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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