Vox Vetting

admin · Post by **admin** » Mon Mar 27, 2006 3:58 am

To those with the Korg AC-30 reissues, I had my first negative experience over the weekend. Outside of a rectifier tube, my 10 year old AC-30 has been a great performer. With the Celestion Blues it has a golden tonality and a shimmering presence that has been worth the sting of its cost used, and the addition of the Bulldogs.

Shortly before Vintage kicked off its first set at a Beatles' Extravaganza, the brilliant channel was, well, lets just say less than brilliant. Now in a Fender amplifier, one would try another channel and perhaps the hard day's night would have been saved. But alas, the "normal" channel had a cardboard box, egg-crate tonality that only someone sleeping could love.

So off to the amp tech it goes this week to repair the brilliant channel along with a modification to the normal channel that allows it to be put to use without lining up a series of treble boosters.

The gig was well received and I have to say that my Fender Twin Reverb did me proud. A couple of Fender lovers couldn't resist on commenting on the durability and reliability of the Fender and the fragility of the Vox. I'll get over it.

leftybass · Post by **leftybass** » Mon Mar 27, 2006 7:25 am

Peter, the guitar player in our band is a Fender man and his stuff has let him down a time or two...they all will from time to time. That's why I have two of the same bass amp, LOL...soon to be three perhaps.

admin · Post by **admin** » Mon Mar 27, 2006 8:19 am

Yes, at least the audience still had the chance to rattle their jewelry.

randyz · Post by **randyz** » Mon Mar 27, 2006 8:29 am

Back in the 1980's when Tom Petty was touring with a Vox backline, I recall reading that they always took 2 or 3 times the amps they needed on the road. And these were the solid-state Thomas Organ-built Super Beatles!

soundmasterg · Post by **soundmasterg** » Mon Mar 27, 2006 9:00 am

Please post when you have the problem figured out and what the cause was Peter. I have my suspicions but I'll hold off on mentioning them now. I've never had a problem with my 1993 Korg AC30 aside from the ghost notes caused by the speakers that I've mentioned previously.

admin · Post by **admin** » Mon Mar 27, 2006 12:55 pm

Thanks for keeping the mystery alive Greg. I think it screams out a bad tube.

rictified · Post by **rictified** » Mon Mar 27, 2006 7:42 pm

I guess now is the time to brag that I can count on one hand and have fingers left over of the amount of times my SVT's have broken down on me in 30 years of using them almost exclusively, and many times 4-5 nights a week. haha! I also had very good luck with Fenders, Marshalls? Now that was another story even though I still love the old non-master volume heads.

soundmasterg · Post by **soundmasterg** » Mon Mar 27, 2006 10:11 pm

Well my SVT had a bad cap or two and was miswired from the factory....but then its an MTI too.

Any tube amp will have problems at some point, but if you maintain them they usually work fine regardless of brand.

Ever seen the chassis pics from inside a vintage Hiwatt Bob? They're the most reliable amps around, and have beautiful construction...light years beyond your run of the mill Ampeg SVT's.

Thats what my guess is Peter, but we'll see.

rictified · Post by **rictified** » Tue Mar 28, 2006 3:47 pm

I blew up a Sound City in one night Greg with incorrect impedance, did that to an SVT for years before I realized it and didn't hurt it, I was the original amp killer in my day, american amps always were far more reliable for me. I've never used a Hiwatt. I've always thought of British amps as very nice sounding, very sensitive to the touch and very delicate. I also destroyed a Super Beatle once also (wasn't mine). The Fenders and Ampegs just kept plugging away no matter how badly you abused them. I think the all time most reliable amp has to be the Ampeg B-15, you can't kill those things, there's a lot of unrestored 60's amps still in use today and people used to play those on ten all the time back in the day.

soundmasterg · Post by **soundmasterg** » Tue Mar 28, 2006 11:01 pm

I just love those old B15's.....one of these days I'll build one for myself or find a good price on one. Sound City's were actually related in a small way to Hiwatts, but they used inferior components and undersized the output transformers like Marshall did, so they blow easily and aren't tolerant of mismatches. Fenders were pretty robust, and the SVT was just a tank. One of these days I'll recap my SVT so it will have its true voice, but I've got tons of other projects to do first.

kevin · Post by **kevin** » Wed Mar 29, 2006 9:24 am

Brian May's on-stage setup currently has three banks of three AC30s (used to be three banks of four before he went to digital effects). Only one bank is used -- the other two are backups. Triple redundancy, like the space shuttle systems. Of course, this is not necessarily because AC30s are prone to failure, but more likely because he runs them so hard and hot (full on -- volume maxed out -- and blasting a treble booster with 26db of gain through it) that the amps can't take the constant beating.

soundmasterg · Post by **soundmasterg** » Thu Mar 30, 2006 12:18 pm

I wonder if the new amps that he has been using instead of the vintage stuff he used to use holds up better?

admin · Post by **admin** » Fri Mar 31, 2006 2:00 am

I received a call from my amp tech yesterday to say that he plugged in the AC30 and that the brilliant channel worked fine. I now strongly suspect the jack and its connection as the culprit.

soundmasterg · Post by **soundmasterg** » Fri Mar 31, 2006 10:54 am

It could very well be Peter. The jack is on the PCB board so it could have developed a loose connection. Have him reflow the solder joints for the jack and/or replace the jack. Its kind of like your car problems...they never manifest themselves when the mechanic is looking at it.

admin · Post by **admin** » Mon Apr 03, 2006 12:21 am

Greg: Your comments to my amp techs observations would be appreciated.

I noticed this evening that the plate voltage on the EL 84's was
about 354 VDC. I wish I would have measured before the cathode resistor change but I'm almost certain it would have been lower. On my AC 30 I have at present a 47 ohm/12W resistor with a 25 ohm/3W pot in series to adjust to
taste. I noticed when I increased the resistance of the pot the plate voltage increased. I know on the schematic (for the vintage) the plate voltage values are 320V. What I wonder (and I know you have some good advisors on your site) is whether the increased plate voltage is stretching it for EL 84's. Some have said the tubes will run cooler. I know when you increase cathode values on preamp tubes the gain is decreased/ it gets cleaner.