AC30HW Filter Cap Mod

A few weeks ago Nik Player told us about his modifications he did on the AC15HW. Because of the fact that the AC30HW and the AC15HW ist nearly the same, I tried his mods at this amp and I really can recommend them. So below you see his mods described and adapted for the AC30HW. Original article of Nik Player is available in his blog.

WARNING - These amps contain lethal high voltages which can remain present even when the amp is unplugged from the mains supply, do not attempt any modifications unless you know what you are doing.

Made changes:
- Reducing the DC filtering capacitor values and using better quality caps.
- Changing to better quality signal capacitors.
- Adding grid stoppers to the Top Boost and Phase Inverter to avoid blocking distortion.
- Reducing (or even shortcut) R17 to give more gain in the Normal channel.

These simple changes really liven up the amp and remove some the modern "stiffness" associated with too much DC filtering.
All components should be replaced with voltage ratings no less than the originals.

Signal Cap Changes:
C13 - Silver Mica 220pF (Bright Switch Cap)
C22 - Silver Mica 100pF (TB Bright Cap)
C21 - Silver Mica 500pF (TB coupling cap)
C23 - Silver Mica 47pF (Treble Cap)
C24 - Mallory 22nF (Middle Cap)
C25 - Mallory 22nF (Bass Cap)
C7, C10, C12, C14, C27 - Mallory 47nF (Stage Coupling Caps original values 100nF)

Filter Cap Changes:
C1 - F&T 500v 22uF
C2 - F&T 500v 22uF (original value 47uF)
C3, C4 - 630v 100nF (original values 10nF)
C5, C6 - F&T 500v 10uF (original values 47uF)
C19 - F&T 500v 30uF (original value 47uF)

Resistor Changes:
R17 reduced from 330K to 160K or even shortcut for maximum gain (neccessary if you want that Brian May playing style)
Add 7.5K 27K resistors to the input pins 2 & 7 of v2 Phase Inverter
Add 7.5K 27K resistor to the input pin 2 of v9 TB channel

I have tested 27K grid stoppers with no detrimental effects to high frequency roll off and these have helped smooth the break-up when the amp starts to overdrive so I would recommend them over the 7.5K.

I used 500V Silver Mica for the small signal caps, 630v Mallory 150M caps for the larger signal caps and 500V F&T caps for the filtering. Resistors are all 1/2 watt. The grid stoppers are soldered directly between the tube base pin and its associated connecting wire. Click on the images below for larger versions, I have highlighted the changes with green and magenta circles.

Pictures and schematics are from AC15HW original from Nik Player´s blog. Position and changed components are the same as on the AC30HW.

Original mod by Nik Player, adapted by Jens Winkler

 

[jattle]

Original AC30-Guide blog comments on this article:

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Josh on 23. May 2013 at 11:38 said:

Great page! Just thought I’d add that I tried 47nf caps on the PI output (C7 and C10) and thought the bass was slightly too light, so increased the value to 100nf and that made it thicker to my ears. I think 150nf was one of the original values for the “normal” AC30 but they don’t do a 150nf Mallory so you’d need to pick a different cap – Sozo and TAD both do one in that size.

Chris on 6. February 2014 at 3:06 said:

For c3 and c4. What is the difference between 10 and 100nF? How does this change the amp sound and feel?

Josh on 10. February 2014 at 12:51 said:

Hi Chris,

the 10nf caps there are in parallel with the first filter cap (47uf) and EL84 cap (22uf). They help by smoothing high frequency B+ ripple that the 47 and 22uf caps wouldn’t be able to deal with (the bigger the cap, the slower the reaction time which means high frequencies get through).

In amps with solid state rectifiers, various IC chips, digital effects etc they perform an important task keeping high frequency noise out of the power supply. In the AC30HW they don’t do much at all! I’ve tried 100nf, 10nf and removing them completely, and I couldn’t hear a difference in the sound or feel of the amp, or how noisy it is.

Zach on 5. July 2015 at 23:16 said:

Do you have a schematic, like the one above, for the AC30 rather than the AC15?

jattle on 6. July 2015 at 10:43 said:

It’s pretty the same in the hw. Just 4 tubes instead of two.
If you have a specific question on the plan, I may can help you.

zeze on 10. April 2016 at 21:14 said:

Hmm is anyone else experiencing problems with the pictures
on this blog loading? I’m trying to determine if its
a problem on my end or if it’s the blog. Any suggestions would be greatly
appreciated.

jattle on 13. April 2016 at 19:09 said:

Not with the pictures but with some tables. I hope I can correct it soon. Can you tell me where you had problems with pictures ?

Don Gallop on 11. January 2017 at 19:04 said:

Power Section to 1964 Spec
R61 & R101 (and 2 more not in schem for AC30) Change from 3.3k to 1.5k (per Vox & Matchless)
R10 & R16 from 10k to Bypass
R79 & R104 (and 2 more not in schem for AC30) from 470R to 100R (per Vox &Matchless)

Hey Guys, I know this is an old thread and possibly worn out by now But… I’ve experimented with all the mods on this page and feel that some of them get you closer to a 1964 AC30 “tone” (and some closer to the Brian May “tone”) but somehow for me, they come up short of capturing the Vintage Vox “FEEL” which to me is the Holy Grail of Voxyness. My Experiments have been done on the AC30HW2 with Blue Speakers and Not the AC15HW2 so there may be some slight differences.

That said… my experience has been that the “magic” happens when the power section is working really hard and there are several things you can do to get the “magic” back into this respectable HW reissue. I’m assuming that the Vox engineers were going after the reliability factor when they made changes to some of the operating parameters of the EL84 power section… but if you know Vox tone and feel, then you know that you’ll need to push the power tubes to their limit before it really sounds like a Vox. So… try the following changes with or without any of the previous mods and it’s guaranteed to put a sloppy smile on your voxy face.

To put the Power Section back to 1964 Spec:
R61 & R101 (and 2 more not in schem for AC30) Change from 3.3k to 1.5k
R10 & R16 from 10k to Bypass
R79 & R104 (and 2 more not in schem for AC30) from 470R to 100R

Blessings,

Don

Rene Wehrmann on 13. December 2021 at 19:07 said:

Hey Don,
wow your comment is 4 yrs gone, but maybe you‘ll see it..
what does it mean when you say put R10 and R16 from 10k to bypass?
Did you remove and put a wire in?

Blessings
René

VOX on 15. December 2021 at 20:11 said:

Hey René,
maybe I can answer this. Bypass means putting a wire in instead of R10 and R16.

All the best
Jens

Bjoern on 2. November 2021 at 20:29 said:

Hi Jens and Vox AC30 friends,

I found this info very helpful and I am going to apply the changes to my ac30hw. I just got one question to be on the safe side:
C3, C4 should be replaced by 630v 100nF (original values 10nF). Is the increase from 10 to 100nf correct?
And is there a cap brand you recommend for this replacement?

Thanks a lot in advance!

All the best
Bjoern

VOX on 8. November 2021 at 19:16 said:

Yes that´s correct