Spotlight on Tubes

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This article provides some information about tubes in conjunction with the AC30 amplifier.


Quick Facts
  • The AC30 is a cathode based amp with auto-bias. This means, you do not have to rebias your amp after you've exchanged tubes - just exchange them and you're done.
  • You should always get a matched set of power tubes.
  • Depending on the actual model, accessing the tubes may or may not require removing the amp chassis
Initial Thoughts
Most amps can be improved by changing tubes. In many cases, factory tubes are not the best sounding ones available and the tone can be significantly improved by upgrading to better tubes. However, it might not be an easy task to find the right tubes for your amp and your personal needs. Even different AC30 models may react differently to different tubes. Moreover, there are a lot of tubes available and the possible combinations are barely limited. You may follow other peoples experiences and best practices, but if you want to find the right tubes for your taste, you should consider a decent amount of time to try and experience for yourself.

Effects of Tubes
Different tubes may not only change the tone, but also the amount of gain/distortion respecitvely the amount of headroom. Usually, the most significant improvements in tone can be achieved by changing the preamp tubes. But also the power tubes and rectifier tube (if you have one) have an influence. Different power tubes, the EL84 in the AC30s, can make the sound thinner or more full. That´s the main difference when we tried different factories (those were TAD, Mullard (new), JJ and EH). The rectifier, the GZ34 in AC30s delivers the DC the power tubes use. The main way they affect the sound is by the different output voltage they deliver to the amp. In our experience, the difference can be more than 50V between different factories.

Those tubes mentioned above, especially matched quartetts of EL84, tend to be expensive so you maybe don´t want to buy more sets than needed. Good news, preamp tubes (exept NOS (New Old Stock) tubes) are in general cheaper and have more impact to the sound. But this is also the bad news about it. To really improve your sound, it is necessary to buy different factory tubes, more than you will need in the end. If you want to use other types of tubes be sure they match the specifications and the socket-layout.
The number of preamp tubes in the AC30 varies between the models. Top Boost and Normal Channel mostly use ECC83/12AX7 tubes. If you´re not shure have a look in the manual.

Let´s say we got 3 tubes for the channels including the phase inverter tube. Than you need at least 5 different tubes as basic for experimenting. Good ones we can suggest are TAD Premium, Tung Sol, Valvo (NOS), Telefunken (NOS), Genalex Lion, Mullard, Silvania. Try to get selected low noise tubes, it´s always worth the money.
After you got some tubes, take your time to experiment. Set each tube to a different position, one after another and you will hear while playing if it is better or worse. But be careful while changing tubes and switch off the amp each time you chance them and only catch the tube and no other components inside the amp. Let them cool down a bit, they can get very hot.
Write down the combinations that improve your sound and then take these combinations and check again to find out which one is best.

In general:
- the tube in V1 affects the sound most.
- high gain tube means in general more distorted sound and lower gain tubes result in a cleaner sound.
- low microphonic tubes are suggested because noise always becomes more after the sound passes another tube.

Basically, tubes not last forever but we must differ between preamp, power and rectifier tubes. If you like the sound and nothing is getting microfonic oder noisy, there is no need to change the preamp tubes.
The rectifier tube only needs to be changed if you feel the amp is not keeping its tone when playing hard chords. The volume may lowers when playing hard chords. This can be because the tubes are not getting enough voltage from the rectifier when stressed. Another indicator is a blown amp fuse.
Power tubes need to be changed from time to time. They slightly lose their brilliance and the amp sounds muddy or they fail completly. Depending on how often you play and how hard you push the tubes you need new ones after 3 months to 3 years. We recommend to change them at least every 3 years. You may not notice the lose of brilliance but you may be surprised if you plug in new ones how open and airy the amp sounds.
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Original AC30-Guide blog comments on this article:

Sjors on 31. December 2014 at 18:24 said:

Very nice article! I have a Vox ac30cc2x myself, and I am just about to change all the tubes. On this moment there are 3 sovtek 12ax7WC preamp tubes in it, and 4 JJ el84’s with a JJ GZ34S rectifier tube.
In the article it says that different power tubes can make the sound thinner or fuller, according to your experiences with TAD, Mullard (new), JJ and EH. But can you please explain what brand of these power tubes sound more thin vs full? For example: Do the Mullard el84’s sound fuller then the JJ’s? And what is the difference in sound (fuller or thinner) between the EH el84’s and the TAD’s?

jattle on 3. January 2015 at 11:00 said:

As I tried the TAD sound more fuller, more compact and the mullard (new) mor airy.

Sjors on 23. January 2015 at 18:13 said:

Thanks for your reply! I went with the new Mullards el84’s, I am going to install them within 3 days, I’m curious if they are vastly different then the JJ’s that were in there

jonny on 8. June 2015 at 17:12 said:

I’m about to change my factory tubes on an ac30 cc2. i want something that will give me more clean headroom and a little more full sound without the breakup. doubt i can get too much more but would you have a recommendation?

Andy Cooper on 26. April 2016 at 18:30 said:

I always found Sovtec to be affordable and able to keep the sound intact at higher volumes. Pretty much anything in that price range will beat the factory fitted tubes.

Hugo on 17. September 2015 at 6:58 said:

I have an AC30 C2X with the alnico blue speakers. It happens from time to time that it sounds like a beast and even cuts over a small-medium PA, but sometimes like tonight I pushed all the way up and simply it didn’t cut through the mix (in a small bar) it sounded muddy and had a lot of feedback. Does it means that I need new tubes? Can somebody recommend ? I have haed the amp for 4 years and never changed tubes, but need to mention that I play 1 o2 times a wee. Thanks, cheers

Benfatbass on 14. April 2016 at 22:00 said:

Working recently on an old Vox AC30 I notice that the powertubes are way to hot setted with the common Cathode resistor of 50 ohms . it is a very bad design technically since the tubes are pushed over their limits , it is better to change this resistor to a bigger value like 82 or even 100 ohms in that way your tubes will last much longer . soundwise this will not effect very much , maybe you can upgrade the cathode resistor to a bigger 1000 uF to correct for the somewhat thinner sound you would get by changing for a higher value cathode resistor . Best solution would be to split cathode resistors in pairs so 2 x 220 or 270 ohm would do perfect and this would prevent your tubes from early failure , fact is that with one common cathode resistor for the 4 powertubes , if one tube fails all tubes will suffer and get way to hot becoming red hot tomatoes even your amp can burn out and start a fire . This is one stupid fault in ALL tube voxes .it makes of course the small headroom sound and the typical distortion of early saturating the powertubes , but it is not a good situation. in fact each tube should have its own cathode resistor and capacitor to have a healthy situation . I am 50 years tube tech and it amazes me that such bad designs are so popular . look at Fender with the 6L6’s at 25 mA idle class AB amps , they survive for ages .. and yes it’s different in sound , but I love more the Fender then the Vox . Every competent tech that sees this diagram must start laughing ! Cheers , Ben, 68 yr young Tubeamptech.

Andy Cooper on 26. April 2016 at 18:39 said:

I just put my 1971 AC30 in for repair today and heard all about this problem with the cathode resistor. When the guy opened the amp my cathode resistor was a molten blob on the end of a piece of fuse wire.
Having said that, I’ve had the amp for 32 years, and this being the first time the resistor in question has blown, I wouldn’t say it’s that fatal as flaws go.

Eric Folts on 7. November 2016 at 20:17 said:

Dealing with tubes on my AC30 right now, it’s a 2014 and I’ve already had issues with it blowing fuses and changing the tubes twice. It is quite possible I have a dud, but still for the price of this amp I was not expecting to deal with all of this.

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