Vox Treble Circuit

[rogerbayliss]

Hi

Maybe a question for Charlie this one.

I was reading about how several vintage amps used capacitors wired across the master volume to provide a bypass for trebles and in some cases mids depending on capacitor values used. Often a bright switch enabled the feature.

Is this how the brilliance circuit of a Vox was implemented ? which seems to be the case according to an engineer from Origin Effects.

Rog.

[rogerbayliss]

I may have found an answer here

VOX LINK

[rogerbayliss]

Looking at the panels on those old AC30's, it is clear that the brilliance circuit was in fact the name Vox gave to the Top Boost circuit , so I get that now . But an AC30T or AC30 Treble was done in 1964 which is different . That amp had the extra capacitors on volume and phase invertor which is clearly what I was intending to ask about.

I think the Brilliance circuit on the AC15 though was different with bass cut to accentuate highs and mids ?

I think the use of brilliance instead of top boost later on AC30 top boost versions along with frilliance on no top boost AC15 is where my confusion came from.

[Charlie Hall]

Hi Roger,
You understand it all about right.
By master volume I think you mean the channel volumes as early amps didn't have master volumes.
The Treble or T versions had a resistor and a capacitor on each of the 3 volume controls. Normally there would only be a capacitor so when the pot is at maximum, the capacitor has no effect. But on these amps the resistor was in series with the pot and the capacitor connected across the input of the resistor and the output of the pot so there was still some treble boost even with the pot at maximum. The downside of this arrangement is that the pot can never pass the full amount of signal so these amps will not be capable of as much overdriven sound as the non Treble amps.
I don't think there was ever any treble boost at the phase inverter but different capacitors at the input and/or output would alter the amount of lows that would be passed to the power amp. Larger value capacitors were used here on the bass amp versions.
The top boost amps had two extra stages, one a gain stage, the other a follower that was simply an impedance converter, derived from one double triode ECC83 driving the bass and treble controls which then passed to the phase inverter. The brilliant channel that was used for the top boost had a capacitor across the brilliant channel volume pot, and no resistor.
Regards,
Charlie

[rogerbayliss]

Many thanks for the tech info Charlie , great explanation. Yes I did mean Channel volume. I understand the resistor and capacitor would form a simple RC type filter to pass certain frequencies as well which is what it appears to be.

I did get myself confused at first, but later understood that Vox originally called the AC30 Top Boost the Brilliance channel , before it got to be the Top Boost channel. The AC15 also had a Brilliance channel which was not same and that was what my confusion was when I asked this question.

Yes the resistor in series would definitely reduce the available signal to the output resulting in reduced gain, that is understood. I read the Marshall amps with single volume and some Fender amps had capacitors across the output volume to pass more trebles and that Vox had done this too, which led to my interest in what Vox did.

I don't know if the Shadows ever used the AC30T issued in 1964 as Vox article indicates , or whether it lasted long in reality.

Obliged

Rog.

[Charlie Hall]

Hi Roger,
I had heard that The Beatles used the Treble version at some point. I don't know about The Shadows. I once had a 1964 grey panel black diamond cloth AC30 which was a T version without top boost, so I think it did last a while.
Many Marshall amps have a very large value capacitor on the High Treble channel. These are the amps that have a lot of mids boosted as well as the treble.
Regards,
Charlie

[scanners]

Charlie, I ve never played an early AC 15 or 30 , did the brilliance switch just add treble without cutting bass ? I have the first Chinese AC 30 with all the bells and whistles it has a brilliant switch on normal channel that adds a lot treble without cutting bass. I was interested because the Shadows sound is more about cutting bass than adding treble
Matthew

[Charlie Hall]

Hi Matthew,
The brilliance switch on the original amps added no treble, it cut a lot of bass.
Regards,
Charlie

[rogerbayliss]

Hi again Charlie,

Regarding the brilliance switch, I just wondered what degree of cut to the bass . Did it act like an High Pass Filter ? What frequency did it cut up too ?

Roger

[Charlie Hall]

Hi Roger,
The brilliance switch is simply a high pass filter. I don't know the exact frequency but it is very high, probably somewhere between 500 Hz and 1000 Hz. It removes so much that the treble frequencies that remain have to be reduced with the cut control. I did work out that the frequency of the cut control is somewhere around 750 Hz. That of course is not a simple low pass filter except when the cut control is set for the least treble.
Regards,
Charlie

[Charlie Hall]

I did a quick calculation based on the capacitor and pot value and it worked out to 1273Hz. Including the phase inverter input impedance, although very high, might make the frequency a little higher still.
Regards,
Charlie

[rogerbayliss]

Thanks for your answer Charlie, wow that is quite a cut ! Did the Shadows use it ?

[Charlie Hall]

I believe The Shadows must have used it. It instantly gets you close to the sound of the early hits, except for Apache and Quatermasster's Stores when the earlier 1959 AC15 was used. Others are aware of this too.
Regards,
Charlie

[bernardj]

hi Roger, Charlie

It is quite likely that the filter point is around 1200 hz...

But what is important is also the slope of this filter which is undoubtedly at 6db / octave
such a device does not correspond at all to the spectrum of Hank's guitar that I have been able to identify.
the cut-off point is rather around 300 Hz with a slope probably greater than 6 db, but it must be remembered that in some Aroad titles to add 6 db more with the RS106

there is also a phonomene which corresponds in the AC15 -2 and even the others has a strong reading around 1760 hz

For 2 years that I try to understand the tonal balance of Hank I have a doubt that his AC15 amp conforms to the official Vox diagrams ... but I could be wrong .... lol ...

Bernard
Thanks google trad

[fenderplucker]

Don't forget that the Meazzi that Hank used also affected the frequency response, with a cut of about 3dB at around 250Hz.

Paul.

Edit, I meant 3db, not 6dB.

[rogerbayliss]

Thanks for contributing Paul and Bernard

[Charlie Hall]

Hi Bernard,
You must consider the effect of the cut control too. This can remove so much treble that more mids remain than anything else and this is the sound most of us are trying to get. This will no doubt change the low roll off point too, since the roll off point has to be referenced to the main frequency content.
It is easily possible to overthink all of this.
The Meazzi Echomatic 2 had a roll off near to 100Hz so while this will modify the tone spectrum a little more, the main content is relative to the mids.
Regards,
Charlie

[fenderplucker]

Charlie, Bernard,

Just to clarify the low frequency cutoff of the Meazzis, I presume that Charlie has based his figure on the nominal values of the 4n7 interstage coupling capacitor and 500k pot. However, the resistor networks that follow the pot also load the capacitor so that the cutoff can be as high as 200Hz, depending upon the level pot setting. I haven't measured the actual value for an Echomatic 2, but for tube tape Meazzis (that admittedly have different resistor networks) I have measured 3dB cutoff values from 150 to 250 Hz at typical level pot settings. Long Toms have smaller interstage coupling capacitors (typically 2n2) and so the cutoff is even higher and can be up to 300Hz.

[bernardj]

Charlie, Fenderplucker

Thank you for your sharing of experience. I am very happy that you are interested in my contributions.

My work proposes new hypotheses based on the knowledge of the material of the time: guitars, amps, Abbey road equipment (before the Beattles!) But also an understanding more and above all close to that of a sound engineer than of a guitarist or of an electronic technician.

It may seem original and I force myself to share with other specialists the fruit of my research that I show whenever possible with sound examples, some of which I have already posted here

Regarding the Vox filters we are talking about the question for me is not whether when we cut the bass and treble only mids remain ... because if it was the right solution everyone could have a sound that looks like Hank's.

A technician friend who repaired a large number of equipment from this period (Vox AC15 2, 3 Selmer, AC30 4, 6 Meazzi with drums...etc..) and carried out a large number of measurements on this occasion told me that the bandwidth of the plateau Meazzi he had in his hands was reduced. which effectively lets the mediums come to the fore.

It also tells me that the original components of these machines are of very unequal quality. Do not forget the Meazzi intervenes only partially in the tonal balance of Hank as well as in the saturation.

But by applying what is commonly accepted on this subject one can then conceive that to play in public with a 15 w amp by cutting the bass and treble we can obtain Hank's tonal balance while allowing him at the same time to play with a sound pressure adapted to that of the group.

I myself have an Ac15 H1TV heritage and this approach to the pb does not work ... the analysis that I carried out with the Bright filter - which joins your measurements Charlie - shows that it "cut" way too high (1000- 1200 hz) by removing the very nice thickness that is in the sound of Hank

I must also that all the analyzes which I was able to make by isolating as well as possible the strat of Hank show an "unnatural" accentuation of the frequencies towards 1700-1800 hz followed by a very rapid fall after the action of the RS 127 (a 2.7 or 3.5 k) in this period of musical production of the Shads

These analyzes show, starting from E 6 string, that the gain is reduced from at least 12 to 18 db.
This represents a differential of more than 30-32 db between the high point which is located around 1700-1800 Hz and the low point of the curve which is therefore located on the E 6th strings

Such or such valve or hp can not explain this phenomenon. No official Vox diagram mentions this device which would seem to be the result of a customization ... especially as the filtering possibilities at Aroad did not allow by themselves to generate such corrections ..

I am starting to publish part of this work on YouTube for the moment about the AC30 4 and 6.
www.youtube.com/watch?v=8dI7Ak4lOoA

Unfortunately It is in French for the moment but we can see the diagrams and the work on the curves.

Sometimes, I tell myself that if we do not admit the presence of a customization for medium frequencies in Hank's amp the enigma may last at least another 60 years .... lol

regards
Thanks google trad
Bernard

[Charlie Hall]

Hi Bernard,
Something not often considered or even known about is the harmonic content caused by non linearities of the EF86 and other valves in not only the guitar amplifier but other items of recording equipment. I can also tell you that the non linearities are far greater in the 1959 EF86 circuit compared with the 1960 version. The Apache sound will always have something missing if this is attempted with a different amplifying system.
I have done some experiments of my own with this in the past. If the harmonic content is there, added by the EF86 especially, then within sensible ranges, the sound is quite well established regardless of precise EQ settings. I think this is an important consideration for anyone trying to recreate these sounds.
Regards,
Charlie

[fenderplucker]

Another factor in the overall guitar tone of the original recordings was the "pop" equalization used in the Redd.37/51 recording desk that provided a shelving bass boost and cut (max around 8dB at 100Hz) and peaking treble boost and shelving treble cut (max 10dB at 10kHz). Unfortunately it is not known what settings were used but with both the bass and treble equalisation in effect there were a very wide range of tonal possibilities.

Paul.

[bernardj]

Paul, Charlie
Thank you for your answers


Paul :
No doubt, the shelving in the pop module of REDD played a role. But the joint and very particular interaction of these 2 low and high shelving (rather 5 khz for the high shelving of the pop module) may not have an important attribution in Hank's sound. because most of the time its bandwidth is limited to 3.5 in the treble ... but of course these 2 filters may have played a role in limiting the bandwidth without however explaining this very strong presence around 1700-1800 hz

In the Vox AC 15-2 the corrections only work to remove the treble (cut) a bit like a tone of the strato .. But the official diagram does not allow to control an strong addition of gain on a band of frequencies around from 1700_1800 Hz

Charlie :
Thanks very much for all the details.
I myself could hear when I changed the valves of my AC15 H1TV heritage several sets of excellent quality valves. it is true as much for the amp as for the pre-amp there is a significant incidence according to the chosen valve.

But overall what I've heard focuses more on how the valve will start to saturate and how its content in harmonic enrichment will behave during this saturation. the production of major accident on the gain of a specific frequency band did not occur to me. But it is true that the valve which I listened to were not those of 1960 but only what one can find better after having compared them with several marks at the current manufacturer. I had chosen for the EF86 Sveltana, EL84 Valvo
Rectifier EZ81 RSD and 12 AX7 Sylvania

Regarding the customization of Hank's amp, Bruce himself, hadn't he declared that Hank's amp had something special in the midrange so that we could hear it better '' ?

Regards
Bernard

[Charlie Hall]

Hi Bernard,
It is correct that there is nothing in the Vox amps that would boost this frequency range, nor with the Redd EQ. However, it might have been possible to boost around 2KHz with the use of the optional Pultec EQ in the studio. Although the Pultec EQ is entirely passive components and has no gain at all, it is used in conjunction with booster amplifiers.
My experiments with valves were with the differences I heard in the clean sounds well below saturation levels. Of course, the saturation characteristics can vary too. As for the EF86, I have tried several Mullard EF86 valves. One of those was of a completely different internal construction but every single one of the Mullards sounded exactly the same as far as I could tell with my ears. Someone who was with me observed exactly the same results as me.
Regards,
Charlie

[fenderplucker]

Hi Bernard, Charlie,

I ran a circuit simulation on the Pultec EQP-1A upon which the REDD equalizer was based and it can produce a peak at around 1-2kHz with the bass cut set at 30Hz and the treble cut set at 20kHz. However, being simple single pole R-C filters, the peak is fairly broad with the -3dB points at about 400Hz and 3.6kHz.

Paul.

[Charlie Hall]

Interesting. I must admit I had never noticed this particular frequency band on Shadows recordings, not that I ever actively studied any in this way. I have casually noticed a lot of activity around 3KHz on some of the recordings. Probably more to do with the amp's speakers than anything else.
Regards,
Charlie

[fenderplucker]

Regarding the Pultec equalisation, adding another low frequency pole from the Meazzi and another high frequency pole from the Vox cut control (assuming the normal channel)can lead to a sharper overall response peak. Conversely, having the bass and treble controls on the Pultec on full boost produces a dip of around 10deB at around 1kHz -2kHz, so just about anything is possible! it is a pity that there were no engineer's notes from those early recordings to indicate what, if anything, might have been used.

Paul.

[philc]

I think it was the case of just using what was at hand in the control room to get the required sound, nothing really jotted down because there was no reason to do so, it changed a little when the Beatles came along with what gear was used and track bounce down sheets etc.

BTW, it's not so much the top end that interest me, it's the mid-high's as these seem to be in abundance with the early recordings.

Phil

[bernardj]

Phil
I agree with this approach. but as Paul says, we have no notes on the subject.
When we speak about Pultec we must think that it is the diagrams of the devices which were `` inspired '' by Pultec most with resistance / capacitor filters therefore passive which did not allow drastic action.
if we could be in the cabin in the 1960s (lol) we might be surprised at how little material is available because studion 2 was reserved for chamber music where the corrections are not electronic but acoustic.
The material, in compensation was of a very high-end audio quality, without concessions. There is almost no chance that there was an Eq Pultec made in USA in the Cabin in this time

Paul
I agree that we do not have enough documents and to my knowledge, I could not find the diagram of the Pop filter (can you have that?) and I looked for a lot of solutions with Universal emulations Audio (REDD mixers 1,7, 3,7, Eq Pultec) without being able to recreate the characteristic phenomena of Hank's bandwidth or a deep trough towards 1k 1k2 is immediately followed by a powerful rise towards 1700 ... sometimes up to at 2.5 then a rapid fall.

The problem is that this is due to a ''differential'' that only modern devices can provide. The ones in Studio 2 weren't '' designed '' for that and the filter action while being musical were above all very '' friendly '' lol ..

this is why we can think of the presence of a particular device that Dick Denney considered to allow Hank's guitar to ''stand out'' from the group.

Mr. Addey's genius along with that of D Denney and N Parramor was intended to make Hank's guitar 'resonate' and the mix is ​​actually treated as a 'trio of guitars' where Jet's bass is 'Deep' '' to put Hank's guitar whose bass is '' cleaned '' with the complement very well dosed in timbre with Bruce's Gibson (from Cliff ..lol)

Charlie
I have never heard of EF 86 Mullard lamps with large plate .. undoubtedly this influences the sound because of course I heard the differences between several brands when I did this for my AC 15 Heritage.

there are still avenues to explore because Hank's signal may very well have been processed on 2 channel strips, or may even have suffered with the help of a small cable the doubling on the 2 inputs of his ampith differents settings. .which gives a lot of possibilities...

However as an artist, Hank like all great musicians, would not have accepted that one transforms his sound to the opposite of what it was in his amp ....
the characteristics of its curve in the sixties show during several months of this time the "incidents" of which I spoke

Once again thank you for talking with me, because I think the subject is not finished and the experiments that I can carry out with the EQ2 as I show it on YouTube seem promising.
No doubt one person cannot explain everything ... but several ...

Regards
Bernard

[Charlie Hall]

Hi Bernard,
I don't know that the EF86 plate size varied. The difference I saw with the Mullard valves was with the cylindrical metal surrounding part. Most of them have what looks like perforations or fake perforations but there was one that had a solid shiny metal part in place of the usual perforated one.
Regards,
Charlie

[bernardj]

Charlie

thank you very much
I will find out more about it and of course, I will say what I can learn from it
Regards
Bernard