My Vox AC-30

[admin]

My AC-30 is a Korg Reissue from 1996, which makes it 10 years old sometime this year.

It has never given me any trouble, except for a rectifier tube malfunction which tripped out a couple of fuses until I figured out what was going on. A two fuses and a new tube later and all is well.

In the interest of avoiding a meltdown, what preventative maintenance should I consider at this point?

[soundmasterg]

Well my Korg reissue AC30 is from 1993, and it still works fine. I too had the same rectifier tube problem, and resolved it just as easily. I also had some badly ghost noting Celestion Blue speakers, and I solved that by sending them to Ted Weber, who lightly doped the cones. The amp still has a very slight ghost note when dimed, and it is probably a bad coupling cap somewhere leaking DC, but it is a very minor problem.

I plan to replace the electrolytic caps at some point in the amp, but since I use it regularly, I'm not too concerned about them being bad yet. On your amp, depending on its use, the electrolytic caps should be replaced within the next 5 years or so. Other than that, unless a problem develops, just replace the tubes as needed and you should be good to go for quite awhile.

[glen_l]

There's an easy mod to convert the biasing to pre '64 spec, which is a little kinder to the amp. It will run a little cooler, and the el84's will last longer. The amp will break up a little earlier, which a lot of people like.

All you need to do is locate the pair of 100ohm/5W resistors, and replace one of them with a 390ohm (min 1W)

[admin]

Thanks Glen. I cannot see any down side to this modification. Sometimes breaking up is hard to do, but this should make it easier.

[admin]

Greg and Glen: I am going to have an amp tech carry out the mod to convert the biasing to pre 1964 specifications. I am wondering if either of you have any additional recommendations while it is in the shop.

My 1996 Korg Reissue performs very well, however, I am not taken by the rather bassy tonality (muffled to some) of the Normal Channel. Is there a mod for this that you have read about or tried? The only way I am satisfied with the Normal Channel at present is if I use considerable treble boost.

If there are any other modifications that you think are worth trying on the AC30, to improve functionality or tone, I would be interested in hearing about them at this time.

[admin]

Glen: Are you saying to replace two of the four 100 ohms with 390 ohms? Each EL 84 has one (screen grid r's), right?

[soundmasterg]

The normal channel was always kind of dull and uninteresting on the AC30, at least with the 12ax7 models, which is why they came up with the top boost. The early AC30/4's used an EF86 in one of the channels and it sounds similar to the top boost channel but thicker and more gain. When they took that channel away and just had the 12ax7 normal channel, people complained and they came up with the top boost as a mod and later had it incorporated into the amp. There is not a way to easily change the normal channel to more gain and more brightness and have it sound good without adding another tube stage and some tone controls, which there isn't room for.

The biasing mod would mean that you are changing the cathode resistor value on the EL84's to some other value. The originals had one 50 ohm resistor and a 250uf cap to ground from all four cathodes of the power tubes. Currently all four EL84's share a common cathode resistor in effect, although they used two 100 ohm resistors to ground in parallel to get 50 ohms, and there is also a 220uf cap. If you change to two seperate cathode resistors, appropriately sized, then you can pull two power tubes and run at half power, but as a consequence it sounds different in normal mode than stock. Going to one 100 ohm and one 390 ohm resistor in the same hookup as they are now gives you something entirely different. You would have to do some math since those components are in parallel to see what the end result would be. You'd also want at least a 5w minimum since it is a cathode resistor and is dissipating a lot of heat.

The 100 ohm screen resistors are actually a bit undersized for a screen resistor, unless you are always using a good quality tube in your amp...and by "good quality" I mean a known good NOS type like GE, Mullard, Valvo, etc. None of the new modern EL84's are consistant enough to always be safe with a screen resistor value that low. A better value there would be a minimum 1k, and I think Kevin O' Connor recommends a 2.2k as a screen resistor for EL84's, but I'd have to check my books again. As you go up in screen resistor value, it gets more compressed and dirtier, and also more lifeless. I like the smallest one I can get away with personally and prefer the 100 ohm sound in my AC30. I always make sure to use a good quality tube in it, and when I use modern stuff, I watch them closely for the first couple hours of use to make sure they will hold up.

You can find which resistors are which on the schematic for the RI at http://www.schematicheaven.com/ . Look at the power amp schematic and you'll see the different 100 ohm resistors Peter.

The filter cap values are different in the Korg compared to vintage, and changing those to vintage values could get you closer to vintage tone. Beware though that the amp will hum more and ghost note more if you do. I prefer the modern values myself.

The other mods people typically do on the Korgs to make them sound better is to use 1/2 watt carbon comp resistors in place of the 1/4 watt carbon film resistors in some places, and to replace all the signal caps with a good modern brand like Mallory 150's or Sozos. The last one people do is to get good transformers in there instead of what came with the amps. Some of the Mercury Magnetics, OEI, Heyboer, or the new Vox Albion clones that they are putting in the CC amps would sound better. Its a lot of soldering to do all of that. I suggest talking to Don Butler about what mods are available. He's at http://www.tone-man.com/. He is THE guy around for AC30 mods.

[admin]

A wonderful answer Greg. Thank you. I have some thinking to do.

[nic37]

A 10Ohm with a 390Ohm in parallel as Glen pointed before, would give you a total resistance of 79/80 ohms and will run the output tubes cooler.

[glen_l]

Yes, Nicolas gets what I mean't. I was referring to the common cathode resistor. By changing one of them to 390ohm, it brings the total cathode resistance back to approx 80ohms, which is in fact the original value in the AC30/4 and AC30/3 up until late '63 (which would include the AC30's used by the beatles).

Stay with the 100ohm screen resistors on the el84's as that's the value AC30's were always made with.

You could reduce the main filter caps from 22uF/450v to the original JMI value 16uF/450v - sprague atoms are good for this. You may could even change the 220uF cathode cap to 250uF/50v (sprague atom again), which was the original value. These are pretty fine points though. I doubt there'd be much perceptable difference.

If you'd like to brighten the Normal channel, you could always get the Treble version components put around that section. It consists of a 330K resistor and a 220p cap.

One final thing that should be done to any Korg AC30TB is changing the cathode resistor on the second stage of the topboost section from 56k 1/4W to 56K 1W. The 1/4w is well under spec.

These are all pretty cheap mods. However, changing the Output transformer, audio path caps, and audio path resistors starts to get very expensive