Vox AC 30

soundmasterg · Post by **soundmasterg** » Thu Apr 21, 2005 8:24 pm

Peter, the Top Boost is just another channel with added gain and more frequency shaping. Brian May doesn't really use it from what I've heard, but most players I know use the Top Boost channel. If you go straight into the amp, it will usually sound much better because there is more gain on tap if you need it. Every one of the AC30's channels except the Top Boost channel is very simple with no tone shaping except for the cut control, which is in the power amp section of the amp and just shunts highs to ground. The Top Boost added more gain stages, and a Fender blackface tone stack with a mid dip, but Vox used different values for it which limits the bass quite a bit more than the Fender. If you were going to build an amp to duplicate a Vox sound, then the order I put things in in the previous post would be most important to achieve THAT sound that the AC30 does.
Many people have said that Vox come up with the Top Boost to emulate the EF86 channel that was on the earlier ones and was taken off. They received complaints about it and came up with a more reliable way to acheive it than just putting the EF86 back in. EF86's tend to be microphonic, but sound GREAT.

admin · Post by **admin** » Thu Apr 21, 2005 11:59 pm

Thanks Greg. My understanding, which is limited, is that Top Boost is another gain stage inserted between the volume control of the original first
gain stage and the original second gain stage of the Normal/Bright channels. So, if I understand it correctly, in practical terms it is not another channel that can be plugged into independently of the three standard Bright, Normal and Vibrato Channels. So did Korg include the Top Boost gain stage in their 1990s Vox AC30 reissues?

I have heard the Top Boost described in many different ways, from simply the treble and bass controls to a gain stage designed to allow for greater volume on the Bright Channel?

soundmasterg · Post by **soundmasterg** » Fri Apr 22, 2005 12:32 pm

The original AC30/4 had an EF86 channel which had high gain and really nice tone. It was considered the "bright" channel. It also had a "normal" channel which was just a 12ax7 stage and was just like the current normal channel. They got rid of the EF86 channel because the EF86 wasn't reliable enough, especially in a combo. So that became the AC30/6, but players liked the sound of the EF86 channel and wanted more gain and a brighter tone. So the top boost mod was invented and initially they added it to the back of the amps, like in the Beatles first AC30's. Eventually it was put into production and incorporated into the amp as part of the bright channel, where it has remained since then.

Without Top Boost, the AC30 is a very simple amp with differently voiced channels, but not much in the way of tonal change possibilities because all you have is a bright cut control in the power amp. It just shorts highs to ground as they go from the phase inverter to the power tubes.

With the Top Boost, they added it to the bright channel in between the first stage and the second stage, and it has a tone stack like a Fender Blackface amp, with a mid range frequency notch that cuts out some mids. Vox used different values than Fender and Marshall, and Vox's choice of values limits the bass and doesn't have as much insertion loss as the Fender setup, resulting in a brighter and more powerful "jangly" tone. Now, with Top Boost you have the previous cut control, but you also have a treble and bass control, and more gain on tap too. To make the Top Boost work, it adds the tone controls and two 12ax7 stages, one setup as a gain stage, and the other is a cathode follower to provide a low impedance input to the tone stack and phase inverter. Marshall used the same gainstage/cathode follower setup on almost all of their amps, and Fender used it on the tweed bassman among others.
I pretty much think of the "bright" channel as the Top Boost channel, as it is in there and won't go away! But initially the Top Boost was an add on with treble and bass controls and a gain stage and cathode follower. Korg's 90's reissues from '93 to '03 or '04 or whenever they stopped are as accurate to the original AC30/6 with Top Boost as you can get without building a PTP original copy yourself or getting the expensive AC30HW that Korg did a couple years ago. The Korg reissue is the same circuit completely, but they added a standby switch, and increased the filter cap values just a tad, which reduces hum and maybe makes the amp have a little more bass response too. If I was to need another AC30, I'd get a Korg reissue in a heartbeat over an old one due to reliability issues. I haven't seen the new ones yet so can't comment on them too much. Nowadays though, with learning about amps like I have been, I would just build myself one I think. If you were interested in building a kit, Ted Weber at Webervst has a well respected AC30 kit that is reasonably priced.

BTW, my name is spelled with only two G's, not three!

Greg

admin · Post by **admin** » Sat Apr 23, 2005 1:59 pm

Thanks Greg.

raider · Post by **raider** » Mon May 02, 2005 2:53 pm

I have the Korg Reissue Vox AC-30TBX
and its a great sounding amp. I'm glad
that I have one it sounds good with my Gretsch
but I don't get the same sound as with a
Rickenbacker. It just seems that Ricks and Vox
for me is the perfect setup.

admin · Post by **admin** » Tue May 03, 2005 1:41 am

Lou: What year is your Korg Reissue?

soundmasterg · Post by **soundmasterg** » Tue May 03, 2005 9:45 am

Peter, in case you or anyone else are looking for a Korg reissue for themselves, sometime in 1995 they had a wiring mod come out from Korg which changed the grounding and reduced hum. I have a copy of it somewhere here. Anything made before mid 1995 would not have had this mod done and it should be done. My Korg reissue is a 1993 model with Celestion Blues. Only problem I've had over the years has been ghost noting speakers. (cone cry, which the Celestion Blues are known to do)

raider · Post by **raider** » Sun Sep 09, 2007 3:35 pm

Peter, sorry it took so long to give you
an answer. Here is the S/N M-2003-50-0416-B
I think it a 2003 it has the Blue Speakers
you may know more than I do. Don Bulter my
know by the S/N number.

Greg do you use your AC-30 on gigs? Right now
mine just sits on its stand and I use my Tone
lab more then anything else. That way I don't
have to listen to my wife saying turn it down.
One thing that I like about my AC-30TBX is this
if I'm using my Rickenbacker, Gretsch or my
Steinberger GR4 they all sound good.
Lou D

admin · Post by **admin** » Mon Sep 10, 2007 12:31 am

Better late than never, Lou. I appreciate your response and 1993 is it.

Have you ever tried the THD Hotplate? It will allow you to have the tone you want at a volume that will please your wife and yourself.

admin · Post by **admin** » Mon Sep 10, 2007 12:32 am

Greg: I have always wondered what causes tone cry. Can you explain the cause and the cure?

raider · Post by **raider** » Mon Sep 10, 2007 4:06 pm

Peter did you mean 2003 and as for the
THD Hotplate I've looked into them and
its on my list just as the Jangle Box is.
Lou D