L2L / Super eXtreme / UHG-style amp build

Update May 6th: See this new post on what I turned this build in to. This build had too much gain and was too unstable.

Update April 16th: Finishing touches – see the bottom of the page.

Update: I couldn’t leave it alone… I thought I was done, but instead realised it’s better to have the 2nd gain in the usual position between stages 2 and 3, instead of between 3 and 4 as I had it.

This was a very quick change to make – removed one interstage divider, swapped the wiring to G2 around, and adding back in a divider. Before choosing values for the interstage divider I used a 1M pot test the values – came out close to 820k and 220k, very similar to Merlin’s UHG.

Additionally, I added in a bright cap across the G1 pot of the UHG circuit – this brings things very close to the Marshall 2203/04 preamp.

I’ve updated the below to reflect these changes. I’ll re-do the scope traces at some point in the future too – as I guess there may be more changes to make…

After extensive preparations and prototyping, I modified my previous build.

The result is a ~5W amp with the following switching options:

  • ‘UHG’ or ‘L2L’ – when in 4 stage mode, this activates the negative feedback around the 2nd stage
  • OT negative feedback – select whether the power tube cathode is grounded through the OT or direct to ground as usual
  • Selectable bypass options on the 3rd and 4th stages as per the original N5X design
  • Tone stack bypass, again as per the N5X

Background

The initial goal was to improve the frequency response of the L2L-style local feedback and make it a bit more versatile.

Along the way I realised it would be nice to have the local negative feedback switchable and, as a result, basically fell in to having a more Super-eXtreme-style build. Whilst testing this, I took inspiration from Merlin’s UHG design to achieve the voicing I was looking for.

A tip I got from the AX84 forum was to use LED biasing of the local NFB (the 2nd) stage to remove a lower frequency pole from the overall response. Trying this improved things (to my ear) and simplified my layout a bit. I also then tried LED biasing on the 1st stage and liked what I heard – I presume I’m getting maximum gain from the stage thus hitting the 2nd stage a bit harder than through partially bypassed cathode biasing.

I borrowed the idea of grounding the power tube cathode through the OT from the Super eXtreme and HO options guide. This is switchable. Also, I added in adjustable bias.

Changes to N5X

This is all achieved by taking the N5X and

  • making a new preamp turret board,
  • drilling a few extra holes in the chassis,
  • coming up with a custom dual gang pot by combining 470k log and 1Meg lin dual gang 16mm alpha pots,
  • doing lots of fiddly switch wiring, and
  • spending lots of time prototyping or simulating in LTSPICE

Pictures

Schematic and Layout

N5X2-mk2-v4
Contact me if you want the layout / board plans.

LTSPICE Files

Coming Soon…

Scope Traces

TO BE UPDATED
For scope traces of both the L2L-style and Super eXtreme / UHG modes see this page. Input signal was 1Vpp, 500Hz. Readings were taken at each grid in turn.

Sound Clips

TO BE UPDATED

Oscillation / Squeal & Hiss at High Gain Settings

There are two issues:

  1. With the gains dimed the amp squealed… Between 3 and 4kHz, and it could be triggered by tapping the chassis. Turning down either gain would stop it. The pitch could be controlled by the tone controls and MV.
  2. It also hisses at an annoying level when the gains are above 80%. I measured ~10mVpp at the 1st grid and ~1.2Vpp at the power tube grid… (The amp is pretty much silent with nothing plugged in.)

For (1), I spent a while chopsticking to no avail. I then used an unconnected oscilloscope probe and noticed that when it was close to the 1st stage grid the squeal would reduce. I thought this was due to the components positioned around that location, but no amount of moving helped or changed the tone. I then tried grounding the centre pin of the socket. No difference.

I then switched to using a socket with a shield for the 1st valve. This seems to have cured the squeal. Although I can’t then look at the lovely valve glow… UPDATE: This turned out to be a microphonic valve – an old Mullard. Using a new JJ or TAD solves this.

For (2), I’m used Vishay ultra low-noise resistors. Seems to help. The amp is quieter than a Blues Deluxe with a Tube Screamer in front of it when at similar gain sounds.

Output Transformer Negative Feedback

I implemented the negative feedback featured in the Super eXtreme and mentioned in the High Octane modifications document. Doing frequency sweeps to generate a bode plot using this feedback on and off (in either mode – L2L or Super eXtreme) I see no difference. And hear no difference as well… Perhaps I need to use the 16 Ohm tap. Or perhaps it’s the position of the capacitor to ground – maybe the bottom of the ‘normal’ Rk and Ck then needs to go to the OT.

Power

I calculate the ‘clean’ output of the amp to be 0.4W before the waveforms start to distort… So not much clean volume at all, which goes along with what I experienced, hence my other build.

The distorted output comes in at 4.35W. Close to the nominal 5W rating.

Next Steps

For there will be many… A couple of issues: a small bit of flubby bass and weird ‘scratchy’ distorted sound behind the note when at higher gains.

Using an EQ pedal and cutting 100Hz solves the flubby-ness. So I may replace the IR LED for the 1st stage bias and go back to a regular Rk and Ck. Likely 2.7k or 1.8k and 0.68u.

The scratchiness could be any number of things; cold joint, too much gain, etc.

Finishing Touches

I’ve installed the faceplate and also (borrowing an idea from Rob Robinette) a 5″ speaker to the head. This allows me to practice easily. It sounds quite good when used for cleans and light crunches – but heavier stuff becomes a fuzzy mess. Also, as the speaker is not that sensitive, I can have the master volume higher getting some nice power amp distortion.

All in all, I’m very happy with it.

© 2024 Tristan Collins
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