Upgrades to the (made in China) AC50 and AC100

VOX China P1020388Hello and welcome back to the blog. This week we discuss an upgrade project for Sydney indie band guitarist, Cameron Birt. Cameron and the boys have built their guitar sounds around the Chinese-built VOX AC50 and AC100 amp heads, but they are well aware that these amps could be significantly improved from stock with some important component upgrades.


VOX China P1020384

We put together an upgrade project scope and quotation for the band, which would deliver maximum results for the $$$$ spent. Our recommendations were accepted and orders placed to source the agreed upgrade components.



VOX China P1020385

Our agreed upgrades were as follows:  output valves – current production Mullard EL34, a pair for the 50W and a quartet for the 100W, suitably matched and the amp rebiased accordingly. Preamp valves – current production Mullard 12AX7 (‘large plate’) for the V4/PI stage positions, and Mullard 12AX7/CV4004 (‘short plate’) for positions V1 to V3 inclusive, in both amps. At the time of this project, Evatco (Queensland) had a good stock of these valves. Transformers – replacement of output transformers and filter chokes in both amps. Power transformers left in situ.


VOX China P1020386While we install and recommend Mercury Magnetics transformers for their lush tone, for this project we chose to use transformers and chokes by Magnetic Components of Chicago, USA, marketed as ‘Classic Tone’. This choice achieved a cost-effective solution. The chosen transformers provided the closest physical dimensions as well as the required electrical specifications.

VOX China P1020387For the 50W head, we installed the Classic Tone part # 40-18025, and for the 100W head the part # 40-18026. The latter is the same transformer used in the Landry made in USA boutique grade heads, previously discussed in this blogsite. For both heads we installed the part # 40-18058 filter choke, which required the drilling of new mounting holes.

We avoided the complication of replacing capacitors in the signal path for this project – upgrading components on double sided boards is time consuming and not much fun at all. Apart from that, there is insufficient space allowed for our favourite caps. Magnetic Components provide detailed dimensions, etc, on their website which is of great assistance with these projects. Never underestimate the impact that a good filter choke can have on an amp’s performance. We have installed the Marshall style 3H (3 Henry) choke by either Mercury Magnetics or Classic Tone in many many projects now, always with positive results (yes, even the dreaded Marshall JTM600/JCM600 series amps).

We should point out that the recent production (China) VOX AC50 and AC100 have absolutely nothing in common with the famous amps of the 1960’s that the British Invasion groups conquered the world with. The current amps are definitely ‘modern’ designs with multiple gain stages and channel switching, more akin to a Marshall amp than a VOX. The end result on these two amps was a big “thumbs up” from Cameron and the boys with the VOX amps sounding great and exceeding expectations. We offer world-class service/MODs/upgrades on all VOX amps in 2016 and beyond ! Thanks for checking in to the blog – see you next week. IR.

2 Responses to “Upgrades to the (made in China) AC50 and AC100”

  1. Henrique Rodrigues Says:

    Hi there. I am the owner of an AC50CPH head as well. I still use the stock Tung-sol preamp valves and 2 JJ in the output section. I don´t know how better the new current production Mullards are. Could try one day. I don´t know how to compare the stock transformer to the one you have replaced too. Big improvement ? My transformers remained the same. But the secret for my real improvements in this amp are where you didn´t go : Inside the PCB board !!! You have to look into the schematic, trace it, understand it, and then you will see all the hassle is well worth it. Soldering is very time consuming, specially when you have to test and re-test different resistors and capacitors here and there and reassemble everything again and again.Try different guitars, etc. So many hours. But in the end, it was so worth it, specially for Channel 2 (and the fat switch as well….)
    I have opened a topic in TDPRI called “Vox(ey) mods AC50CPH”. Have a look. It took me a few months to trial and error and many hours. But it was well worth it !! Sharing it with all the AC50 lovers. Cheers

    • ivanrichards Says:

      hello, thank you for your comments which I have published, BUT you should keep in mind that this blog described work carried out to customers amps in accordance with an agreed scope of works and an agreed project budget; MODs to the PCB were considered but rejected as being less a less effective use of the customers budget (obviously I am charging an hourly rate); the customer employs both these amps as gigging & recording amps and was in the best position to compare the ‘before’ & ‘after’ performance of the amps; the customer was more than happy with the end result, which apparently was a significant improvement; however, every customer has their own particular priorities, which we identify with considerable communication before we commence the project; thanks, IR.

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