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Vox AC30 Vib/Trem Channel

12/5/2005 1:49 PM
Rick Erickson
Vox AC30 Vib/Trem Channel
I couldn't decide if this was a repair or modification problem so I decided to just post it here.  
I have a customer who has discovered that he can make the Vib channel distort at a low volume setting on his AC30 by cranking a high output pickup into it. I tried to explain to him that these amps were not designed to be used with those pickups and even my original JMI AC30 amps will distort with a HB pickup. Further investigation reveals that the volume control for this channel is at the end of the preamp & vibrato circuitry, rather than after the first stage as one might expect. Has anyone here dealt with this problem? Does anyone know what amp Vox took this vibrato circuit from? I'm assuming there must have been some inspiration from an existing design somewhere. Knowing the source might help give me some insight on how to deal with this. For the record this is a recent reissue AC30TB/6.  
12/5/2005 2:22 PM
Steve A.

    I bet that the customer hasn't tried the low gain input for the Vibrato channel. So the volume control is at the non-inverting input to the PI... Sounds like a lot of crunchy fun to me! ;)  
    By "high output pickup" do you mean an active one, or a passive one?  
    Another solution might be to use a lower gain tube for the Vibrato channel input- a 5751 or maybe even a 12AU7 or 12AY7.  
Steve Ahola
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12/5/2005 3:21 PM
Chris/CMW amps

Hi Rick  
Most of my customers do use the TB-channel...  
Did check some old and reïssue schematics...  
Maybe try replacing that volume-control with a 470k resistor/etc. and convert R25 (1M grid to cathode/etc.-point resistor) with a 1M log pot so you can adjust the level/gain after the 1st stage io. just before the PI.  
Hope this helps!  
Ps Couldn't find a source for that vibrato-circuit yet but I know the topboost-channel/eq-section was copied from a faulty Gibson GA77 schematic, hence the strange bass-pot-grounding :)
12/5/2005 3:54 PM
Greg Simon
Vox came up with the vib/trem circuit themselves, and its really a trem/more elaborate trem circuit anyway rather than a trem/vibrato. My Korg reissue AC30TBX will gain up pretty quick on the trem channel also, but it doesn't sound nearly as nice as the top boost channel, and thats mainly because there is only 1 gain stage before the PI on the vib/trem channel. The normal channel is the same way, with only 1 gain stage before the PI and just sounds cold and muddy compared to the top boost channel. I prefer the top boost channel myself, and usually use the vib/trem channel or normal channel only for effect in the studio.  
12/5/2005 5:18 PM
Rick Erickson

Thanks for the responses on this one. My idea of high output is a full size Gibson HB. I mainly play single coil Strats and Tele's, and even my Les Paul has Firebird style mini hb pu's instead of the P-90 type it had originally. I had never noticed a problem with the Trem channel before this, but I rarely use it either. One interesting use for it is to couple it with the TB channel and set it for Vibrato, which is another interesting point - the RI's have the Trem/Vib marking backwards on the amp. Maybe they were all produced like this, I don't know but it drives me crazy. I have changed it on a couple amps thinking it was just a production error, but then I realized it's probably like Fender saying Tremolo bar on the Strat and Vibrato channel on their amps. My ear hears what appears to be a pitch variation in Vibrato (Tremolo according to Vox) mode. Don't know if that's really what's happening but it's close enough for me.  
I have toyed with the idea of moving the volume control but unless the customer wants it done I'm not going to go that far with this. I have reduced the gain of the first stage a bit and increased the supply voltage to the Vib/Trem channel. This seems to be working pretty good. Not the way I would design it but what the heck... For the record - that 3 pos. Speed switch also annoys me. I replaced it with a 3MRA (10% Taper) in my grey top amp and now have a variable speed trem that goes much slower than a stock Vib.. I have noticed also that many of the RI AC30's have rather unstable ceramic caps in the oscillator circuit and the osc. speed will vary considerably with temperature changes. A can of freeze mist and a heat gun are very handy troubleshooting tools for this kind of work.  
12/5/2005 11:37 PM
Steve A.

    I don't know if you noticed but Vox has discontinued that AC30 model in this thread and have come out with a "CC" version which is pretty slick! They got rid of the Vib/Trem channel, but added a global tremelo function (which goes with the global reverb and global treble cut control).  
    There are seperate input jacks for the Normal channel and the Top Boost channel, but there is a small toggle switch to jumper them together. For the Normal channel there is also a small toggle switch for Brilliance, to emulate different AC30 models. The Top Boost channel has a switch on the tone stack, for a classic AC30 sound or a more modern sound.  
    More switches on the back for the techie types: two different cathode resistors on the output tubes for 22 or 33 watts. And two different bypass caps for a modern (44uF) or vintage (22uF) sound. FX loop with a -10dB/+4dB switch. 8/16 ohm switch to match the speaker load.  
    I tried one of them out for about 10 minutes at GC and they sound really nice! Especially the way that you can blend the two channels...  
Steve Ahola
12/6/2005 12:06 AM
Greg Simon
The initial builds of these new chinese made AC30CC amps were spotty, with lots of QC issues. I haven't seen inside one yet to see how they're made. I've heard good and bad on that count, but will reservce judgement until I see one in person.  
Regarding the trem on the AC30, this following stuff is a post I saved from Ampage from like 5 years ago, and it explains how the circuit(s) work.  
There are two circuits working together -- the oscillator and the modulator portions. The oscillator is very simple so let's tackle this first. It's a straight-forward phase shift oscillator, which works by placeing a series of phase shift networks around an amplifier. The phase changes so that the negative feedback becomes positive and -- viola -- we have an oscillator. The output is sent via the depth preset trim to a polarity or "phase" splitter. The two opposite polarity signals are used for the modulator.  
Okay, the modulator. Starting at the input jack, there is a traditional preamp stage (common cathode) with a split load, which drives the input of another polarity splitter. The cathode output is fed to a pair of band-pass filter networks, which are tuned differently. The plate output feeds two different high-pass filters. One each of the high-pass and band-pass filters are summed to provide two different outputs. Each output consists of one high-pass signal combined with one band-pass signal. Each of the final signals has very different frequency response and phase characteristics.  
So we have an input signal, which is turned into two signals that have different frequency response and phase characteristics. Those two outputs are re-combined in V9, which also has a split load. Remember our two, opposite polarity oscillator outputs? They are fed to the grids of V9 as well so that when one half of the tube is passing signal, the other isn't. The effect of this is to fade or pan from one of the filtered outputs to the other. The resulting sound is what Vox called vibrato but which is really just phase shifting with some tone shaping.  
The Vib/Trem switch shorts the audio going to one of the grids to ground, which removes one of the phase shifted signals entirely. At this point, you have a slightly colored sounding signal turning on and off through V9 or, tremelo. Note that the rate doesn't change when you switch from vibrato to Trem. The switch isn't connected to the oscillator's feedback network.  
There's only one more thing to talk about, which is the output of V9. It feeds FOUR series high-pass filters and then the volume control. The purpose of the high-pass filters is to remove DC from the output signal so the circuit wouldn't thump.  
That's all there is to it.
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