VOX AC-50 technical help

[trdlasvegas]

Has anybody out there ever worked on a AC-50? I got a problem on the Brillant channel that has me currently stumped. It is too thin sounding. I have just about replaced/checked everthing in it, in fact I even found a wire soldered in the wrong place when they built it 30 years ago. But the Brillant chan still sounds too thin.

How about it - any Vox pros out there?

Tony

[Wayne Alexander]

Send it to Don Butler. http://www.tone-man.com

[MJ Harnish]

The first question to ask yourself is "Is this the way the amp was meant to sound?" If so, then it isn't an issue of what's wrong with the amp but rather what you can do to make it sound better. "Sounding thin" is a bit of a hard one to simply "fix" since a lot depends on your definition of thin: Does it lack bass? Does it lack mid-range? Too much high end?

Off the top of my head & without looking at the schematic for the specific values:

My first suggestion would be to consider increasing the size of the coupling caps in that channel; try bumping them up a value or two, swapping out one at a time, and see what happens. You don't want to increase the size too much or you'll lose all your definition and just end up with a really muddy sounding amp.

Another possibility is to try increasing the size of the cathode decoupling cap (the electrolytic bypass cap) on the input stage of that channel. Increasing the value (don't go over 25uF or you'll end up with flatulence city at higher volumes) will increase gain as well as add a lot more lower frequencies.

[trdlasvegas]

I'm sorry for not being more specific.

I don't know what a AC-50 is supposed to sound like. But however I would think that the Brilliant channel should sound like the normal channel except brighter. Like switching the bright switch on a Fender. This one has a lack of low end, hense the "sounds thin".

As for changing the value of the caps, it is not my intention to modify the tone of this amp but to correct it to the orginal way it was intented to be.

I have measured all the caps in the signal section and they are more than close to the values they are supposed to be. Also I have changed all electrolytic caps in the amp so I know that is not the problem.

Since most of you guys seem very knowledgeable, I thought maybe someone has either worked on one of these or know what it should sound like.

Tony

[MJ Harnish]

I took a look on the schematic found on schematics heaven and I noticed one major difference between the two channels: The brilliant channel uses a 500pF coupling cap after the first stage (right before the volume pot) compared to the normal channel's .022uF. That is a HUGE difference and I would expect the brilliant channel to be a lot thinner and brighter sounding as a result. So my guess is that the channels sound they way they should. If you wanted to experiment you could just swap out that 500pF cap for something bigger (maybe start with a .0022 or .0047). That would make the two channels much closer.

[trdlasvegas]

Thanks MJ.
I looked at that also with the scope and noticed that I was looseing all the low end after that cap. Since the Chan 2 pot had been changed I thought maybe the RC network formed by that cap and the pot was not correct. So just to make sure I swapped the pots and sure enough same result. That's when I looked at your post and realized that 500pF cap they have in there must be correct and this what they want it to sound like. Interesting enough there isn't a bright switch like shown on the schematic, the other small cap is permantly attached across the pot. Sure does give it a lot of highs as compared to the first channel.
Tony

[Wally]

Tony, you may have tried this and youmay have already made your changes to the Br. channel, but I like to chain channels together on this type of amp. The result is a more complex sound than if you change the amp to where the channels are much alike. Just a thought......Jimi liked to do this with his Marshalls, which have channels that are very different. BF Bassman amps work well with a jumper, also. :)

[Dacious]

I think you might find that sound is by design - the Beatles and other Britpop bands were driving Vox development and they were just trying to get the same Rickenbacker/AC30 trebly jangly sound only bigger. Fender was focussed differently making guitars and amps with the fat 6L6 bottom end.

Go to www.voxtalks.com and ask over there - I'm sure a Vox nut can tell you for sure.

[buglecorps]

In the early 60's in the UK ..Highend american guitars were somewhat hard to come by and VERY $$.. only artists with record contracts could afford the better US instruments. Most working bands used instruments imported form Germany (Hofner) or lesser US instruments (Kay, Harmony ect).. think of Keef ot the first Stones album with that funky Harmony.. Most of these instruments had PU's that lacked highs, AND were Hollow or Semi solid. As Dacious has pointed out the chord based sounds of the time favored highs! Early Vox amps (fawn and esp. TV style) were VERY dark and Jazzy sounding, ..the bright channel on later amps was voiced to give a little more shimmer to the guitars of the day. I think it sounds as it should..try it with a Gretsch or better yet one of the new budget Ibanez Artwhatever semis for some retro fun.