Review: Tube Rolling The ALO CDM

Sylvania 6021 Brown Label


  • Tight sound and yet very musical and airy presentation
  • Vocals/voices sound very full and seducing
  • More detail and better depth/layering in total
  • Wider sound stage. Stock tubes sounds thinner compared to this one
  • Bass goes deeper with better layering
  • 40°C


Sylvania 6021W Green label


  • Bass has slightly better layering but goes just as deep. Bass body is almost equal to the stock tube
  • Shows more detail, especially noticeable in the treble with better extension
  • Mids have an airier and smoother presentation
  • Smoother touch to it overall
  • 42°C



My fav tubes are the Mullard 6112, the Sonotone 5719 and the brown labeled Sylvania 6021. These tubes are great and even superb with slower paced music, jazz, blues, classic. If you listen more to faster music where a lot is happening at the same time I would suggest the stock tubes or the Motorola 6BF7.

With tubes there is no certainty they will sound exactly as I described. Where they were made, when and how all has an impact on their sound. It’s very well possible your tubes measure stronger than mine, and that can also impact the sound. At the same time that’s the beauty of tube rolling. If you have time and feel like experimenting, I can guarantee you that tube rolling will bring you a lot of fun. Pay attention though, it can get rather addictive.

Most of these tubes, except for the sold out Mullards and Sonotone (I think), can still be bought directly from ALO Audio. I suggest to get the brown labeled Sylvania 6021 from their shop before they sell out. A hint though: ALO sells the PCBs separately, so if you can solder and have a nearby tube seller, you can get your replacement tubes yourself. ALO also mentioned it is very hard to find large numbers of these tubes to sell. That means you have to keep checking their shop to see if new tubes have popped up. That also means that the tubes that are in the shop now can be sold out any moment. So my second suggestion is to buy a couple of the PCB boards and find some tubes on the web or at a local dealer. They just might have that sold out tube you’re looking for.

I’ll be keeping one of these CDMs here with me. I think I’ll load it up with the Mullard 6111 for now. I still have one pair of single triode and dual triode PCBs left, so if needed and if available I can test a particular tube for you if there are several requests for the same tube. I hope you enjoyed reading this short overview as much as I enjoyed listening to all these tubes.

Happy Tube Rolling!

4.6/5 - (16 votes)

Lieven is living in Europe and he's the leader of the gang. He's running Headfonia as a side project next to his full time day job in Digital Marketing & Consultancy. He's a big fan of tube amps and custom inear monitors and has published hundreds of product reviews over the years.


  • Reply October 3, 2015


    Wow no comments yet? Well I liked this article – the CDM sounds like a winner of a niche device. I’m curious how it would work with my Ortho-type headphones.

    • Reply October 3, 2015


      Thank you, it took a lot of work. ALO was supposed to link to here from their site but they still haven’t.

      I really enjoy the HE1000 with the CDM, I haven’t really tried my LCD2 and LCDXC with it. And both my EL-8s are boxedup (not that exciting)

      • Reply October 3, 2015


        Have you listed to the MD Alpha Prime at all? – Do you think that would be a good match?

  • Reply October 17, 2015


    How does it pair with with the HD650?

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