Vox AC30 Top Boost (1968)

ASCIIFinally I've put up some photos from my vintage Vox AC30 Top Boost. Maybe they're of some use to you.

The quest for the once broken vib channel is solved, The .1 uF cap at the tremolo on/off switch was broken and so was the cathode bypass capacitor. I did acquire some wiring schemes from http://www.voxamps.co.uk that helped me a lot to understand what is going on in this amp. As I was searching for information on when and where the amp was built I think I have come to some conclusions with the help of some people at the Vintage Amps Bulletin Board. You can read about my amp being discussed in this thread.

Update [2003-11-02]

crackling_1.jpgcrackling_2.jpgObviously something in the preamp section is broken for all channels. There is a crackling noise just like the one you would expect from an old pot when you adjust it. Unfortunately it's not that simple. Turning the pots does not seem to be influencing the crackling noise except for the level of volume it crackles out of the speakers. I sampled the sounds of it and put it up on this site. Please take the time to listen to those 20 seconds of noise.

Crackling Noise at full volume:

Update [2003-12-03]

voxac30.jpgI managed to fix the crackling and background hum with great support of the people at the Vintage Amps Bulletin Board. You can read about it in this thread. Crackling on all channels was caused by broken cathode bypass caps and the 8uF and 32uF electrolytics. After the replacement of those the brilliant channel was still crackling because the 220k plate resistor had seriously gone up in resistance. Replacing that one solved the crackling problem. Still there was hum even at zero volume and I measured the four 1.5k resistors at the el84 tubes (power stage). With 2.1k they were way too high, so those needed replacing, too. Now the amp is really quiet. I replaced a couple of other resistors that had gone up, too just to be on the safe side.

ef86bread.jpgHaving fixed that I couldn't help fooling around with an EF86 pentode that is to be found in early Vox AC30/4s. I built the small preamp section on a breadboard according to the AC30/4-EF86 schematics found at http://www.blueguitar.org and tapped it into the circuit, replacing the first gain stage of the normal channel. This modification resulted in great gain improvement (yeah right, some kind of gain gain ;-)) and somehow richer/thicker sound + the tube just looks great. Sorry for the bad quality of the breadboard experiment image, I just didn't have even a half decent camera lying around.

Update [2003-12-04]

Went to buy some material for a bracket to fit the EF86 into my amp. The design is similar to the boost bracket for treble and bass. You can look at another gallery how I did it.

Update [2003-12-08]

vox-open.jpgAfter some experiments with the bracket resulting all in more or less audible hum I finally managed to bring hum down to zero :-) I had help from some friends from the plexi board again and you can read about it in this thread. It seems to be important that the ground wire's path from the bracket has to be carefully chosen as not to result in being a source for hum. Another modification obviously widely know to amp techs is the heater balancing mod where two 100R resistors (one for each side) are tapped to the ground after lifting the ground from the heater chain near v1. I made a gallery of everything I did to the amp in order to sort out the crackling (read above), the overall hum and the special hum caused by the ef86 modification. Feel free to have a look. If you have any questions, please email me at tim@tubegeek.de.

Update [2004-05-18]

Had time to play the amp in a practising room and crank it up some. Nice! The EF86 modification sounds damn good and the tube is just a little bit microphonic. Guess I should have mounted it with rubber grommets right from the beginning.

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