Whom of our talented builders would be able to build the following...

brooom

So Jim Kelley recently released a new version of his Jim Kelley line amp. The principle behind the amp is that it does a sort of "re-amplification", using the following topology:

-> pre-amp -> cascode dual diferential amplifier -> phase inverter -> power amp ->
 
The cascode dual diferential amplifier, basically acts as push pull output stage, which is being fed by the pre amp and generating some of the harmonic distortion content and amplification and then passing that signal to the actual power amp stage. Also along with this "unusual" stage you get a master gain control, which effectively works as a master volume, basically allowing you to have some nice tones at really manageable volumes (no need for attenuators).

My rather "naive" idea, would be to build a pedal that has a similar circuit (could be using FETs rather than valves). With two modes:

- A cascode diferential amplifier only mode, which you'd be able to use in an amp's FX loop (essentially placing it after the pre but before the inverter), to achieve a similar result to the one the Jim Kelley achieves.

- A preamp + cascode diferential amplifier mode, that could be used as a regular preamp/overdrive pedal, to slam the frontend of the amp.

Is there anything out there, that does this? And if not, do any of our talented builders, want to have a try? I'd happily commission one.

Winny_Pooh

Yes but does it sound good in practice. There's always the Revival drive if you want a detailed amp in a box to then stick into an amp.

brooom

Well the default mode, to be used in an FX loop, should sound good in practice yes, as essentially it would be the same type of application/topology as the one in the Jim Kelley amp, which does sound good and functions as it shoud.

AS for the preamp + differential amp mode... it's a bit of a question mark, and in all honesty I don't have a clue. But I'm willing to try it.

I'm not a big fan of the Revival at all... too many options. I'd like something that sounds good with some amount of flexibility, but realistically a volume, gain, treble, bass and mode switch is all I'd like to see there.

Also I'm not looking for an amp in a box.. This is a slightly different concept. Re-amping in a box, if you will.

Winny_Pooh

IMO you may over thinking it. If this concept was the route to tonal nirvana then it'd be out there already. You can already use something like the fryette powerstation or attenuators for similar jobs. 
All the best guitar sounds we reference are decades old and so is most of the gear used, just stick the right guitar into some nice pedals and a great amp, done.

digitalscream

Without understanding the really technical details, isn't this similar to the idea that Vox had with their original Valvetronix amps, with the 12AX7-based power amp before the solid state output stage?

thecolourbox

digitalscream said:

Without understanding the really technical details, isn't this similar to the idea that Vox had with their original Valvetronix amps, with the 12AX7-based power amp before the solid state output stage?

Yes I think so

brooom

digitalscream said:

Without understanding the really technical details, isn't this similar to the idea that Vox had with their original Valvetronix amps, with the 12AX7-based power amp before the solid state output stage?

Yes sounds similar. Although they might have been using a different type of amplification stage for those 12AX7s.

brooom

Winny_Pooh said:

IMO you may over thinking it. If this concept was the route to tonal nirvana then it'd be out there already. You can already use something like the fryette powerstation or attenuators for similar jobs. 
All the best guitar sounds we reference are decades old and so is most of the gear used, just stick the right guitar into some nice pedals and a great amp, done.

Yes of course I'm overthinking, that was the point. But not because I'm after any tonal nirvana. I've already achieved that, multiple times in fact. Basically every time I buy a new piece of gear, I achieve tonal nirvana 

I'm doing it because I'd like to try this circuit. But definitely not because I'm attempting to achieve new heights in guitar sound crafting :P And no it isn't the same as an attenuator or even a power station.

I suppose lack of interest from any of the builders, will probably be the ultimately dictation that it is the ultimate stupid idea.

jpfamps

Out of interest why do you want to use a cascoded diff amp?

smudge_lad

thecolourbox said:

digitalscream said:

Without understanding the really technical details, isn't this similar to the idea that Vox had with their original Valvetronix amps, with the 12AX7-based power amp before the solid state output stage?

Yes I think so

These amps sound GREAT imo, my singer uses one and it's stunning!

brooom

jpfamps said:

Out of interest why do you want to use a cascoded diff amp?

purely because it's the same circuit used by Jim Kelley in the line amp. And I know it sounds quite alright for this application.

Sassafras

I wonder how different it would sound to the millions of OD pedals out there?

brooom

I've had what seems like a million now, most of them tend to sound different to some extent. Of course with a tweak here and there you can also make anything sound like anything else within some settings.

But that's not the point, I'm not trying to create a new overdrive or convince anyone to buy into it.

I'm just trying to find someone that will build it for me, with a limited audience of... me at my living room. So people who have "seen it all" may rest assured, this will go under your radar.

jpfamps

I've had  a brief look into this. 

It seems the idea behind the cascoded diff amp is that as the cascoding shields the control grid from the anode and produces a more pentode like transfer characteristic, and being a diff amp it is push-pull like a valve output stage, so you can recreate the distortion behaviour of a valve amp output stage.

I expect the finesse with this design would be getting it to distort in a "pleasant" fashion.

You could cascode JFETs to provide a similar effect. 

Not sure if this approach is used in any pedals, although the Revival Drive would be a candidate. I've seen inside one of these and it would be a major undertaking to reverse engineer one, so I don't expect this being done in the near future even by the voracious DIY pedal community.

If I have some time I my try simulating a circuit in pSPICE to see if I can get anything sensible out of it.

brooom

Hi,

yes that's the idea. And I do believe JFETs could work.

I'm not familiar with the revival drive, other than the fact that it has more knobs than I care to understand. But it does seem to offer some tricks that could be similar to this.

jpfamps said:

I expect the finesse with this design would be getting it to distort in a "pleasant" fashion.

This is probably the reason why Jim was working on it for some time.

If you're willing to try it out, it would be interesting to see what comes out of it.

jpfamps

To be honest I don't really have the time at the moment.

lukedlb

Inspired by this thread, I’m curious about other applications specifically made for the fx loop. Rather than disrail this thread, I’ve started another:
https://www.thefretboard.co.uk/discussion/160907/pedals-built-specifically-for-the-amp-s-fx-loop#latest