Vox ac15tb > ok with a 16ohm spkr?

[Sea Chief]

My personal amp puts fifteen clean watts. Not sure how much when cranked and overdriving, but it's more than that. I've used my attenuator with that amp cranked into it A LOT. Over 500 hours for sure. Maybe a thousand. And mine is literally jammed into a much smaller chassis than the layout I put together for you. Meaning the unit you build will dissipate heat better (good). I've also used my attenuator a few times with fifty watt (ish) amps but only for short duration amp testing purposes. No failures.

That sounds very encouraging.. especially as I dont use my electrics that much, so more an occasional hobby, mind you it might encourage me to more if I can get into the amp's 'goodies' without n'bors stomping round. I have an idea to trade my ac15 in for a ac30(another tb type) if I can find one & afford it.. so will build it with this amp in mind I think. Two of my resistors in series as you suggested/ make it 50w'able.

cheers Chuck.

[Chuck H]

FWIW I built the resistive version of this attenuator (no inductors or capacitor) into a 4xel84 amp (30ish watts) for a customer and it gets about the same amount of use as mine. No failures. If the resistive version is up to it then the reactive load version (the one I drew up for you) certainly is as the inductors I linked are far over rated for the task.

[Sea Chief]

FWIW I built the resistive version of this attenuator (no inductors or capacitor) into a 4xel84 amp (30ish watts) for a customer and it gets about the same amount of use as mine. No failures. If the resistive version is up to it then the reactive load version (the one I drew up for you) certainly is as the inductors I linked are far over rated for the task.

I think it might be your resistive version I built into a tiny hammond 'mxr diddy pedal' sized box, back open.. I can't think who else kindly put the info up/ how else I built it. It has the 25r rheostat I mentioned, & a few gold heavy resistors, a switch. Do you possibly recognise this recipe-?

So what is the fundamental idea/ design difference between them?

[Chuck H]

I suppose it's probable you built the resistive version of the attenuator I designed. Sorry it doesn't give you any satisfaction.?. In my experience it's pretty good to about half power, but starts to sound a little flat at low volume settings. Not terrible, but workman like and not "satisfying". My own reactive version (the posted design) is much better for me at low volume settings. I hope it is for you too.

[Sea Chief]

I suppose it's probable you built the resistive version of the attenuator I designed. Sorry it doesn't give you any satisfaction.?. In my experience it's pretty good to about half power, but starts to sound a little flat at low volume settings. Not terrible, but workman like and not "satisfying". My own reactive version (the posted design) is much better for me at low volume settings. I hope it is for you too.

Great Chuck thanks again to you for making it available- it'll be a slow process to gather bits but looking forward to building it.

[Sea Chief]

As to it being a slow process.. my goodness, 2 hrs trying to find the 1st component: the capacitor/ scouring all www possibilities (400uf/ 100v seems a standard US cap.. but not so in UK).

Chuck H closest I can find here is 300uf/ 100v bipolar (those blue Alcaps you often find in speaker x-overs). Would this suffice?

(Or anyone give Chuck a break & give their opinion). Thanks- SC

[g1]

470uF or 330uF would be the standard values I think.

[Helmholtz]

You can go as low as 150µF, which would emulate the bass resonance of a 4x12 Greenbacks Marshall cab.

[Sea Chief]

You can go as low as 150µF, which would emulate the bass resonance of a 4x12 Greenbacks Marshall cab.

That's quite an expansive answer.. & as such, I can't understand it! (apart from a very vague gist of course).

So the lower the value, the more bass it doesn't tamp down & let's through.. is that anywhere near the avenue your gesturing down?

[Helmholtz]

That's quite an expansive answer.. & as such, I can't understand it! (apart from a very vague gist of course).

So the lower the value, the more bass it doesn't tamp down & let's through.. is that anywhere near the avenue your gesturing down?

Just so much:
The purpose of a reactive load is to present a load to the amp which closely resembles a real speaker. A speaker has an impedance peak at its bass resonance. The resonant frequency depends on the enclosure. A closed Marshall cab raises the resonance of a Greenback to around 120Hz. In an open back 1x12 or 2x12 the speaker resonant frequency is lower.
At the bass resonance sound is boosted but drops rapidly below the resonance.
Means that a 4x12 cab produces a lot of sound level around 120Hz but is not able to reproduce as low frequencies as a 2x12 open back cab.

A 150µF with a 12mH choke gives a resonant peak at 119Hz resembling the impedance a 4x12 cab. A 400µF cap would give a resonant peak at 73Hz, closer emulating an open back cab.

[Chuck H]

So, consider whether you want to emulate a closed back or open back cabinet. If you want 400u (ish) you can always parallel a 300u (or 330u) with a 100u. And even though the design indicates a 100V cap you can actually use anything rated for 50V or more.

[Sea Chief]

So, consider whether you want to emulate a closed back or open back cabinet. If you want 400u (ish) you can always parallel a 300u (or 330u) with a 100u. And even though the design indicates a 100V cap you can actually use anything rated for 50V or more.

But i don't know what difference is between open or a closed back cab! I think Ive only ever played combo amps, with open back two panels. I mean the shame.. like whistling on a tuesday.

[vintagekiki]

About Open or Closed Back speaker cab

https://www.youtube.com/watch?v=KtEJw77t72E

https://www.youtube.com/watch?v=STaL-wMlbds

https://www.youtube.com/watch?v=ts_P67uKXQA

https://www.youtube.com/watch?v=BLqc71IdKAs

https://celestion.com/blog/open-back-or-closed-back-cabinet-which-is-for-me/

[Chuck H]

But i don't know what difference is between open or a closed back cab! I think Ive only ever played combo amps, with open back two panels. I mean the shame.. like whistling on a tuesday.

Well then... To keep things as you're used to them you would choose to design for an open back.

Otherwise... You should go play through a closed back cabinet and see if that's your new thing. Bonus, it comes with the need to acquire more gear

[Sea Chief]

Well then... To keep things as you're used to them you would choose to design for an open back.

Otherwise... You should go play through a closed back cabinet and see if that's your new thing. Bonus, it comes with the need to acquire more gear

I don't or will never be in a position to use a closed back cab you see.. it makes no sense for me, even if I might 'slightly prefer one to the other' I cant help but think if one's more bassy (I assume this is the main difference- I don't know) then if you have a tone knob on the amp (on the guitar too).. then I can't really understand why closed back cabs exist.

So to just to boil this back down. Will using a 300uf cap instead of a 400uf.. make it emulate more a closed back cabinet then-?

So.. that means -less- bass. Is that all there is to it, or am I missing something?

thanks, SC