Vol knob basics.

[Dave H]

What 'Brilliant' means too (Ive never seen an AC30 with a Brilliant channel.. have you?) I've no idea either, but, the marketing bumf here is meaningless twaddle telling me absolutely nothing as to what a 'top boost' actually is.

The 1960 six input AC30 had three channels Normal, Brilliant and Vib/Trem with a single Tone control. Later it was decided to add Bass and Treble controls to the Brilliant channel. A lot of signal voltage is lost when the usual passive tone stack is added so it was necessary to add an extra 12AX7 to “Boost” the signal back up. The Brilliant channel has now become the Top Boost channel. There's nothing special about this channel. The added 12AX7 makes it similar to a 5F6A Bassman. Bright, Brilliant and Top Boost are just terms Vox has used to mean treble.

It's worth noting that the standard guitar amp tone stack isn't like a HiFi amp tone circuit where the frequency response is flat with the tone controls in the mid position. The guitar amp tone stack is “scooped” with the controls central. i.e. there's a “scoop” or mid dip in the frequency response between 500Hz and 1kHz even with the mid control on 10. The closest setting to a flat response would be Mid 10, Bass and Treble 0

[Fluoroscope 5000]

The 1960 six input AC30 had three channels Normal, Brilliant and Vib/Trem with a single Tone control. Later it was decided to add Bass and Treble controls to the Brilliant channel. A lot of signal voltage is lost when the usual passive tone stack is added so it was necessary to add an extra 12AX7 to “Boost” the signal back up. The Brilliant channel has now become the Top Boost channel. There's nothing special about this channel. The added 12AX7 makes it similar to a 5F6A Bassman. Bright, Brilliant and Top Boost are just terms Vox has used to mean treble.

It's worth noting that the standard guitar amp tone stack isn't like a HiFi amp tone circuit where the frequency response is flat with the tone controls in the mid position. The guitar amp tone stack is “scooped” with the controls central. i.e. there's a “scoop” or mid dip in the frequency response between 500Hz and 1kHz even with the mid control on 10. The closest setting to a flat response would be Mid 10, Bass and Treble 0

...also true for the TB stack knobs a 0, but with a 15dB boost with a 100k R2 value! Assuming the TB circuit is b4 the master volume, you could get mucho preamp OD at low volumes.

[Sea Chief]

The 1960 six input AC30 had three channels Normal, Brilliant and Vib/Trem with a single Tone control. Later it was decided to add Bass and Treble controls to the Brilliant channel. A lot of signal voltage is lost when the usual passive tone stack is added so it was necessary to add an extra 12AX7 to “Boost” the signal back up. The Brilliant channel has now become the Top Boost channel. There's nothing special about this channel. The added 12AX7 makes it similar to a 5F6A Bassman. Bright, Brilliant and Top Boost are just terms Vox has used to mean treble.

It's worth noting that the standard guitar amp tone stack isn't like a HiFi amp tone circuit where the frequency response is flat with the tone controls in the mid position. The guitar amp tone stack is “scooped” with the controls central. i.e. there's a “scoop” or mid dip in the frequency response between 500Hz and 1kHz even with the mid control on 10. The closest setting to a flat response would be Mid 10, Bass and Treble 0

Hi Dave, cheers for that. It makes more sense but seems strange for vox to harp on about this tb channel - exactly as if it is very special- when in fact, it is not then. It seems you need a failrly in depth knowledge of circuits, in order to understand what it means. This is very peculiar, from simply a players ' perspective. I dont know how most who likely bought them, could have had much understanding then of what this channel was all about.

So by adding just a treble and bass, which seems almost obligatory to me to have in any decent sized tube amp, and, as I just simply see they seem to have (PR, deluxe reverb, marshall, all mid sizes fenders & up including my twin reverb).. vox deem it suitable to market this normality as something 'added' , even 'special', when its just totally normal. I don't quite understand it. I thought it was some form of added layer of distortion (& indeed the rangemaster info, does suggest perhaps so). But a PR, or DR doesnt have any extra layer of distortion, they're in fact inherrantly clean afaict, especially a twin reverb, both with - two- sets of treble & bass controls. !Double Whammy inputs! why didnt they market them like so? Because, Id suggest, they are just par for an amp, add no extra distortion at all, nor any extra boost facets afaict. Lord I will never get this amp thing. Doesnt mean I dont love the sound as much as the next though, so I can only plough on.

Thanks SC

[Dave H]

So by adding just a treble and bass, which seems almost obligatory to me to have in any decent sized tube amp, and, as I just simply see they seem to have (PR, deluxe reverb, marshall, all mid sizes fenders & up including my twin reverb).. vox deem it suitable to market this normality as something 'added' , even 'special', when its just totally normal. I don't quite understand it. I thought it was some form of added layer of distortion (& indeed the rangemaster info, does suggest perhaps so). But a PR, or DR doesnt have any extra layer of distortion, they're in fact inherrantly clean afaict, especially a twin reverb

Well, the Top Boost circuit was 'something added' originally. It was a metal plate with a 12AX7 and two pots which had to be fitted to the back of the amp. It's inconvenient having to grope behind the amp to adjust the Bass and Treble pots so as the add on was popular it was incorporated on the control panel. It was 'special' for an AC30 which hadn't had bass and treble controls up to that point. Back in the day old British amps were kind of dull and jazzy sounding, not what was required for the new fangled Rock and Roll hence the need for treble boosters.

The Twin Reverb output stage is a lot harder to overdrive than an AC30 because it uses 6L6s and negative feedback. AC30 has EL84 and no negative feedback meaning it needs a much smaller signal at the input to the PI to drive it to clipping than the Twin does.

[Fluoroscope 5000]

There only seems to be a blackface era TMB tone stack in the TSC app. Previous Fender TB stacks either didn’t have a Midrange control and were configured for less mid-scoop, or were configured for less mid-scoop when the Midrange control was added, as in the case of the Bassman TMB stack from which the Marshall JTM45 TMB stack was derived.

The Vox TB tone stack has as much mid-scoop as the Blackface era Fender TMB stack. That reduces distortion a good bit. I can’t follow the AC15TB schematics that well, but it looks like only one triode of the the first ecc83 (12ax7) is used, and probably the cathode output of which drives the TB circuit, which then drives either of the next ecc83's, one of which might be for the Normal channel, both of which then drive the PI ecc83. I think the two other ecc83's are used for the Tremolo and Reverb. However it is configured, there isn’t as much preamp gain as the total amount of tubes might imply, and the TB ecc83 may not add any midrange gain. Tube gain also depends on the V at which they are run, although tubes distort more at lower V.

Those that do get more overdrive from an AC15TB might set the T: 0-1 & B: 1 for up to 5dB midrange increase, and then set the Tone Cut to 0 (or very low) to compensate for the loss of Treble. Jumping the channels with a Y cable or passive ABY box should drive the midrange even harder. All that with a higher output HB pickup would considerably increase distortion b4 the Master Volume stage...but I thought we weren’t discussing this here anymore?

[J M Fahey]

The VOX tone stack has a built-in Middle control hidden in plain sight, the Bass control does double duty.

Roughly from 0 to 6 or 7 boosts Bass smoothly; from 7 to 10 murders Mids, because wiper shorts Mid resistor, introducing a deep Mid notch and making tone control scooped.

We don´t recognize VOX as "scooped" only because it also cuts Bass and low Mids *drastically* , what remains is Brilliance/Chime.

I use that clever tone control a lot all over the place: Guitar/Bass ... even simple PA amps because of its simplicity and good results, only I design proper values according to my requirements and also *drastically* lower impedance to sensible levels, such as 25k or 50k pots

50pF treble cap? What were they thinking? Just wiring parasitic capacitance must vary sound A LOT.

Another reason for "not 2 amps sounding the same"

[Sea Chief]

Almost correct. The only change would be that you'll omit the 47k resistor (the one you weren't sure about) and the new 330p capacitor will be in series with the 100k resistor instead of parallel with it. Just like the modified drawing I posted.

Hi back over here Chuck.

I'm just going back over the thread (yup, I do re-read threads, often a few times in this case). I've just spotted your modified-post new cap configuration: you did it so subtly & so well I couldn't spot any changes made! Ok reading your explanation makes sense now.

I'm still trying to source this B250k Bourns tone pot. Is "No Load" the type to get-? maybe a CTS would be easier to find: I think you menrtioned they have the same taper % figure.

Thanks, SC

[Sea Chief]

Well, the Top Boost circuit was 'something added' originally. It was a metal plate with a 12AX7 and two pots which had to be fitted to the back of the amp. It's inconvenient having to grope behind the amp to adjust the Bass and Treble pots so as the add on was popular it was incorporated on the control panel. It was 'special' for an AC30 which hadn't had bass and treble controls up to that point. Back in the day old British amps were kind of dull and jazzy sounding, not what was required for the new fangled Rock and Roll hence the need for treble boosters.

The Twin Reverb output stage is a lot harder to overdrive than an AC30 because it uses 6L6s and negative feedback. AC30 has EL84 and no negative feedback meaning it needs a much smaller signal at the input to the PI to drive it to clipping than the Twin does.

Great thanks for this info Dave H. I've heard of this negative feedback & heard that the 6L6 is synomymous with a cleaner sound, so the Twin reverb makes more sense now. I dont like the sound of those early jazzy dull sounding ac30's then.

[Sea Chief]

There only seems to be a blackface era TMB tone stack in the TSC app. Previous Fender TB stacks either didn’t have a Midrange control and were configured for less mid-scoop, or were configured for less mid-scoop when the Midrange control was added, as in the case of the Bassman TMB stack from which the Marshall JTM45 TMB stack was derived.

The Vox TB tone stack has as much mid-scoop as the Blackface era Fender TMB stack. That reduces distortion a good bit. I can’t follow the AC15TB schematics that well, but it looks like only one triode of the the first ecc83 (12ax7) is used, and probably the cathode output of which drives the TB circuit, which then drives either of the next ecc83's, one of which might be for the Normal channel, both of which then drive the PI ecc83. I think the two other ecc83's are used for the Tremolo and Reverb. However it is configured, there isn’t as much preamp gain as the total amount of tubes might imply, and the TB ecc83 may not add any midrange gain. Tube gain also depends on the V at which they are run, although tubes distort more at lower V.

Those that do get more overdrive from an AC15TB might set the T: 0-1 & B: 1 for up to 5dB midrange increase, and then set the Tone Cut to 0 (or very low) to compensate for the loss of Treble. Jumping the channels with a Y cable or passive ABY box should drive the midrange even harder. All that with a higher output HB pickup would considerably increase distortion b4 the Master Volume stage...but I thought we weren’t discussing this here anymore?

I wish I knew what this 'mid scoop' was all about. I don't know what I'm meant to be hearing, a signature 'fender' sound, or something.. but I can't distinguish a mid-scoop sound to a no-mid scoop sound.

Anyway I think your TB set up idea here, is based upon an AC30.. which has two channels, plus a 'Tone Cut' knob? (I've no idea what this is).

This 15TB only has one channel (two inputs), no Tone Cut. So I think it's a sort of bastardised Top Boost circuit perhaps. Thx SC

[Fluoroscope 5000]

I wish I knew what this 'mid scoop' was all about. I don't know what I'm meant to be hearing, a signature 'fender' sound, or something.. but I can't distinguish a mid-scoop sound to a no-mid scoop sound.

Anyway I think your TB set up idea here, is based upon an AC30.. which has two channels, plus a 'Tone Cut' knob? (I've no idea what this is).

This 15TB only has one channel (two inputs), no Tone Cut. So I think it's a sort of bastardised Top Boost circuit perhaps. Thx SC

The Tone Cut knob rolls off the high end above ~2kHz after the PI tube, but I'd assume your 15TB doesn't have any high end roll off since it has no Tone Cut knob. You can still try the tone stack settings I suggested for more midrange (less "scoop"), and the R2 value change if the circuit board allows it.

[Chuck H]

Hi back over here Chuck.

I'm just going back over the thread (yup, I do re-read threads, often a few times in this case). I've just spotted your modified-post new cap configuration: you did it so subtly & so well I couldn't spot any changes made! Ok reading your explanation makes sense now.

I'm still trying to source this B250k Bourns tone pot. Is "No Load" the type to get-? maybe a CTS would be easier to find: I think you menrtioned they have the same taper % figure.

Thanks, SC

Ah! Yes. I've been out of the loop for a while and failed to register that both Bourns and CTS make "no load" pots now. I've always made my own by scratching the track (as shown in the drawing). Either the bourns or CTS "no load" pot would be perfect. Make sure the one you get is a 250k audio taper as they're also offered in linear taper. You can get either on *bay uk right now (I just checked). Wire it up with the 510k resistor across the two outer lugs as shown in the drawing.

[Sea Chief]

Ah! Yes. I've been out of the loop for a while and failed to register that both Bourns and CTS make "no load" pots now. I've always made my own by scratching the track (as shown in the drawing). Either the bourns or CTS "no load" pot would be perfect. Make sure the one you get is a 250k audio taper as they're also offered in linear taper. You can get either on *bay uk right now (I just checked). Wire it up with the 510k resistor across the two outer lugs as shown in the drawing.

Just done the rewire: I found an Alpha pot only one with a 10% taper/ quality fine for this gtr.. so done that A250k in now for my tone pot. Scratched the track where you said to, to make a no load pot (I think this was the idea), bunged in my 220pf cap on the vol pot.

Sounds great, the vol seems now all bunched up in the top half of the rotation, but this is ok I think. The tone seems even all along now, no sudden dips.

It does lose some treble rolling the vol down tho, I think I maybe put a higher value cap on the vol pot before, in order to resolve this-? (but I do think 680pf as it was, was getting too bright rolling vol down : so a cap mid value between 220 & 680pf.. I assume is the optimum idea).

Thanks alot- SC

[Fluoroscope 5000]

Just done the rewire: I found an Alpha pot only one with a 10% taper/ quality fine for this gtr.. so done that A250k in now for my tone pot. Scratched the track where you said to, to make a no load pot (I think this was the idea), bunged in my 220pf cap on the vol pot.

Sounds great, the vol seems now all bunched up in the top half of the rotation, but this is ok I think. The tone seems even all along now, no sudden dips.

It does lose some treble rolling the vol down tho, I think I maybe put a higher value cap on the vol pot before, in order to resolve this-? (but I do think 680pf as it was, was getting too bright rolling vol down : so a cap mid value between 220 & 680pf.. I assume is the optimum idea).

Thanks alot- SC

It will be different with the 680pF wired in series with the 100k R. You could try it, but a low C cable might actually be needed for better linearity. It will also give the tone knob more tonal variance from high to low.

[Chuck H]

Sounds great, the vol seems now all bunched up in the top half of the rotation,..

Yep. That's because you're hearing the taper now. Very gentle at the lower end of the adjustment which gives a lot of control for higher gain stuff. Since you don't seem to need that I have a good solution for both your volume pot taper and your brightness issue. Go back to the old style bright circuit you were using (with the cap and resistor parallel in circuit), but use a higher resistance value for the parallel resistor. I think a 510k would be fine if you have extras of that value from the tone circuit. Otherwise a 470k or 680k would probably be ok too if you have those on hand. And I might choose a 330p cap since the 680p was a little bright for you. Better yet, if you have 220p caps on hand you could just use two of those in parallel for 440p. You can wrap and solder the caps and resistor together before adding them to the volume pot so that you only have one extra lead to insert for each lug hole.

The 510k parallel resistor for the bright circuit will make the volume taper behave somewhere between where it was and where it is now.

[Fluoroscope 5000]

Yep. That's because you're hearing the taper now. Very gentle at the lower end of the adjustment which gives a lot of control for higher gain stuff. Since you don't seem to need that I have a good solution for both your volume pot taper and your brightness issue. Go back to the old style bright circuit you were using (with the cap and resistor parallel in circuit), but use a higher resistance value for the parallel resistor. I think a 510k would be fine if you have extras of that value from the tone circuit. Otherwise a 470k or 680k would probably be ok too if you have those on hand. And I might choose a 330p cap since the 680p was a little bright for you. Better yet, if you have 220p caps on hand you could just use two of those in parallel for 440p. You can wrap and solder the caps and resistor together before adding them to the volume pot so that you only have one extra lead to insert for each lug hole.

The 510k parallel resistor for the bright circuit will make the volume taper behave somewhere between where it was and where it is now.

I realized I got the response of higher value R in the series wired treble bleed backward, so I deleted that portion of my suggestion. I agree that 220pF to 440pF is better than 680pF, but ~220k R with the 220pF C for the parallel treble bleed shows the most linear response using the Guitarfreak database app with the output being down ~12dB when the Volume knob is at 5. Higher R only reduces the output below the peak more, and lower R increases it, and there is more treble loss if the cable C b4 the first preamp is too high. I don't know all the details, but think it has to do with the C value on the bleed vs the C of the cable in relation to the pickup impedance with the added R as the volume is turned down, as well as the impedance and "Miller" C of whatever input the guitar is plugged into. Even going from a typical ~360pF 12' store brand cable to a ~200pF cable could make a substantial difference in the linearity so the C value on the bleed would also be less critical. Shorter low C cables were the norm for a reason b4 the long coil cables became popular in the late '60s.