the orange box.

Here’s a shot of the grid resistor in its new location right on the grid pin of the output tube:

 

Grid resistor wiring

Grid resistor wiring and heatshrink

Eye-candy

I noticed some blue glow in the output tube. I’ve read enough to know not to worry about this either as it’s apparently normal. But I used it as an excuse to take a long exposure shot of it in relative darkness. This was a 20 second exposure…

An Electro Harmonix 6L6 in an AX84 P1 eX

An Electro Harmonix 6L6 in an AX84 P1 eX....ain't she purdy?

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What does an AX84 P1 eXtreme sound like?

From the posts in this blog you might know already that my first (and only, so far) amplifier build was a P1 eXtreme, which is a design from the AX84 ‘movement’. It has these characteristics:

  • Single-ended, i.e. Class A output stage, using a single pentode.
  • A two-stage triode-based preamp. My build is using a single JJ Electronic ECC83S (12AX7).
  • A simple tone stack sitting in between the two triode stages.
  • Capable of using multiple 8-pin tube types. My build is using a JJ Electronic 6V6 pentode.

Since I built it I’ve been playing it as much as possible, trying to get to know it’s sound and feel. The only decent amp I’ve ever owned prior to this was a Marshall 2204 50 watt head, which in itself is lovely, but it is way too loud for good tone to be had at home. This was the major reason why I built the P1 eX in the first place. But the question I am most asked is “what does it sound like?”. So this post is to present some recordings I did this weekend.

P1-eX with Tokai Les Paul copy

P1-eX with Tokai Les Paul copy, my 2×12 cab with the SM57 placed scientifically ‘in front’

Setup

I have a very crude recording setup at home. My ‘office’ is an 8’x10′ room with wood panels and a reasonably high ceiling, which is terrible for recording. But these didn’t come out too badly I suppose. Here’s the setup:

  • Apple iMac with built in audio input.
  • Nicely cheap Behringer mixer acting as mic preamp.
  • Shure SM57 placed ‘somewhere in front’ of the guitar speaker.
  • A custom build (I made it myself) 2×12 cabinet largely modeled on the THD design, with Celestion G12T-75 speakers I got off eBay years ago for $50.

The ‘made-up-as-I-went-along plan’

I decided to start with the master volume up full, and the preamp gain down low. Then I’d play a bit, crank the preamp gain a bit, play a bit…you get it. The idea being that one could hear the entire range of the amp from quite to full-on cranked. I also wanted to show the difference between the typical Les Paul type guitar and the typical Strat type guitar.

Recording One – Les Paul

About 5 years ago my wife bought me a fantastic Tokai Les Paul. It’s a pretty damned good copy of someones notion of what a ’59 sun burst would be like. I love it. It plays like butter. (Towards the end listen for my apology to my wife…apparently a cranked P1 was enough to completely freak-out the dog). Pickups-wise I started with the neck and made my way through both pickups on and then the bridge only. As is fairly typical when and amp is cranked you end up with a good tone on the bridge, but then you get mush when you switch to the neck.

Click the orange/white play button for the audio clip: 

I was standing about 4 feet away from the amp which was the sweet spot where feedback was easy to control….right up until it was dimed. You can hear at the end (if you last that long) where she just wanted to squeal whenever I stopped playing. So it must be said, this amp is great just below ‘full-on’. The recording doesn’t do justice to the feeling of being in the same room as this amp, but we’ve all been there, right?

Recording Two – Steinberger

This recording is with my Steinberger. It has the so called ‘7’ configuration of pickups, which means an 89 in the neck position, an SA in the middle and an 89 in the bridge. The 89s have the ability to turn off a coil and pretend to be SA pickups. This means I use this for strat-like tones rather than Les Paul tones. So this recording starts off with the neck pickup in single coil mode. In fact I never use humbucker mode on this recording at all.

Click the orange/white play button for the audio clip:

While I like the tone of a strat-type guitar with this amp, I am still so very biased towards Les Pauls. i.e. if Eric Clapton calls me right now and said “I need another guitarist tonight” I’d take my Les Paul.

Recording Three – Les Paul + a lame tune

This is a simple jam track that I made with my son Dylan. He was noodling around on the keyboard and created this lovely haunting chord pattern. I then ruined it by putting drums and bass on it. Anyway, the ‘solo’ guitar is the Tokai in the middle position, with the neck volume backed off a bit, and the P1 preamp gain at about 80% and the master volume at about 30%. Same recording setup otherwise, in fact the picture above was taken right after I recorded this.

Click the orange/white play button for the audio clip:

I love this tone. It’s what I always end up dialing up on any amp and the P1 gets me this tone at very low volume. That was the whole point right? 🙂

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Bias adjust for my P1 eXtreme

Variable cathode bias option for P1 variant amplifiers

Variable cathode bias option for P1 variant amplifiers

As my P1eX is pretty much done (I have to revisit the grounding scheme as there’s a fair amount of hum when cranked) I’ve been meaning to get around to trying different output tubes to see what differences I get in tone. Initially I chose the 6V6 tube which should give me a “Fendery” tone…whatever that means. Admittedly it does sound “Fendery” when it’s doing “light crunch” with a Strat in position 2 or 4, but when it’s cranked it gets “Marshally” very quickly. Not that I’m complaining.

So here’s some tubes I can try , with their recommended cathode resistor values:

  • 200R / 5W – KT88, 6550
  • 330R / 5W – EL34/6L6
  • 400R / 5W – KT66
  • 600R / 5W – 6V6

Right now the amp is cathode biased, so I must also add the fixed bias modification. This will allow me to measure voltage across a 1 Ohm resistor going from the cathode pin to ground (therefore giving me current). See the diagram to the right for the circuit. It’s quite simple really.

UPDATE

I was having difficulty finding a 1kOhm/2 watt potentiometer, but I have located a part here on the Newark site.

I can get “bias tip jacks” from the Hoffman site with no problems.

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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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Beginning assembly of P1eX

The quiet before the storm. Note the pile of baggies from Tube Depot...most handy!

So the previous week, in an effort to have everything ready for assembly, I order the remaining parts. This amounted to all the resistors, capacitors and diodes that make up the circuits on the board. Finally, I needed tubes of course. I ordered from these folks:

  • Tube Depot – They have all kinds of manufacturers in stock, and the shipping is fast and high quality. Each component arrived in its own ziplock baggie with a decent label. Very cool for a newbie like me.
  • Eurotubes – I decided to go for a 6V6 in the output section and Eurotubes was the only place I could get a JJ 6V6 in a one lot. Every other place wanted to sell only pairs, and their prices were high anyway. Naturally this site is a big proponent of JJ, but others online have sung their praises. If these work out I may retube my 2204 with a set from these guys.

Before starting I laid everything out on my dining room table, and made sure I had plenty of light. To ensure accuracy I had both the schematic and the layout diagram available. And so as to maintain diplomatic relations with my significant other (i.e. not burn the table), I used a print out of the AX84 theory document for the P1eX as an “earth friendly soldering station”. Fitting, really.

So, now it was time to begin attaching stuff to the turrets. I started with the wires that go under the board as this made total sense (and the guide told me to). I had already drilled holes next to the appropriate turrets so that I could keep the solder joints above the board, but still route the wire underneath. Apart from the build guide recommending this, I was also keen to never have to remove the board once built as having previously repaired my Marshall 2204 it was a royal pain to have to remove the PCB to de-solder components.

Under-board wire

A wire routed underneath the board surfacing to attach to the turret

Bare turrets

Bare turrets with adjacent holes waiting for wires. The larger hole is for the chassis stand-off.

After all these were in place, and making sure I had marked these as ‘done’ with a hi-lighter on the layout diagram, I started on the right hand side of the board with the power supply diodes. As I worked my way through the layout I was troubled by a few things:

  • Progress

    Progress. The first few joints drank solder like it was going out of fashion.

    Is there a “rule of thumb” to help decide where/how to solder a component? i.e. do you put caps/resistors/diodes inside the turret, and wrap wires around the turret? Or is it the other way round? I did both as I saw fit, but it’s not particularly neat and tidy.

  • Do you wrap as many components around each turret all at once and then apply solder? Or do you do them one by one? Again, I did both, but the latter meant components were heated multiple times, therefore went under more thermal stress.
  • How do you stop the turret itself wicking all the solder away? Early on I found that if the turret got too hot it would wick solder until you filled it up. And in fact when I turned the board over there were quite a few lumps of solder that had fallen through and pooled on the paper I was using as an ad hoc insulating surface. I think this is down to technique. Later on I got better at keeping the heat on the parts as little as possible which meant I could minimize the amount of solder. This must’ve been due to the lower part of the turret remaining below solder melting temperature, thus no wicking.
  • The ground bus along the top of the board cried out for a single wire to be connected from turret to turret. I decided to use my 20 AWG solid core wire (as I had been using that all along) but it didn’t come out looking especially nice. I’ll do it differently next time…perhaps thinner wire?
99% done

The board with a few missing/incorrect bits. Note the wiggly copper ground bus along the top

Along the way I realized I had screwed up my order to Tube Depot. A missing component here, a slightly-wrong component there etc. Never mind, I can go to Radio Shack during the week as it is only 5 minutes walk from the office, whereas from home it’s a 20 minute drive. Normally this would’ve irritated me, but I still had plenty of interesting stuff to do after soldering all the remaining components and wires.

So time to put the board aside and do some work on the chassis. The day before I had sprayed the amp’s chassis with a black ‘Rustoleum’ hammered-finish paint. It came out surprisingly well seeing as I was rushing (over excited) and humidity and temperature weren’t ideal. 24 hours later the paint was dry so I started adding pots and lights etc. This part was easy as I had already ‘dry fitted’ the parts a couple of weeks ago. I definitely did not want to have to drill anything out again once assembly had begun.

Once I had the transformers in place I decided to proceed with the filament wiring. I had some green and black solid core wire, so I used that to make the connections. Twisting the green wires from the power transformer wasn’t so easy as they are stranded. Stranded wire doesn’t seem to like keeping its form once twisted. We’ve all experienced a neatly coiled wire that transforms into a tangle once you let go of it. Luckily though the run from the PT to the pilot light is short. Twisting the black/green solid core was a breeze in comparison and I soon had plenty of wire to connect the pilot light to the output tube and the first preamp tube.

Filament/heater wiring

Green wires from the PT, to the pilot light, then my green/black wires going to the tube sockets.

Filament wiring at the sockets

Filament wiring at the output tube socket.

Overall I was very pleased with the weekend’s progress. It’s really starting to look like an amp now! I’ll have the missing/wrong components in order this week, and I might even be able to get some sound out of it within a couple of weeks, time allowing.

PT/OT

The PT and OT in their new homes. You can see the full effect of the hammered black paint.

Chassis so far

Chassis so far. I must order some proper knobs for it.

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