#1
I recently bought a 2nd hand ac30c2 amp, and it was working perfectly, maybe in need of a tube change, but otherwise no issues. I have taken the back off once to clean out the amp since it was full of dust and spider webs, but it was still working after this. Last night I was playing the amp fine, but this morning when I turned it on, the top boost channel wasn't working. The normal channel works fine, but the top boost is only audible when both the channel and master volumes are cranked, and even this is about the same volume as an unplugged electric guitar. Does anyone know what may have gone wrong?
#2
Quote by mtolhurst3001
I recently bought a 2nd hand ac30c2 amp, and it was working perfectly, maybe in need of a tube change, but otherwise no issues. I have taken the back off once to clean out the amp since it was full of dust and spider webs, but it was still working after this. Last night I was playing the amp fine, but this morning when I turned it on, the top boost channel wasn't working. The normal channel works fine, but the top boost is only audible when both the channel and master volumes are cranked, and even this is about the same volume as an unplugged electric guitar. Does anyone know what may have gone wrong?
Take all the tubes out one at a time, spray some Deoxit (CAIG Laboratories D5, it is a miracle fluid) on the pins, and re-seat the tubes. Make sure they are pushed in well. Be careful not to touch any of the circuitry and do this all with the amp unplugged from mains power. Also, I like to take most the screws and washers out from the back, I just keep 3 main ones in on each side of the amp. Keep the rest in a small bag.

If that does not help, then take a spare for-sure working 12AX7/ECC83 and try it through the preamp positions V1-V3. When the problem is solved with a spare in a certain position, you know which tube to replace. V2 is likely the culprit, as the Normal Channel is just (mostly) affected by V1. It is always good to have a spare preamp tube and a spare matched set of power tubes. In fact you might want to go ahead and order a whole new tube set for it just to be safe, and use the current tubes as spares. If you need recommendations on what brands I can suggest some.

I recently did some research on the AC30C2 as I have the same lovely amp, particularly regarding something called a cathode followers. Generally the CF helps drive tone stack circuits, the Top Boost circuit in this case. In the AC30C2 (and most other AC30's) V2 is a cathode follower position and as such some brands of tubes cannot handle the workload. So if you check V2 and it is the culprit, be sure to write back and tell me what brand the tube was. Replace it with a JJ 12AX7.

Also, feel free to post/browse in our dedicated VOX thread, I made a longer post about the V2 and cathode followers just recently: https://www.ultimate-guitar.com/forum/showthread.php?p=33747269#post33747269

Also some info on the C2:
http://www.voxshowroom.com/uk/amp/ac30c2.html
http://www.voxshowroom.com/uk/amp/ac30c2_hood.html
Last edited by Will Lane at Jan 3, 2016,
#4
Quote by mtolhurst3001
But wouldn't the normal channel also be affected if it was a tube issue?
No, according to the VOX Showroom (2nd link) the normal channel circuit does not really run through the V2 position tube. It might a little bit but not enough to matter in this case.

The Top Boost relies heavily on V2, though, so it is most suspect. V3, the phase inverter, is shared between the two circuits as well as V1. V2 is the tube position most isolated to just the Top Boost circuit.
Last edited by Will Lane at Jan 3, 2016,
#5
Ah, thanks. I'll try swapping the v2 tube somewhere else to see if it is the problem since I have no spares lying around, and get back to you. Thanks again for the help
#6
^Okay. I would suggest swapping V1 and V2, then. Do not leave it like that for long, test and cut it off. If my suspicions are correct then, when placed in V1, both channels will be affected as it is a shared position.
#7
That was it, the amp gave no output at all with the tubes swapped. All of the preamp tubes were the stock ones labelled "12ax7b China". Thanks for your help man.
#8
Quote by mtolhurst3001
That was it, the amp gave no output at all with the tubes swapped. All of the preamp tubes were the stock ones labelled "12ax7b China". Thanks for your help man.
No problem, glad I could help! Those are most likely the stock tubes so they are probably a bit old, especially if the amp was showing age (like dust) when you got it. I would replace V2 with a JJ 12AX7/ECC83.

For V1 I like Tungsol 12AX7. V1 generally has the most impact on your tone. The Tungsol seems to be very clear and opens up the amp some. For V3 a Sovtek 12AX7LPS. For the power tubes JJ EL84, not the EL844.
Last edited by Will Lane at Jan 3, 2016,
#9
Im looking at a set of 3 jj12ax7s and 4 jj el84s, since it is cheaper to buy that way (damn aussie gear prices), and I might look into the tungsol later, and keep the jj as a spare