Find a pair of 12AX7s
for your ZOOM G9.2tt on eBay
There are Vacuum Tube purists out there who might make quite a song and dance routine out of selecting just the right 12AX7 pair based on output specs and the 'timbre' of a given tube's sound. However this is not a high-end audiophile pre-amp. Seriously, this device still utilizes the old RC4558 dual op amps (I didn't realise they still made those shitty things).
The reality is that the vacuum tubes are there to introduce some distortion. If you run your G9.2tt on the clean side (as I do), well, those tubes aren't going to be contributing all that much to your sound. That said however, it's nice to have it when you want it.
So my recommendation is that some reasonable quality (and reasonably priced) tubes will do nicely.
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The first thing to mention is that the YouTube video (that seems to come up first in searches), done on some guys mobile phone is way wrong.
In the video a whole bunch of 'unnecessary undoing' happens, including the removal of metal shielding around the tubes - nope - two screws at the bottom of each 12AX7 tube mount circuit board will allow them to be easily removed and replaced. It really is that simple.
Of course to get inside the G9.2tt in the first place, well lets just say, if you are not the patient type ... let someone else do this.
Before going further it might be prudent to ask the question ...
Do the Zoom G9.2tt vacuum tubes actually need replacing?
Firstly, new vacuum tubes do make a difference to the sound. But realistically, not a radical difference. In fact, slightly better clarity in the form of enhanced brightness is about it. (Just FYI: I own two G9.2tt's and have replaced the tubes in both.)
The real question is how do you know whether the vacuum tubes 'need' replacing. Sadly the answer is ... unless they have actually ceased to function, there really isn't any way to readily know.
Removing and visually inspecting the vacuum tubes may indicate whether they have seen a lot operational hours (by the amount of 'silvering' at the top of the tube), but that is hardly definitive. If the tubes happen to be unlabelled or labelled CHINA, they may well be the original vacuum tubes shipped with that G9.2tt ... there's a hint.
Since the G9.2tt has been around since at least March of 2007, this is equipment that could easily be around years old. Just to say that most G9.2tt's you are likely to encounter will probably be due to have their 12AX7s replaced (if they haven't been already). Some sellers on eBay rightly make a point of mentioning that they have replaced the vacuum tubes.
Factor in approximately AUD $60 to $100 for a pair of 12AX7s, and another $50 to $100 if you get the G9.2tt professionally serviced/repaired. In short, a service + two new vacuum tubes could easily add AUD $200 to the cost of a G9.2tt. Keep that in mind before you rush out and buy that $300 eBay bargain.
If you have decided to go ahead with this madness, and you have already acquired some shiny new 12AX7s, then ...
There are no less than 26 screws that have to be taken out to remove the bottom panel and expose the circuit boards and 12AX7 vacuum tube mounting boards.
And just to keep life interesting, those screws (you know this is coming right?) ... they're are NOT all the same ;-))
Yes it's true, there are a few that go down the sides (3 on each side) that are just a little longer than the others. It's a real good idea not to get them mixed up. See the photo/diagram below for reference.
So the hardest and slowest part of this whole process is getting the case open. No Joke. You could use a powered screwdriver, but if you damage the screw heads ... well, not a good look.
First however ... unplug all leads.
This thing is heavy, if you drop it with any leads connected, something is sure to break and/or snap off. Also, if nothing is plugged in, it can't be powered up and you won't get an unexpected electric surprise.
The up-side is that all you need to complete this task is a pair of new 12AX7s and a Philips Head (star) screwdriver, and I think I already mentioned the patience. A well lit place to work (and/or headlight) and a small container for those screws will also be handy.
For a quick reference, the pic below shows the location of the 6 longer case screws and the mounting screws for the 12AX7 mounting circuit boards (click on image for larger version).
From here removing the small square circuit boards that hold the ceramic vacuum tube mounts is easy, just two screws each. Once undone, pull the circuit boards gently toward the front of the unit, then up and out of their enclosure.
Each vacuum tube is held in place by a ceramic mount. Remove the tube by gently wiggling it back and forth while apply some outward pressure. Try not to force it. If you damage / break the ceramic mount this project can go from DIY to 'Off to the Service Shop' in the blink of an eye.
Also, don't be tempted to unplug the connector cable to the vacuum tube mount PCB. This only creates more work and may have unintended consequences. It is not necessary to do this to replace the vacuum tubes.
Once the vacuum tube is removed, simply insert the replacement (they can only fit one way), with even gentle pressure.
Check the PCB (Printed Circuit Board) that the ceramic mount is attached to for signs of heat damage (usualy indicated by a darkening / browning of the PCB). I say this primarily because, given that you've gone to the trouble to crack this thing open, you might as well make sure all is well while you are there.
Heading off problems is easier than repairs after something has been cooked. You may for example find a bunch of dead bugs around the tubes (attracted to the glow). Vacuum out the inside of the case as required (gently!).
All being well, slip each of the PCBs back into it's niche and replace the two retaining screws. When complete, flip it over, plug it in and make sure it's all working. If all is good, disconnect from power again. Pat self on back, job well done .... now put the bottom plate back on ... guaranteed to take at least some of the smile off your face ;-)
But - that's it! Take care with what you're doing and all should go smoothly.
Your comments and contributions are greatly appreciated!
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Editor's Note: Your contributions are appreciated.
Incept Date: Wizard - 160803
Last Update: Wizard - 180417