Vol knob basics.

[Chuck H]

Again, regarding the AC15TB, there's not a ton of clipping available, but there IS some. Below is a demo where a guy claims no pedals and is just taking the amp through it's paces to show what it can do on it's own. He's using a strat, so, similar guitar output to what you should have with the tele. Distortion demo starts about 1:40 and goes to about 2:25. If you're not getting this level of distortion then you're doing something different, possibly habitual, that is reducing gain.

https://www.youtube.com/watch?v=NvOhu8EmFTA

As mentioned before by myself and John H. Plug the amp into the attenuator, turn the master volume up full. Also keep the amp tone knobs up around 5. Keep your guitar volume and tone up full. Now your amps first volume control (the one that ISN'T the master volume) will be much more sensitive and you should be able to induce at least the level of clipping in the above clip. Since you have the attenuator you could just start with all knobs, Preamp volume, master volume and guitar volume up full!!! Again, keep the tone knobs up too. These are also gain controls, just that they reduce gain above or below a certain frequency.

I just can't explain it any better. The power tubes will clip. But you have to drive them. THat's what the attenuator is for. It's there so you can use amplifier settings you couldn't normally use because it would be too loud. I still think you may be doing something habitual in your setup or method that is reducing gain. So I'll say again... Guitar up full, amp volume up full, master volume up full, tone knobs at or above 5. Use the attenuator to quiet things down from there.

Only NOW can you use your guitar volume to lower the distortion level.

[Sea Chief]

Again, regarding the AC15TB, there's not a ton of clipping available, but there IS some. Below is a demo where a guy claims no pedals and is just taking the amp through it's paces to show what it can do on it's own. He's using a strat, so, similar guitar output to what you should have with the tele. Distortion demo starts about 1:40 and goes to about 2:25. If you're not getting this level of distortion then you're doing something different, possibly habitual, that is reducing gain.

https://www.youtube.com/watch?v=NvOhu8EmFTA

As mentioned before by myself and John H. Plug the amp into the attenuator, turn the master volume up full. Also keep the amp tone knobs up around 5. Keep your guitar volume and tone up full. Now your amps first volume control (the one that ISN'T the master volume) will be much more sensitive and you should be able to induce at least the level of clipping in the above clip. Since you have the attenuator you could just start with all knobs, Preamp volume, master volume and guitar volume up full!!! Again, keep the tone knobs up too. These are also gain controls, just that they reduce gain above or below a certain frequency.

I just can't explain it any better. The power tubes will clip. But you have to drive them. THat's what the attenuator is for. It's there so you can use amplifier settings you couldn't normally use because it would be too loud. I still think you may be doing something habitual in your setup or method that is reducing gain. So I'll say again... Guitar up full, amp volume up full, master volume up full, tone knobs at or above 5. Use the attenuator to quiet things down from there.

Only NOW can you use your guitar volume to lower the distortion level.

Hi Chuck,

yup thanks for this: but again it's going back a step: I -cannot- get the amp (same one- useful clip this) to sound like this with the attenuator. I know how you dig into strings the distortion will increase, or you should be able to use your vol control (the pipe dream situation I cannot achieve). You simply turn both the vol & MV knob up flat out on the amp (tone pots say midway), on the attenuator you turn up max. That should get me in 'a similar area' of distortion to the clip (post 1m 40). Not smack bang on no, but let's just say 'similar' for now. But no, it does not do so. It simulates only having the MV at perhaps level 6, nothing more. Therefore there is no distinction really, between this sound & taking the attenuator out, & just using the amp with preamp vol max, & MV at 2. No distinction much at all. Therefore it makes no sense to strain the tubes/ strain the amp/ adding the attenuator.

Ive turned the amp up once briefly to see -if it is indeed capable- of sounding like the clip (past 1min 40).. & so loud I couldn't tell, as I was panicking, but I dug into strings & did hear distortion, but it was painfully aggressive in not a big room, it was awful. Not a good test really. But I think a big dose more distortion than with the attenuator: so that was the only result needed from the test. The amp I think then -IS- capable.

So I was thinking of building the cap attenuator, but have a huge noise issue last week/ amp not working currently, scaring the living daylights outta me.. so the idea on hold, as I'd be terrified if MV full with any attenuator, of this suddenly happening: tho whether the attenuator would tamp this noise along with the whole amp level (should it sudenly occur again) I don't know.

I know the attenuator should let me get "something approaching" the clips amp dimed tone- but as I've said it just will not. So I'm left with nowhere to go. Apart from adding a pedal: which is why I was asking about the 'vol knob clean up thing' in what situations this facet can happen. Can it/ should it be able to happen with a pedal?? does the pedal needed certain aspects to it to be able to achieve this facet?? does it need tubes in it itself maybe?. Back to the very same position, asking the very same questions as 35 yrs ago!

--

I will do the rejig of the tone pot & cap as you suggest.. & see if it makes any difference on this 'level of distortion' aspect, but I don't believe it can/ could.

Thx again SC

[Fluoroscope 5000]

So you do have an AC15TB. You will need to crank at least the Treble to overdrive the amp section, but that won’t affect much below ~4kHz. The Vox TB circuit is even more mid-scooped than a Fender. The Duncan Amps "TSC" app shows that increasing just the R2 value from 10k Ohm to ~50k would increase mid-gain by ~9dB, or by just ~6dB with ~30k. The amp would be much more mid-focused like a Marshall when the knobs are at noon, but still become very scooped when they are up high, and you can get a useful spectrum tilt with the bass at ~9 if you adjust the Treble up and down. Download the app to see what I mean. It’s a simple mod if you take proper safety precautions and feel confident doing it. So, is the speaker an AlNiCo Blue?

You could also try a pair of JJ ‘EL844’ tubes. They break up 20% sooner than EL84, and you can just swap them in without having to rebias or anything. Still gonna be pretty loud by the time they break up, and JJ’s can sometimes develop rattle in a combo.

Some higher gain preamp tubes might be worth trying. The best new production 12ax7 is the new EHx 7025 (Mil-Spec 12ax7). It is very low noise and breaks up smoothly like a Mullard with more harmonics than a JJ ecc83S, although I think the JJ ecc83MG is a nice tube. It may not be higher gain than what’s in there now, but they are cheap enough that it might be worth buying a few. The EHx 12ax7LPS is a good phase inverter tube (for the last slot) with a nice squishy breakup sound. Someone might have put in some wrong tube types at some point, like a low gain 5751, 12AT7 or 12ay7 that at least might be worth replacing. The schematic specifies to use all 12ax7/ecc83.

Amp caps can leak over time. Although that might create more distortion, it might be worth having that checked and at least replacing any electrolytic power supply caps with some equal value reliable new ones. That might cost some serious cabbage unless you know a decent tech in your area. You could find a video on it and do it much cheaper yourself.

No, you don’t need a tube pedal to get a nice gain clean-up sound, but I don’t know which Vox pedal you have or if it cleans up well or not. You could try reducing the pedal gain and turning the output knob up high to drive the amp input section harder. There are some really nice transformer-based OD pedals that clean up really well. The Hudson 'Broadcast' is well worth the money. Made in the UK, it does anything from sweet sensitive overdrive to full-on fat fuzz with a vintagey juicy sound I find ideal. There's also a dual gain knob w/boost switch version for a bit more cash. I’d honestly sell the Vox pedal for one of those. It might be the best option if you think you’ll need to spend any real dough. Good attenuators can cost 2~4x as much. Attenuators sound flat if not a well-designed reactive load, although there’s a simple/cheap mod you can do to an amp to sink 90% output power to ground without affecting the impedance load:

https://robrobinette.com/Generic_Tub...%_Power_Switch

You could just wire in the parts without a switch if you don’t want to drill the chassis. 1.5W is plenty loud if you don’t need to be heard over a drummer.

[Sea Chief]

Fluoroscope 5000

I didn't know you had to turn the treble knob up to get distortion- I've never heard of this before?! (yet another mystery). If so, then in order to achieve parity tonewise.. you'll have to turn the bass up full too though surely (I can only imagine). Which leads me to the Q of why wouldn't you have said 'both tone knobs need to be @ full' not just the treble. Yet another mystery.

And continuing on with the mysteries, it has a 'top boost', which of course.. is an utter mystery to me. I google it, it has something to do with an extra tube in preamp (that's all I can determine, which then I have to surmise -might- be fabricated to add some extra layer of overdrive-?) Top? is utterly meaningless [treble something-??]. Boost? I can only envisage might equate to a bit more distortion-? but who knows. But as I'm struggling to get much distortion from this amp, with a rectifier, 2x el84's, & 5x 12ax7's (far more than Ive ever seen in a 12-15w amp).. it's simply yet -another- mystery to me as to why. And another mystery is folks here saying 'oh that amp doesn't distort too much' (how on earth, can an amp full of tubes, plus, with this ridiculous mystery 'top boost' god only knows what it is, not produce much distortion?? how? how on earth can someone design it so?? mystery after mystery (yet another mystery; with it a nicely printed manual all heaping kudos on it's "top boost" [whatever the dickens it actually is: it doesn't say] "ooh its just like the 60's amps" waxing on: but turn the amp up & there is no such extra layer of distortion whatsoever, in fact, as people concur with on here, 'that amp won't distort too much' I can agree with). I am utterly bewildered in every step I take with regard to distortion, every direction I read, everything I buy/ everything I try.

So yes I have this amp (yes it has a Vox Blue), but I can't use it up loud (police), I can't use it with an attenuator (there either isn't enough distortion in the amp for the attenuator to let through, mysteriously if this is so of course, or the attenuator just doesnt work, or it doesnt work in conjunction with my gtr wired as it is.. or a combination of some or all 3 of these), so I'm resigned to pedals. I can't afford another £150 on a distortion pedal. So, I don't play the amp (because unless its at least MV 6, it's pretty stale sounding to my ears). It sits there. Year after year. So it will be up for sale once I can remedy it's gremlin. A real shame I haven't had 5 proper minutes with it, in 2 years Ive had it. Hugely frustrating. It just sits pretty in corner of the room.

Another mystery (a good one folks!) is.. very coincidentally, some kind chap gave me a 5w tube amp (yep, just a Randall Rd-5 metalhead thing, broken but fixed asap with fantastic help on here). Its got 2x 12ax7's. And enough distortion to sink a battleship. How is it produced? why is there so much of it? where does it come from? is it 'tube' distortion, or solid state distortion added to a tube circuit? what is this extra tube? (is it a "working" top boost perhaps??!) this amp, this freebie out of a dumpster it very almost was, is vastly more enjoyable than my ac15! This is ABSURD. Not because I'm a metalhead (no, the opposite) but because if I dial the gain down to 3 (another mystery as to what gain is- Ive asked, but don't understand it) I get a very playable sound. Amidst all this frustration with distortion etc, it is handed to me! A remarkable thing. I'm making a nice cabinet for it, & keeping it.. so thank god, at last I do have an amp with distortion I can play after 35 yrs!! not because I chose it from knowledge from all the decades of building & playing & owning many amps, on the contrary, a complete out of the blue fluke. Floors me.

So I'm flummoxed by it all. I cannot make head nor tail of any & every aspect of it! Maybe it's just meant to be this way, to save me many £100's!

Thx4readMyspiel- SC

[Fluoroscope 5000]

I said you “AT LEAST have to crank the Treble” to OD the 3kHz+ range of the amp section. The Bass range only goes up ~3dB from noon to max though, and there will then be a ~30dB midrange notch! That’s why it’s called the “Top Boost” circuit, and why I recommended the TB circuit R2 change. Yeah, maybe just sell it if it’s not what you want?

[Sea Chief]

Chuck H Hi Chuck, is this bournes pot correct? https://www.ebay.co.uk/itm/254954728...0AAOSwe2RgZDaT

Thanks SC

[Sea Chief]

I said you “AT LEAST have to crank the Treble” to OD the 3kHz+ range of the amp section. The Bass range only goes up ~3dB from noon to max though, and there will then be a ~30dB midrange notch! That’s why it’s called the “Top Boost” circuit, and why I recommended the TB circuit R2 change. Yeah, maybe just sell it if it’s not what you want?

Hi Flour50,

trouble is I dont understand the 3dB/ 30dB information, nor mid-scoop (Ive never understood what this means, 'fender mid scoop'?) so I couldn't understand most of your post. I'm really sorry for this, as it's kindly given from you with consideration U& time: but if it's over my head there's little I can do.

If a guitar sound is played to me & I'm asked 'is this a mid-scoop sound'?.. I wouldn't know how to answer it. I wouldn't know what I'm meant to be listening to. It obviously suggests maybe a midrange tone (pot?) is ommited, or something along/ akin to these lines, but I have a fender twin with a midrange tone control, so how could a fender with all 3 tone controls be a 'mid scoop' sound, if they have 3 controls for the player to determine themselves what sound they choose.

Yet another mystery I'm afraid.

--

So the Top Boost, is to do with the treble control alone-? it adds "a dose of distortion" simultaneously with turning -up- the treble control? If so this would solve one mystery! this seems to be what you were saying, but my Q about parity with the bass control (or surely you just get OTT treble doing this) unfortunately I cannot understand your reply. I can only understand the basics, hence vol knob basics title of the thread.

I have to simplify way down:

If you had the amp, & you turned the treble up to max to get the Top Boost (whatever it is), & you got the expected increace in treble (I assume you do, turning up the treble control.. don't you? or maybe you don't neccessarily maybe-?? I'm totally lost)... then where would you set the bass control-?

Thanks SC

[Sea Chief]

Could anyone explain what the word Top (as in Top Boost) is in reference to? top of the chain of something in the circuit? top of the treble section of the tone pot? (a special pot maybe? surely they dont make them or I'd have heard of it).. or just top used a general word for treble? IE top = treble, bottom = bass? I'm baffled by these two words.

Thanks, SC

[Sea Chief]

Fluoroscope 5000

I'm looking at my manual for this amp again, it's got the usual stuff features/ what knob does what etc.

What I'm surprised by is for the treble control, it only says "adjuts the higher frequencies or treble in your tone, from soft & full at one extreme to bright &cutting at the other". But no mention of the Top Boost facet relating to this control. So I'm completely confused as to what it is. The manual says "This amp design based on the Brilliant pre-amp channel of the classic 60's AC30 Top Boost. Hence the AC15 offer both the bright "chime-like" clean sound and gutsy, smooth full volume overdriven tone that has made the AC30 such a legend".

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.

Anyway the amp is a thread digression, apologies. It's just onwards from vol controls > using to 'clean up' a distorted tone > distortion in general & my confusion with it > my amp currently, which doesn't seem to produce much of it > what the holy feck a top boost is!

An unwarranted avenue I've led you down. I'll go over the graph replies again, see if I can understand some of it.

Thanks, SC

[Fluoroscope 5000]

Fluoroscope 5000

I'm looking at my manual for this amp again, it's got the usual stuff features/ what knob does what etc.

What I'm surprised by is for the treble control, it only says "adjuts the higher frequencies or treble in your tone, from soft & full at one extreme to bright &cutting at the other". But no mention of the Top Boost facet relating to this control. So I'm completely confused as to what it is. The manual says "This amp design based on the Brilliant pre-amp channel of the classic 60's AC30 Top Boost. Hence the AC15 offer both the bright "chime-like" clean sound and gutsy, smooth full volume overdriven tone that has made the AC30 such a legend".

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.

Anyway the amp is a thread digression, apologies. It's just onwards from vol controls > using to 'clean up' a distorted tone > distortion in general & my confusion with it > my amp currently, which doesn't seem to produce much of it > what the holy feck a top boost is!

An unwarranted avenue I've led you down. I'll go over the graph replies again, see if I can understand some of it.

Thanks, SC

Brilliant = bright/trebley

I think the Top Boost circuit was created to mimic the Dallas-Rangemaster Treble Booster unit popularized in the ‘60s, although I don’t know how similar it actually is. I think the Treble Booster unit boosted in a lower freq range.

Download the Duncan TSC (Tone Stack Calulator) app here: http://www.duncanamps.com/tsc/download.html

Check out the different tone stacks and move the sliders. Click on and change the R2 value of the Vox tone stack to 50k and see what I was talking about. I don’t know how I can help you if it doesn't make sense to you.

[Chuck H]

Chuck H Hi Chuck, is this bournes pot correct? https://www.ebay.co.uk/itm/254954728...0AAOSwe2RgZDaT

Thanks SC

Maybe? This is not their "guitar pot" line so I can't be certain of the taper. We're looking for a 10% @ 5 audio taper pot. Which their "guitar pot" line is. FWIW there's a Bournes 250k "guitar pot" advertised at the bottom of that page. Though it doesn't have the fancy brass bushing. Chances are good (?) that the pot you posted is a 10% taper because whomever commissioned them troubled to get that brass bushing. But it's impossible to be sure because they don't explicitly say it.

[Sea Chief]

Brilliant = bright/trebley

I think the Top Boost circuit was created to mimic the Dallas-Rangemaster Treble Booster unit popularized in the ‘60s, although I don’t know how similar it actually is. I think the Treble Booster unit boosted in a lower freq range.

Download the Duncan TSC (Tone Stack Calulator) app here: http://www.duncanamps.com/tsc/download.html

Check out the different tone stacks and move the sliders. Click on and change the R2 value of the Vox tone stack to 50k and see what I was talking about. I don’t know how I can help you if it doesn't make sense to you.

Flour,

you're being hugely helpful, even if I don't understand a certain portion of it right now; I shall come back & pore over yours & Chuck's replies/ whole thread when I have a work time-off (approaching- woohoo!). That seems like a fantastic app thing, likely made more simple for dummies too. A ton of info to get me teef into.

You'd have asked the Q by now why tf is this idiot on MES & building twin reverbs if he cant understand the basics?! a reasonable Q, & I need to study/ get my game up to par admittedly. The info in this thread will be a terrific resource.

I've overstretched & conflated subjects (& moaned about my amp sorry).. so I'm grateful for replying on my amp xyz/ put up the clip (Chuck)/ & dipped into the Top Boost 'subject'.. which is better I just do a fresh separate thread on. Then you can leave up to other kind folks (hopefully) to try explain as you're undoubtadly exhausted with it & me on this thread. I'll look into the dallas rangemaster to see what all that's about (a pedal if Im not mistaken).

--

So back on track: Chuck's pot & circuit rejig ideas, trying to gain (clever distortion pun) clarity on the graphs, & your low-emission cable suggestions.. & only things related/ relevant! V.grateful- SC

[Sea Chief]

Maybe? This is not their "guitar pot" line so I can't be certain of the taper. We're looking for a 10% @ 5 audio taper pot. Which their "guitar pot" line is. FWIW there's a Bournes 250k "guitar pot" advertised at the bottom of that page. Though it doesn't have the fancy brass bushing. Chances are good (?) that the pot you posted is a 10% taper because whomever commissioned them troubled to get that brass bushing. But it's impossible to be sure because they don't explicitly say it.

Aha ok, I'll sniff out the 'guitar pot' line then.. but it was a decent 1st trawl, for an idiot!

Ok so plan is: rehash with the B250k bournes for the tone pot + 220pf (silver mica I have to hand), try this/ rejig to 330pf if neccessary. Then look into low-emission cables, what I have/ if suitable. Then add a few ceramic pf caps 'onto the pickups' as last tweaks.

I can ask the seller in meantime if they know the taper % of the pot in my link. Thanks, SC

[Fluoroscope 5000]

Flour,

you're being hugely helpful, even if I don't understand a certain portion of it right now; I shall come back & pore over yours & Chuck's replies/ whole thread when I have a work time-off (approaching- woohoo!). That seems like a fantastic app thing, likely made more simple for dummies too. A ton of info to get me teef into.

You'd have asked the Q by now why tf is this idiot on MES & building twin reverbs if he cant understand the basics?! a reasonable Q, & I need to study/ get my game up to par admittedly. The info in this thread will be a terrific resource.

I've overstretched & conflated subjects (& moaned about my amp sorry).. so I'm grateful for replying on my amp xyz/ put up the clip (Chuck)/ & dipped into the Top Boost 'subject'.. which is better I just do a fresh separate thread on. Then you can leave up to other kind folks (hopefully) to try explain as you're undoubtadly exhausted with it & me on this thread. I'll look into the dallas rangemaster to see what all that's about (a pedal if Im not mistaken).

--

So back on track: Chuck's pot & circuit rejig ideas, trying to gain (clever distortion pun) clarity on the graphs, & your low-emission cable suggestions.. & only things related/ relevant! V.grateful- SC

I didn't know you've built amps. Changing a resistor in the tone stack of your AC15TB should be a breeze for you. Using a ~100k for R2 will increase mid gain by 12dB so the response will be nearly flat when the knobs are at noon. The mid and bass gain will then go up another 3dB when the Treble knob is at 0, and you can roll off some low end with the bass knob. That's a 15dB total mid gain boost! You could even wire a 10k, 33k & 100k R on a 3-way switch for a 6dB &12dB mid boost. All this assuming the amp R2 value can even be accessed. I do agree that keeping this thread focused on the guitar circuitry makes sense, and the TSC app may help you with that too. Also, "low-emission" cables? FTW? Low-capacitance, yes.

[Chuck H]

Ok so plan is: rehash with the B250k bournes for the tone pot...

B250k would be a linear taper in the Bournes nomenclature. I think the "B" moniker has been used to signify audio in some Euro regions historically, but it typically indicates a linear taper in our genre of amps and guitars today.