Odd noise then cutoff in AX84 P1 extreme

[simonallaway]

This weekend I noticed oddities in my P1 extreme (single-ended EL34). With the amp running clean and quiet, I play a single note and let it sustain. A kind-of fuzz builds up underneath the note and then the entire signal cuts off. Playing the note again will 'bring it back' sometimes with this cutoff, and sometimes not. I'm trying to find a way to reproduce it every time.

I haven't had a chance to do the usual voltage checking and exploration for bad solder joints, but I thought I'd post to see if this is recognised behaviour. I could not find anything in the archives that seemed similar.

I've tried it with a 6V6 (after changing the cathode resistor) and I got the same result.

Thanks in advance.

Simon

[tubesornothing]

P1 extremes are notorious for oscillations.  Chopstick is the order of the day.  Push around wires, make sure grid resistors are right on the tube sockets.  I seem to recall a build guide specifically for dealing with oscillation issues, somewhere on the ax84 site.

[simonallaway]

Interesting. I'll try the chopstick method.

Also, looking at the schematic, I see that there are no grid resistors on the input stage of the P1 Extreme. So I plan to install something between a 10k - 68k directly on the socket to see if that helps.

Thanks for the pointer!


EDIT... fixed broken link.   sluckey

[tubesornothing]

As for grid resistors, it might help on V1, but I was thinking more on the power tube.

[simonallaway]

That makes sense. For some reason I was focussed on the input stage, but I should rewire that one too. The layout according to the AX84 docs has the power tube's grid resistor on the main board. (see bitmap below)

So is it the length of the wire from the grid resistor to the grid or the location that is the issue? Or both?

[tubesornothing]

It could be the issue.  Try chop sticking around first, moving wires about checking solder joints.  If that doesn't fix it, also try preamp tube sheilds.  Pay special attention to any large signal wires next to sensitive wires - e.g. the OT wires close to the signal input to the power tube grids.

Failing that, then try moving the power tube grid resistor to the socket.

Its a pain to find these oscillations, but they can be found and dealt with.

[jojokeo]

Grid resistors should always be on sockets w/ min to no lead length, not on board. Keep grid wiring shielded or as short as possible for all runs. Also try not to mix chassis ground w signal ground.

[simonallaway]

Just to close the thread, I finally had time to address this 'odd noise' issue. Here's what I did:

1) 'Chopsticked' around the amp. The trouble here was that I could not reproduce the original behaviour, so all this did was reduce a bit of hum. So a partial win there.

2) When I built the amp I took a shortcut and used shielded cable from a stereo RCA plug cable. This left me feeling dirty, so I bought some decent shielded cable from Doug and replaced it. This may have reduced some RF too.

3) Wired my input jack properly. I realized when replacing the shielded wire that the input jack was not wired to take advantage of the Cliff jack "switch". So now the tip shorts to ground when no guitar is plugged in. Cool.

4) Added a shield to the preamp tube. I cannot tell if this has improved hum. If it has it's small.

5) I did not touch the grid resistor's location as I wasn't able to get the amp to produce the same noise.

So I haven't been able to get it to oscillate at all. This leaves me thinking that it might have been due to physical oscillation/resonance due to the fact that I had the bare chassis on top of my speakers. Still, I got to poke around inside and learn some more.

Thanks everyone! (here's a picture of it running with a 6L6 glowing sweetly)