Sub-mini tube and solid state hybrid amp

[dorrisant]

I just ordered an oatley kit (K281) two days ago... probably should have ordered two. Now I have to wait 3-4 weeks. The kit is a tube/solid-state, pre/poweramp hybrid (5 watts). I need a schematic so i can start modding.
Anyone care to help?
I need to know what component values should change in order to achieve some guitar-type gain (min and max values in what places). Even if I have to get more tubes and surrounding components, I will.
I'm thinking my project will need to have more gain stages. Preferably with the same 6418s.
I have an old Kingston solid state combo amp with an 8" 8 Ohm 24 watt speaker and a bad IC in it... about $13 US for the chip or... a little more $ added and i could have a sweet little "sleeper" amp. A little polish and this thing could look like it just came from the store.
I would like some advice on where and how to place a tone stack... and maybe if anyone has any thoughts on any reverb driver/recovery circuits... or even a tremelo (with the same 6418 tubes). If not, are there any other tube similar in size/voltage that would do the trick?
I have more questions but will wait for some type of response.
Any schematics would be much appreciated.
I will post pictures as the project goes along.

Here is a couple of links about the kit:

secure.oatleyelectronics.com//product_info.php?products_id=838
diyaudioprojects.com/Forum/viewtopic.php?f=9&t=2020

[jcgss77]

Welcome, dorrisant. Someone with more clout here can welcome you more officially.

Really, I am not sure if anyone can help you with your questions unless you can post a schematic. Perhaps someone here has the schematic, but if not I would recommend posting the schematic once you get it. I am not sure if I am just talking out my butt, but I think that volume would be better situated further down the chain, I believe it would be more effective that way. Again, we need a schematic. That kit does sound nice, though.

Also, I would take stock of what pedals/preamps you have before you go fiddling with the build, after all that does come with a PCB. Maybe with your setup, you will have quite a little monster there.

[dorrisant]

Thanks for the reply. I may just have to wait till it gets here... maybe not. I already sent a schematic request to Oatley... Maybe they will come through. Until then I will be chomping at the bit.
Will post a couple of pics tomorrow...

[newey]

Welcome, dorrisant. Someone with more clout here can welcome you more officially.

Since when does the Greeter have clout?*

But anyway, Hello and Welcome, Dorrissant!

And yes, a schematic will be necessary for help with any mods.

According to the links you posted, this is an audio amp for home audio use, meaning the input will be designed for a line-level source not for a guitar pickup. This can be modified to a high-impedance instrument level input. Or, you can resolve to always use a pedal with a buffer before the amp input.

Adding a tone stack is probably easily doable, again, once we see a schematic. Adding a tube-driven reverb and/or tremolo adds a whole different level of complexity. You might be better off to just get the amp suitable for guitar use, and experiment with tube trem or reverb as an outboard unit (as in a pedal). I've looked at building a tube tremolo pedal, and the circuit looked way beyond my meager abilities- scared me off DIY and into looking at buying one!

*For our non-Yankee friends, a definition of "clout" may be in order:

Mike Royko on "Clout"

In Chicago, "clout" is not a something one has, a "clout" is someone.

[Yew]

If it helps, ive put a guitar through a surround sound system with soem sucses, and a distortion pedal .. would post soem clips but ive sold the pedal

[dorrisant]

Here it is...

Schematic:

...and just in case I don't have this skill of posting pics down yet... the link:
s946.photobucket.com/albums/ad304/dorrisant/?action=view¤t=K281schematic.jpg

The Kingston combo I mentioned from before:

Here is a pic of an old Kenwood power supply chassis:


The reason I am thinking about just using the Kenwood chassis is because of the vibrations from the speaker may cause unwanted ringing in the preamp tube(s)... Any thoughts?

Just to clarify... I probably will just try to achieve a guitar preamp/power amp by itself, whether as a combo or as a stand alone head. If anyone has any suggestions (or schematics) about reverb/vibrato/effects loop then i would certainly consider this in the near future. First things first though... good overdrive tone.

Thanks for all your thoughts and ideas so far...

^{Edited by newey to fix .jpg links}

[dorrisant]

About that pic...
They can put a man on the moon but...
Oh well. At least the links work.

[newey]

Dorrisant-

I fixed your photos, but I think I got the wrong one- back of the amp instead of the Kenwood chassis.

To post photos from Photobucket, just copy and paste the img tags right off Photobucket- Last one on the left next to whatever photo you want.

The link, in fact, doesn't work- your Photobucket account is pass-protected, so if you never logged out, you can access it via the link but we can't- just takes us to the Photobucket sign-in. But the link isn't needed since we can now see the schematic.

Others will soon jump in with some ideas for adding the tone stack and gain stages.

[dorrisant]

Kenwood Chassis:


Ok... now that I have that all sorted out...
Thanks to newey. I hope I will be able to help someone else out as much as I have been helped...

I do see some discrepancies between the schematic and the bill of materials, particularly the value of R2... don"t see it on the BOM.

Anyway, I redrew the schematic and scanned it as well, so here it is.



Feel free to tell me what you might think looks wrong with this. I just tried to copy the original .jpg to make it easier on the viewer.

Thanks again to all for being patient... I think I'm getting the hang of this.

[jcgss77]

Oops...guess I inadvertently called some of us a clout...Bad JC!!! Just don't leave a scar with the beatings...

First of all, I am by no means an amp technician, unlike others of us here. What I am suggesting are just some ideas...

First of all, what kind of sound are you looking for?

Newey already mentioned the need for a pre-amp. You need one. If you want it on-board, I would just google until you find a schem you like. The easiest route, by far, is using powered pedals, just plug 'em in, and rock!

It seems you posted while I was typing-the only thing I can say is build it, and see what happens. See what the sound is like, and then cater to it from there.

[dorrisant]

I guess I got ahead of myself... cart before the horse... amp before the preamp...

I like another of Oatley's kits - K270. Maybe I should use one of these in front of the K281.
(Schematic on the last page)

secure.oatleyelectronics.com/files/K270notes.pdf

If I could put in a tone stack somewhere in there, I would be about half way there. I'm not concerned with it sounding exactly like a Fender/Marshall/Vox/Whatever... just need to get some tube type overdrive with the range being from very clean to relatively hard overdrive. If I knew how much the component values could be changed for a tone stack and the bias of the tubes, I would breadboard it and adjust to suit taste. I'm not trying to confine myself to a specific plan any more than that. If it would work out better to hand wire a preamp on a turret board and feed this into the K281, then that is what i will do. If the general consensus is that the K270 kit would work just fine with some modifications as the preamp feeding the K281 then I will do that. Like I said before, this could be a combo or a head... whichever works best. I'm going to forget about reverb and vibro for now.

As far as "just google it"... some of this sub-mini tube stuff gets a little less discussion than standard tubes. I want to push this envelope (I like the pun) a little farther and stick with the 6418 or similar low voltage requirement tubes. I want this for my amp now and to be able to put similar circuits in stompboxes in the future.

Hopefully with your continued help and comments we will all learn more. Plus I may end up being the envy of my fellow bandmates.

[newey]

The preamp looks like it would give you the Hi-Z input you need.

Just to refer you to our archives, Dunklefalke was working on an onboard tube preamp, 9V powered, in this thread:

guitarnuts2.proboards.com/index.cgi?board=Amps&action=display&thread=4775

This also use these "mini-tubes". I don't know what ever happened with that project, but there may be some info for you, or at least a cautionary tale, in that discussion.

[jcgss77]

Ok dorrisant, I think I have something you will like. This is a dual submini preamp which I personally have not built, I am currently building the valvecaster with a 12at7, but am awaiting my breadboard so I can troubleshoot it properly. Check this out:

www.diystompboxes.com/smfforum/index.php?topic=63479.0

This is quite a read, so get comfortable.

I would also suggest, depending on the number of pins of the tubes and how they lay out on the pcb, that you use a dip or sip socket for the tubes, for ease of changing out/upgrading/whatever. You may have to modify the board to accommodate the socket.

[dorrisant]

Thanks jc... (funny, I think I've said that before).
All I can say is WOW!... That's what I've been looking for. By about page 6 my eyes started bugging out. Then I noticed my slightly elevated pulse... And your right, its gonna be a while. I will still keep posting updates, but I have some research and decision-making to do.

[jcgss77]

dorrisant, once you build your new amp, give us an update, I for one am pretty curious how this will turn out. Maybe some pics and sound clips, too?

[Yew]

Im rather interested in that valvecaster thing, i can see that becoming a pedal soon.. just a shame i cant see how to translate the schematic into a wiring diagram...

[dorrisant]

You've got it guys... Will do... Got to wait on deliveries. Then I will start posing form then on...

[jcgss77]

Sept 5, 2010 5:05:27 GMT -5 Yew said:

Im rather interested in that valvecaster thing, i can see that becoming a pedal soon.. just a shame i cant see how to translate the schematic into a wiring diagram...

A solderless breadboard is your friend with that. I built mine in an old Daphon compressor broke down pedal, and it did nothing. I just bought a 2 smacker breadboard off eBay, from a Chinese seller, and it just came in. And guess what, it works! So I disassembled my Valvecaster and boarded it, and I made progress. The tube lit up! However, it doesn't work still, just getting fuzz through. But it has helped me make progress. I would recommend trying it out, I have got a bunch of schematics for distortion pedals and stuff to try out. Man this is fun!

[Yew]

Also, there is a Point-to-point schematic further down the page

[longo3]

Ive got the K281 coming on over and trying to figure why they have the wattage so low, and what can be done to boost it.

[dorrisant]

Sorry guys... Its been a while since my last post.
I got my kit in and built it in about an hour (great learning/soldering experience for my 10 year old step son). I could have built it in about half that, but he gets a kick out of helping me like that. Had a small glitch... L.E.D. Supplied with the kit was bad... No biggie. All my friends and family who have heard the sound quality are blown away... Common reaction is the disbelief in only 5 watts. I want to see if the end product will keep up with a drummer so I would be interested in getting a few more watts out of the hybrid if necessary... Remains to be seen.
I put the kit in an old CPI DC terminal enclosure. When these units go bad (lightning strikes) we trash them and I can't see throwing the nice enclosures away. I had to remove the heat sink and remote mount it for clearance... Then I remote mounted a new volume pot, in/out jacks and L.E.D..
Now for the preamp... I read the forums from the links shown to me on the first page... Much can be learned there... I ended up going with the "pepper shredder" schematic/setup. I wanted to be able to change things as necessary so I bought epoxy board and turret lugs for a wide open layout. I stuffed this into the stripped Kingston chassis. Sorry I can't post any pics of the progress right now... I will in the next couple days...

[sumgai]

+1 for perserverance.....

[dorrisant]

Ok...got the Oatley kit...built it in about an hour, (would have been less time but I had my 10 year-old step-son helping me solder. This kit sounds great... maybe not absolute hi-fi-nirvana, but it is impressive. Two of these kits used in stereo would be the cats meow for just about anyone. For a simple 5 watt amp, this kit will be loud enough to be heard by the neighbors but i don't know if it will keep up with a drummer... we will see. Overall i would recommend this kit... for or five stars!

Now for the preamp... After much reading and studying i chose the Pepper Shredder.

www.tube-town.net/diy/lov/lov02-peppershredder-eng.html

One of the biggest reasons for this is the fact that it already has a verified circuit with a tone stack. This matches up well with the Kingston amp I want to use. I could make this fit quite nicely.
So... after a short time my package from tubedepot.com arrived. I chose to put the preamp on a epoxy/glass board with turret lugs (like found at tubedepot.com) so that modifications can be made easily. Built the board... wired it to tube sockets, pots, switches, etc...
The Pepper Shredder site said it left the power supply up to the builder... no problem... 16v 1000mA wall wart into a 12from the regulated 12.2v to ground.
After turning on the circuit with no tubes and verifying there would be no meltdown, I shut it off, installed the tubes and fired it back up... this is where I'm stuck.
I have checked through the circuit several times: tube connections at the socket to circuit board, pots to board, switches, etc.. I can't find anything different between my project and the schematic on the website. With the "Kingston" preamp going into the K-281 I am losing signal.
For testing I plugged my phone (as an mp3 player) into the K-281. No problem... Loud and clear with enough volume to make you want to turn it back down a little. When I plug this into the "Kingston" then plug the"Kingston" into the K-281 the signal is muted to the point where it is audible only when the K-281 is turned almost all the way off. The signal is coming through... when I turn the "Kingston" off, there is no sound at all.
I wish I had a voltage chart for the Pepper Shredder... or maybe on o-scope...
I don't think there is a matching impedance problem, but I could be wrong there. Maybe I have a bad tube?
Just throwing this out there for anyone who may have built this circuit...

Sorry... I will try to post some pics tonight... and sound clips when I get it going.
Maybe I will get an o-scope for x-mas! I like this one:

www.seeedstudio.com/depot/micro-digital-storage-oscilloscopedso-nano-p-512.html

Thanks for your help, you wonderful people of the forum!

[dorrisant]

Sorry about the mistyping... I put a 22μF cap on the regulated 12.2v for noise... works great! Now if I could just hear the "noise" I want to hear...

[dorrisant]

"When I plug this into the "Kingston" then plug the"Kingston" into the K-281 the signal is muted to the point where it is audible only when the K-281 is turned almost all the way off."

Replace "off" in the above with "up".... there, that's better.

[longo3]

Kit arrived. I very carefully laid out components and made sure I had everything listed. I put the resistors on the board first bolted the heatsink the the lm1875 and added it to the board. then finished with the caps, tube, led.... The led lights up but no sound comes through. Resistor 13 heats up and the heatsink on the lm1875 gets so hot you cant touch it. The paperwork talks about R14 and R15 being wirelinks but I cant find them in the schematic. Has anybody had this problem?

[longo3]

R13 on closer inspection shows burned middle

[longo3]

removed power supply from board tested it. red and green show 24 volts. Put it back together tried it and the split second i plug it in r13 and lm1875 get to hot to touch.

[dorrisant]

longo3,

There is no R14 or R15 in the current build... I think the notes are referring to jumpers on a previous board revision... and not needed here.
As far as heat... No significant heat here. Nothing you would be worried about with a bare hand. Can you post some pics of your pcb? If necessary, we could exchange voltage levels for comparison.
Let me know if I can help.

dorrisant

[longo3]

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trying to figure out how to post image.
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