1W SE miniature amp

[Cliff Schecht]

This was one of those off the cuff builds that came out of my scrap pile. The inspiration for this build came from the 230VCT 40mA PT I found the other week at my parents house. The case was an old guitar effect (TS808/FET Booster) that I didn't care for and the OT (5k to 8 Ohm) was something I bought randomly from AES a few months back. The tube compliment eventually ended up being a 6021 feeding a 6BM8 running at about 170V.

The circuit itself was originally fashioned to be much like a Champ, i.e. one of the simplest amp designs out there. It sounded good like this too, it had a good amount of bass and enjoyed a good distortion in front of it. But my goal with this guy was to tack on the extra gain stage with the 6021 (I can't stand having unused tubes!) and have a little screamer in the palm of my hand. A few calculations later and I had a third stage added and the sound really surprised me. It does a convincing clean to mean sound and puts out a good amount of juice through a 2x12 or larger. A 1x12 open back cabinet is still too loud for my apartment! I'll post some pictures for now and maybe eventually will get some soundclips recorded.

I'm also looking into developing this into a kit that we could sell at PAiA. This would be a much more polished product of course, but this little amp is a very convincing proof of concept. I'd like to offer different flavors like a hifi and bass version as well. Our first big road block here is sourcing decent quality, inexpensive iron. I don't want to go with an SMPS for the power supply simply because it's not nearly as cool as having a real deal PT and rectifier. I think a tube rectifier is out of the question here but the PT/OT combination really make the looks and (I believe) the sound as well. Edcor sells a well priced power transformer I could use ($20 per) but I can't for the life of me find an OT similar to the one I used (which only cost me $3!). It doesn't have to handle a lot of DC current and the HV is relatively low so I could potentially use a 70V line transformer, but this looks clumsy and unprofessional IMO.

So, to end this rant, does anybody know where to find a similar low cost OT?

[tubeswell]

Looks like you've got 'before' and after' gut shots there Cliff (I see one seems to have the filter caps laid out different to the other)

[rbryerton]

Very cool project! I just built a mini myself, but not that small!

Mine is a 12ax7 into a 6au6, then concertina PI (other 1/2 of 12ax7) into a PP section using 2 6ak6's and it is really awesome.

Takes a variety of 7 pin mini pentodes. Tons of cheap NOS out there (shhhh).

Anyways, my point is, I used a 70v line transformer and it sounds and looks great. Picked up several on eBay for $6 each; I'm sure they could be sourced cheaper, and they look like the transformer you used (open frame etc)

Here's a pic...

[Cliff Schecht]

I quite liked the 6AK6's myself, I did two in a SE parallel configuration a while back and really dug the sound. But I wasn't a fan of the 6AU6, every one I've tried had a nasty high end sizzle/crackle when pushed into distortion that I could never completely eliminate. That was specifically on my dual 6AU6 -> 807 SE amp (first build). The one thing that amp did have was an amazing bloom when pushed into a hard distortion, something that I'm yet to reproduce with any other build.

[FUCHSAUDIO]

http://www.musicalpowersupplies.com/3.html

These guys might work. Or maybe Edcor...

[Cliff Schecht]

Everything they make is like 5W+, just too big for my needs here. I may email the guy about a small custom order though..

[FUCHSAUDIO]

Nothing wrong with oversized. We use a 15-W Weber or Hammond with one EL-34 single ended (for 7 watts) and it sounds huge. Design the circuits to overdrive each stage appropriately, and not worry about output transformer saturation. If you shop around, there has to be low power SE stuff out there someplace. Champs suck because the iron is "to size" instead of over done imho.

[Cliff Schecht]

I won't disagree with what you're saying, but I'm actually looking for some limiting in this application. When the power stage is pushed into distortion, the transformer saturates and cuts out some bass. This, along with the right amount of highpass filtering in the stages, makes it so that the mids are more pronounced and the amp cuts through more (only in ultra-distorto mode). When I roll back the volume, everything should clean up and just a bit more bass will come through giving a nice full sound.

Of course all of this hasn't been quantified yet. All of my lab equipment is in storage right now. I can't take any meaningful data until I get my lab setup again. These were just things that I was aiming for and sound-wise I think I hit the mark with my current design. I may pop inside of it tonight and play with the bypass caps and grid resistors to see if I can really tune in the sound I'm after without having to implement a tone knob (which I might make a user build-time option if a tone control is wanted).

[Cliff Schecht]

Also, I wanted to mention that the 70V line transformer option is starting to look more and more viable. I've never played around with these as SE OT's before but I think if overrated enough, core saturation won't be a problem. I just think it looks kinda cheesy selling a product that uses a line transformer instead of a real deal gapped ferrite core unit for SE amps.

[rbryerton]

Have you thought about doing a P-P version? Just a few more components, another OP pentode and maybe another triode depending on your circuit.

That way the possible core saturation using a 70v line trans as a SE is not an issue. Plus P-P's cooler ...so try a70v regardless and please report back your findings.

I'm having good luck w/the mini one I built, and it's between 0.1w and 10w depending on tube selection & screen voltage (there's 3 settings). I'm currently going through a bunch of 6AQ5, 6AK6, 6AU6(6136), 6ak5, etc tubes to find a nice combo. Fun stuff...