The "Leftover" amp build

[Synchu]

Alrighty.
So I had 2 novals and 1 octal left from previous projects and abundance of other components (it is amazing how components pile up through the builds). Had also a small box, torroidal power transformer (250V out, non center tapped) and a Fender reverb transformer (that got unused at the end).
I figured the above should be pretty much sufficient for a mini amp (in contrast with my last SSS build), and did an initial self brainstorm. Thought of mini Blackface Deluxe and few others, prior to glimpsing into Jim Kelley amp schematic I had.
Quick search over ampgarage got me to the few discussions on JK. I had few refreshing listening sessions over the amazing presentations of the new Suhr manufactured JK.
Both of the above got me thinking on building a Class A/SE version of it (remember - with the available leftovers ).
It required a bit of adjustment of initial plans - I scrapped the intended output transformer (i.e. Fender reverb one) and obtained a SE/5K hi-fi transformer from a local vendor (audioplatinum.com). Turned out very high quality iron. And this was the only item purchased for this project specifically.
The main challenge over the design was the bold Class AB push pull power amp of the Kelley running at rather high voltages (thus definitely higher headroom than SE/Class A) and few LTSpice simulations and Excel spreadsheets calculations confirmed this to a higher degree than I was hoping for.

I also looked at the source of inspiration for the JK amp itself, I guess, - one of the earlier Ampeg SB-12 versions which look later in the project came quite useful.
Fast forward few months later (day job, dissertation due), I got to what I believe is a very useful (at least for me) small amp.

Called it "The Leftover", naturally
Preamp is generally the Kelley amp with some adjusted values, with switchable split load (ala K60; Ampeg SB12 1st stage output is split load as well, though not switchable) at 1st stage. Lost the second stage bypass cap (again as per Ampeg design) as otherwise overall gain was too high and practically there was no clean sound at all.
There are few other adjustments down the road as well.
I kept the cathodyne PI, albeit functionally not needed, it does add its color to the sound (tested without it, brought it back in), using only the anode output.
NFB was adjusted as well.

Current tube complement is:
- 2 x 12AX7
- 1 x 6V6 - the power amp is cathode biased (250R with 6V6) and switchable (adding another 250R in series) for using 5881/KT77/6L6WGB/GC. 6L6GC is quite a bit underutilized in this conditions. I got respectable ~335V at the plate of the 6V6, 1k/220k voltage divider for the screen grid (&1K dropping resistor).

I will look to update the schematic and post it in the next weeks.

Powered up and sound from first time - must say was a bit dissapointed.
Spent next month or so on and off (mostly off) tweaking it and I am now very happy how it turned out.
Great cleanish and lightly overdriven sounds in "cool" mode of the split load and very respectable distortion (hear the Les Paul sample) on "hot".
Very dynamic amp (Strat demo below) and cleans very, very well with the guitar volume. You can crank the amp and go from clean to mean with the guitar vol solely.

The whole "made from leftovers" added to the switches on the front panel (i.e. didn't bother myself to order the push pull pots).
So ended up with 1 channel, Vol, Bass, Treble pots - boost, mid boost, presence, and cool/hot (i.e. the split load) switches.

It will get a nice wooden box at some point this summer.

Enough of that.

Few pictures of the true P2P mess result from the few months of tweaking and experimenting . Despite the mess it is reasonably quiet even at higher gain settings:

[img:640:425]https://farm8.staticflickr.com/7566/262 ... cb97_z.jpg[/img]
[img:500:332]https://farm8.staticflickr.com/7718/268 ... d05763.jpg[/img] [img:500:332]https://farm8.staticflickr.com/7502/268 ... 9ce98c.jpg[/img]
[img:500:332]https://farm8.staticflickr.com/7196/262 ... 68b07d.jpg[/img] [img:500:332]https://farm8.staticflickr.com/7070/267 ... e0706f.jpg[/img]
 
And some sounds (SoundCloud descriptions/comments provide short additional info)

Strat (backing) + Les Paul (lead)
https://soundcloud.com/synchu/left-6v6- ... lp-jb-lead

Strat
https://soundcloud.com/synchu/left-6v6- ... -strat-dyn

Gibson Les Paul
https://soundcloud.com/synchu/left-6v6- ... sion-lp-jb

 
There was some recent jam track posted in another forum, where I used this amp to record the lead with. It is again with a Strat.
https://soundcloud.com/synchu/blues-jz-jim-jam

I am certainly missing a number of details, but it was a fun build and a great experience.
Great little amp it turned.

Niki

[martin manning]

Sounds good! Matching a random assortment of parts together is a great learning experience. I'm interested in seeing the schematic you got to in the end, and do you have a link to the JK schematic that inspired it?

[joeboo88]

that amp is very cool and great playing too!

[Synchu]

Thanks

Martin,
JK schematic is the one on page 2 of this thread (it is attached pdf to one of the posts).
https://tubeamparchive.com/viewtopic.ph ... c&start=15

There are some errors on the schematic - missing cathode bypass cap (and biasing resistor) on the second triode. Bass and Treble pots have been shown as 1M, while if you go to http://jimkelleyamplifiers.com/ (via wayback machine as the schematics have been removed from the live site some time ago) Bass and Treble are 500KLs. I experimented with both and definitely 500Ks worked better.

I'll post mine once updated as well.

Niki

[martin manning]

Great, thanks!

[Phil_S]

I really like this type of project. Anyone can throw money at an amp build and make it good. This is real art. Congratulations on such an outstanding result! It sounds great.

[Synchu]

Thank you Phil!

Here's the final schematic (used LTSpice because of the simulations I did when starting with this). Apologies for the rather busy drawing.
Few notes:
- Power tube cathode resistors switching (i.e. adding another 500R in parallel for changing to 5881/KT77 and total cathode resistance of 250R has not been added, but it is rather obvious)
- Power supply funky look is because with the modeled power supply it took forever for LTSpice to run the simulations. Hence, I simply replaced them with separate voltage sources. Nevertheless, I used 0-250V non-center tapped transformer with bridge rectifier and it produces ~350V at the first cap.

I hope I didn't miss anything else But obligatory wording - use at your own risk, there certainly maybe errors and author disclaims any responsibility for whatever consequences or uses of this schematic. High voltages are present within this device that may cause severe injuries and even death.

Now here is a short strat sample with a 5881 tube. Definitely more headroom and feels different while playing.
https://soundcloud.com/synchu/left-5881 ... sion-strat

Niki

[xtian]

Very nice playing, mate!

[Synchu]

Thanks! Appreciated.
Cheers,
Niki

[tubeswell]

Messy? It doesn't look too bad to me. As longs as it works and sounds good, very few musicians care what it looks like on the inside. Experimental builds are the most fun.

[rp]

Very nice playing, good recordings, amp sounds great - must be that blue cable spiral

[WRC34]

Looks & sounds fantastic!!

[dorrisant]

Awesome projects. Great chops. Thanks for this thorough report.

I'm trying to figure out how I want to build a JK... This inspires!

[Synchu]

Thanks for the good word, guys. Much Appreciated.
Rp you're the second noticing the utter importance of the blue coily cable. The best kept secret to sonic nirvana
Joking aside I am appreciating the amp more and more.
One of my initial ideas was to have a low powered amp for honing my slide skills, being huge fan of Derek Trucks, as cranking a Super Reverb is not an everyday option.
And there's no reliable info on his current rig with dual Allesandro AZZ amps. Would love to see some insight pics of these...
will try to record something slidey, once I am back from this hectic lot of business trips lately...
I did some days ago a short recording using KT77 tube as well. Different harmonics and nice increase in the headroom.
https://soundcloud.com/synchu/left-kt77-stratobast

Niki

[bluesfendermanblues]

Great amp, What tune are you playing - I love it!