Thursday, February 16, 2012

AC15HW1 Modifications (Part2)

After living with the modifications I made to my amp for a few weeks now, I decided to make a couple of extra small changes.

I removed R17 completely which increases the gain of the Normal Channel even further. This also reduces the effectiveness of the Bright Switch as the Normal Volume control is increased. At full volume the Bright Switch will not have any effect, this should work rather well as by that stage the PI (Phase Inverter) is heavily clipping anyway and the Bright Switch was often too harsh at these high gain levels. This is essentially how the Top Boost channel functions too.

I am also replacing C7and C10 with 0.1uF (100nF) values. This is the original value of these capacitors in the amp, but I will use higher quality Mallory 150M capacitors. I had reduced these to 0.047uF during my last modification session because I had those values on hand at the time. I don't expect this to make much of a difference when playing clean but it may effect the gain structure when the amp is over-driven, especially as these feed the power section and the slightly lower frequencies that get passed along to the EL84 tubes can effect their clipping response.

I have also upped the value of the three grid stoppers to 68K.

I really like the extra gain in the Normal channel, it makes it more flexible and does cut some of the brightness back from the Bright Switch so I have kept the original 220pF capacitor value. The changes to the coupling capacitors didn't really make a lot of difference which is what I expected, if anything the amp seems a little warmer sounding but its a pretty trivial difference.

Overall I'm very happy with the mods I made to the amp, they really are worth the effort and it doesn't cost much to change these components so I would recommend it to anyone with a hardwired AC15 or AC30 for that matter.


  1. Hi Nik,
    I implemented most of your mods today before a gig and the amp came to life ,
    Before the mods it was brittle and harsh, almost metallic when it went into clipping, by lowering the filtering with the new caps (I used JJ's ) it took the harshness off, I also changed all the signal caps to original NOS 60's mustard caps, the improvement was very audible. Much fuller and rounded. I left the C7 and C10 as 0.1uf like you did in part 2 , I changed the smaller caps to Silver Mica and to the new values you suggested, And drop R17 to 180K (didn't have 160K) , all that's left to do is change the 10nf caps on the power section to 100nf and I'm done.

    With my LPJ it sounds amazing, such a big crunchy overdrive compared to what the factory amp provides. It's turned a nice amp into an amazing amp and a keeper, Thanks for sharing your mods Nik, I highly recommend the NikPlayer Mods , you won't be disappointed .

    Mark 'Watto' Watson

  2. Your mods look awesome, I'm very tempted to order all the parts for it right now. Any chance you can get a demo of the amp up soon?

  3. Hi Nik, completed the mods a few weeks ago and the difference is amazing! Even the singer noticed! The only thing I don't like now is the Hot mode, it's still too bright. What are your thoughts on taming the highs a little to make it more of a mid-boost?

  4. The Hot Mode switch just lifts the tone stack out of the circuit which is why its bright. You could try putting a small cap across the switch so that high frequencies still get "loaded", you would need to experiment with the value to taste but 47pF would be a good start.

  5. Thanks Nik, I'll have a go when I'm doing the signal caps and report back!


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