Early AC30 Treble vs Bass vs standard version

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Early AC30 Treble vs Bass vs standard version

Postby Milocj on Thu Aug 03, 2017 5:25 pm

Has anybody had the opportunity to compare the Bright channel on all three of these models? I love the normal channel on my '62 better than the Bright channel except for rare occasions. Was wondering how the Bass model Bright Channel compares to the Normal channel when used with a guitar.

It looks like its simply a bypass cap/resistor difference on half of V1 on the bright channel half. Would adding a Top Boost have minor differences with a Treble or Bass model given it attaches right at the volume pot?
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Re: Early AC30 Treble vs Bass vs standard version

Postby dlvoots on Sat Aug 05, 2017 1:41 am

Has anybody had the opportunity to compare the Bright channel on all three of these models? I love the normal channel on my '62 better than the Bright channel except for rare occasions. Was wondering how the Bass model Bright Channel compares to the Normal channel when used with a guitar.


Thanks to another post by Vintagekiki, there is an excellent link on the Vox Showroom site that provides technical details about the three non-Top-Boost AC-30 models (Bass, Normal, and Treble). If you go to the page and look about half way down, you'll find a small table with capacitor values that JMI used for the three models. The link is here:

http://www.voxshowroom.com/uk/amp/ac30_6_hood.html

It looks like its simply a bypass cap/resistor difference on half of V1 on the bright channel half. Would adding a Top Boost have minor differences with a Treble or Bass model given it attaches right at the volume pot?


You are correct about the three non-top boost models, in that the Brilliant channel is re-voiced by changing the values of a few key capacitors. The difference between any of the three Brilliant channels would be audible if you were to A/B compare the three models with each other, but it would not be an "In Your Face" difference in tone. Another way of stating it is that you probably wouldn't be able to walk up to a band and immediately say "That AC-30 sounds like a Bass version" (as opposed to a Normal or Treble version).

Another thing to remember is that on a non-Top Boost AC-30, the Brilliant and Normal channels only use a single gain stage in the preamp, and then the circuit goes straight to the power amp. That means it is difficult to overdrive the amp with only a guitar -- depending on the pickups, the volume may have to be 3/4 of the way up before you got a good distortion voice going. Add a pedal and overdrive? Absolutely. Straight guitar (and keep to a reasonably sane volume)? Not so much.

However, the Top Boost circuit adds an additional 12AX7 / ECC83 to the amp to drive the treble and bass tone stack. And since it is very close in design to a '59 Bassman or a Plexi Marshall, the difference in tone and the ability to drive the amp into distortion is quite dramatic when compared to a non-Top_Boost amp.

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Re: Early AC30 Treble vs Bass vs standard version

Postby DC Jones on Mon Aug 07, 2017 11:36 pm

Milocj,

There is very little difference between the Normal and the Bass versions - the Bass is set up to allow some more lower frequencies by having larger coupling capacitors on the pre-amp.

The treble model, on the other hand, has an identical resistor/capacitor setup applied to each of the channels, designed for the specific purpose of eliminating lower frequencies. It also has a small coupling cap between it and the phase inverter (*Please note - the image is incorrect - that value is .047 and not .47).

I'm with Dave - I don't think you'll notice much difference between Bass and Normal - whether you top boost or not. The treble model, on the other hand, will be a different animal, and a top-boosted one might be a little too much on the treble side.



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Re: Early AC30 Treble vs Bass vs standard version

Postby vintagekiki on Tue Aug 08, 2017 3:50 am

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