Category Archives: Tubes

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.


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.


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.


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


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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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.


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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Circuit board preparation

Bare board waiting for holes

Here's the bare board laying on the scaled drawing. The sizes match up quite well.

This weekend’s fun was all about getting the circuit board ready for population with components. Secondly I had to make sure it fit in the chassis correctly as that was the only thing holding me back from painting the chassis.

Drilling holes using the fence

Drilling holes using the fence made for fast and accurate work.

My first step was to make sure the drill plan for the board was correct. I ended up using the main layout drawing as my template as it was closer to the size I wanted. I ended up getting it copied and scaled by 114% at Kinkos so that it was approximately 12 inches by 3 inches. I cut out the template with a craft knife and used masking tape to stick it to the board. You can see from the template that there are 4 distinct rows of holes (not including a few random holes for wires or the ones to secure it to the chassis). This meant I was able to use the fence that came with my drill press to keep these lines straight. Drilling with a brand new 3/32nd” bit was quick and easy with this material. I made sure the drill speed was nice and high (about 3000 rpm) and used slow pressure. It only took about 10 minutes to drill all the holes. Where I new the holes were for mounting to the chassis, I used a 7/16th” bit.

The middle hole

The middle hole drilled with the board snugly attached to the chassis (temporarily)

Once drilling was complete I needed to make sure it fit in the chassis’s existing holes. I knew there might be an issue as I had used a scaled drawing, and sure enough when one pair of holes were lined up the other two were off by about 1/8th of an inch. I used a spherical grind tool with my Dremel to enlarge the holes in the circuit board. It took less than 30 seconds to make everything fit neatly. With the board attached to the chassis with 4 screws and the standoffs, I used the existing ‘middle’ hole in the chassis to drill all the way through into the board.

Staking tool

The staking tool setup in the drill chuck. The vice bolted neatly onto the bed of the drill press for added stability.

Now the time had come for “staking the turrets” (such a medieval term). All this required was for me to install the staking tool according to the instructions, place a turret in the tool, place the board over the turret using a free hole and then pull firmly down on the drill press. This action squeezes the conical tool into the bottom of the turret causing it to flare out. This causes it to grip the board firmly. I had initially tried to do this by loading up 10 or so turrets into free holes and then flaring them, but they’d always fall out so any time savings I made were lost by searching the bench for dropped turrets. Once I got quick at loading turrets into the bottom part of the tool, and then placing the board over the turret I became quite adept. Total time for all turrets was about 10 minutes.

More staking of turrets

More staking of turrets

All board material, turrets and the staking tool were purchased at Hoffman Amplifiers.

Next steps: paint the chassis (now I have all the holes finished and soldering components onto the board. (must remember to order said components).

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In an ideal world…

…this is what all tube amps should look like:


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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EL34 tube prices

After my previous post outlining how I found that I had a bad output tube, I naturally needed to replace them. My 50W 2004 has two EL34 tubes in the output section, so I must replace them as a matched pair. As I haven’t bought tubes since ’94 I know nothing about the best places to get them. Here’s what I found online:

  • Tube Depot – Wow! Their prices range from less than $30 for a pair of Electro Harmonix to more than $450 for a single Mullard tube. That’s just ridiculous.
  • Tubes and More – Much more reasonable, and they have Groove Tubes too.
  • The Tube Store – Not many in stock, but good prices
  • Sweetwater – All kinds of tubes, with ‘larger company’ return policies.
  • Audio Tubes – More insane prices.

It looks like the market for “vintage tubes” suffers from the same bizarre and irrational pricing justifications that I’ve only ever seen in vintage guitars and so-called high end audio components. There are plenty of snake-oil salesman trying to get us to pay stupid prices for a green marker pen to colour in the sides of your CDs. Or the type that’ll argue that analogue technology as a recording or playback medium is objectively better than digital recording. Here’s a great rant about those types.

In this case I’m going to go with what Mitchell recommends in this book, and get a pair of Groove Tube EL-34M from Sweetwater. They’re supposed to be a “reproduction” of “the classic” Mullard XF-2 “design”, which is apparently “much sought after” (here’s the press release). Let’s face it, my 2204 sounds incredible with the so-called crappy Russian tubes it’s had since ’94, but what do I know.

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