AC 30/6 trem filter give the amp its voice??

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RockinRocket
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AC 30/6 trem filter give the amp its voice??

Post by RockinRocket »

Ive once read that the AC 30/6 from 60-63 trem filter on the other input of the PI affect the tone of the amp. A "mid honk" one would say. This amp in question has a 1/2 a 12ax7 for the normal channel and 1/2 a 12ax7 for the brilliant channel mixed into one side of the PI while the trem connected to the other. Now can one adjust the trem controls to affect the tone of the normal/bright channels?? Affectively using the trem filter control as a way of dumping certain frequancys to ground?

The 1960 ac 30/6 schematic is on schematics unlimited. Will try and post later if needed.
pdf64
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Re: AC 30/6 trem filter give the amp its voice??

Post by pdf64 »

I think that's a tone lizard theory http://tone-lizard.com/vox-myths/

There's some great stuff on his site, and he's been kind enough to respond to a query of mine via email, but I'm sceptical about that particular point, because the tone of the other channels doesn't seem to be affected by the vib channel vol control setting.

It's great to see his site back up now!
RockinRocket
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Re: AC 30/6 trem filter give the amp its voice??

Post by RockinRocket »

Yes it was tone lizard site that I read that on. Im guessing you've played that particular amp with no success adjusting the trem vol for tone affects. There is a you tube demo of a 60s ac30/6 that sounds nothing like the newer handwired and pcb voxs and wasn't sure if the trem had anything to do with it
pdf64
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Re: AC 30/6 trem filter give the amp its voice??

Post by pdf64 »

RockinRocket wrote: Im guessing you've played that particular amp with no success adjusting the trem vol for tone affects. There is a you tube demo of a 60s ac30/6 that sounds nothing like the newer handwired and pcb voxs and wasn't sure if the trem had anything to do with it
Correct, my finding is that there's no appreciable tonal affect on the other channels as the vib/trem channel volume control is adjusted.

Many more modern AC30 models have had the full vib/trem channel in place.

There are various reasons why a 50+ year old amp may sound different to a newly made one of the same design, eg resistor values can drift a lot, alnico magnets will gradually lose charge, mods.
SpeedRacer
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trem channel

Post by SpeedRacer »

The trem channel has a 3-pole HPF which gives that channel a different "voice" than the normal or TB channels. It rolls off a lot of bass and gives you a nice mid-heavy tone.
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Mr. dB
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Re: trem channel

Post by Mr. dB »

SpeedRacer wrote:The trem channel has a 3-pole HPF which gives that channel a different "voice" than the normal or TB channels. It rolls off a lot of bass and gives you a nice mid-heavy tone.
The HPF is there to filter out any "thump" from the LFO isn't it?

Anyway, the notion that the vibrato channel HPF changes the tone of the other channels seems line nonsense to me.
pdf64
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Re: AC 30/6 trem filter give the amp its voice??

Post by pdf64 »

Yes, it seems reasonable to conclude that the HPF is there to get rid of modulation thump.
The corner frequency of each stage (4n7F - 1M) is about 50Hz, though being a 3 stage passive type, the combined roll off may be -3dB at 500Hz. So the tonal balance may be altered a bit.
It would be great if someone with spice etc competence could have a go at modelling this to confirm? As the progressive loading of each filter upon the preceding one/s may affect things.
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martin manning
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Re: AC 30/6 trem filter give the amp its voice??

Post by martin manning »

The 4n7-1M knee is at 34 Hz, and there are three of them. Each C-R has roll-off of 20dB/decade below 34 Hz, so three in series will roll-off at 60 dB/decade. Since the LFO frequency could be around a decade below fo (3.4 Hz), the roll-off there would be ~60 dB. There is a fourth HPF made up of the last 4n7 and the 500k volume pot. The fo there is an octave higher at 68 Hz, so there's another -26 dB at 3.4 Hz (20 dB for the decade plus 6 dB for the additional octave below fo). Overall there is ~16 dB reduction at 82 Hz (low E), so yes quite a bit of bass reduction on that channel.
Mark
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Re: AC 30/6 trem filter give the amp its voice??

Post by Mark »

Hi Martin, thanks for the explanation of the bass response being attenuated on the tremolo channel. I think the second part of the question was does the tremolo channel effect the normal or top boost channel?

What are your thoughts on that?
Yours Sincerely

Mark Abbott
pdf64
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Re: AC 30/6 trem filter give the amp its voice??

Post by pdf64 »

martin manning wrote: Sun Jan 01, 2017 1:09 amThe 4n7-1M knee is at 34 Hz, and there are three of them. Each C-R has roll-off of 20dB/decade below 34 Hz, so three in series will roll-off at 60 dB/decade... Overall there is ~16 dB reduction at 82 Hz (low E), so yes quite a bit of bass reduction on that channel.
I think that the theory for the characteristic of simple passive filters assumes that they are fed by zero source impedance and have an infinite impedance load; so for a string of such filters to have their cumulative response a simple multiple of a single filter, a buffer would be needed between each.

Whereas with the Vox HPF, the later filters are fed from a significant reactive source impedance formed by the preceding filters, and the later filters present a significant reactive load to the preceding filters.

Hence my thought that just adding the theoretical assumed response of single stages may not be adequate and working out the cumulative response of such a multiple stage passive filter may be quite tricky, and modelling software may be good for this type of task.
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Mr. dB
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Re: AC 30/6 trem filter give the amp its voice??

Post by Mr. dB »

Mark wrote: Sat Apr 22, 2017 11:23 pm Hi Martin, thanks for the explanation of the bass response being attenuated on the tremolo channel. I think the second part of the question was does the tremolo channel effect the normal or top boost channel?

What are your thoughts on that?
If the volume of the Vib/Trem channel is turned down, then that grid is shorted to ground at audio frequencies. Filter won't affect the inputs which are connected to the other grid.
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