The Un-stomp Box

[polaris26]

I don't know how in-topic this may be, but I found an alternate use for one of Small Bear's stompbox enclosures.  Check it out and see what you think:
http://www.poconothermionics.com/amp.html  I haven't tried stomping on it, however 

Dave

[stm]

1) IMHO it is perfectly on topic and relevant to guitar FX and DIY.

2) Looks like a nice project, except it is likely to be DEADLY...

Let me explain:

There is no AC mains isolation transformer and the circuit's ground is connected to the AC NEUTRAL, which is in turn tied to the box chassis and to the input jack.  If you plug a guitar in there the NEUTRAL will reach the strings and your body!

Should a Neutral failure occur, or more likely, the phase and neutral be reversed when powering the amp and you could be DEAD.  In my country (Chile) there is no disctinction between the phase and the neutral pins in a power socket, not to mention the fact that if there were any disctinction, the electrician would make sure to wire them crossed at least 50% of the times (probably more due to Murphy's law).

Just something to take into account when playing with tubes powered from a high voltage source.

[axg20202]

I agree. A nice idea, but the execution (ahem) is poor. I wouldn't have a box like this anywhere near me. This should be filed under how not to do things, listed right under the risks of lifting the mains ground on an amp to cure a ground loop. This project is a disaster waiting to happen.

[blanik]

relax guys, it's just a small amp designed to power the radio on the fragile shelf above the bath cause some people like to listed to some radio while taking a bath.... tssss   sissies 

[axg20202]

:-)

Yeah, if anyone is planning on developing this project into a DIY kit, it should come with a wooden broomstick and some Wellington boots as standard, so that the drummer can safely remove the smoking guitar from your soon-to-be-dead hands.

[MarcoMike]

it should come with a wooden broomstick and some Wellington boots as standard

[Dragonfly]

While I applaud your effort, that thing scares the hell out of me !   

Please..for the love of all that is holy...and for the family who loves you...dismantle it and rebuild it with the premise of "safety first". Honestly, I would remove the schematic too, as in it's present form, it is VERY dangerous...and something that a novice might not recognize as "dangerous". Do some further investigation on tube amps, and incorporate some safety into it.

[polaris26]

I would like to point out that in this design, the safety ground (third prong) is attached directly to the metal chassis.  Unless the safety ground in your outlet is somehow connected to the hot line, or not connected at all, you are safe touching the metal case, since it is completely isolated from the other two power lines coming in.  The neutral line is NOT connected to the case at all, it is connected to circuit ground, which is isolated completely from the case, and in a three-prong outlet (hot, neutral, safety ground) it is not likely that you could ever accidentally have the hot line where the neutral is, and vice versa, unless the outlet were wired incorrectly. 

Since most modern signal sources are isolated from the mains anyway, the most that would happen in such an unlikely circumstance would be that you would float the signal source ground up to the hot AC line level.  Not really a safety issue since most signal sources (CD players, etc) are made of plastic and such, and you almost never come into direct contact with circuit ground.  Bear in mind this is only relevant in the unlikey case that your power wiring in your house or wherever you are is wired backward. 

In any case, be it known that I built this for personal use, and besides many radios of the 1940's and 1950's (I own and use several on a regular basis) were made this way (no mains transformer) and are still used to this day.  Most only have two-prong 'polarized' plugs. 

Bottom line:  If it's a real concern, one can always use a cheap isolation transformer and be done with the issue.  The goal was to see if I could squeeze the whole thing into a small case, and even as such I had to mount the OPT's in an odd fashion, but adding a power transformer to isolate from the mains would have to be done as a 'line lump' type of thing (mounted inline with a special power cord), which isn't a bad idea actually. 

thanks for your concerns,
Dave

[frankclarke]

Well good work Dave, and good that there are warnings for people less familiar with safety issues. If I built it, something would come loose and kill me .

[Gus]

Some houses are sometimes wired wrong, some old ones don't have 3 prongs.  Theory is one thing real world is another.

Old not power transformer radios could/would shock you if the plastic knobs came off, ever note they were almost always plastic knobs and plastic cases?

IMO Very dangerous to post on the web and think people will build it correctly.  People sometimes don't even build 9v effects correctly the first time look at the amount that post needing help.

[tranceracer]

Hey Polaris,
Thanks for sharing, I showed the schemo to friend who worked on tube amps and he wasn't to alarmed about the direct connection to the plate.  He did say that it could give you a severe jolt if you're not careful as many here have expressed concerns about.  He said that an isolation transformer is a quick an easy fix to bring down the amperage.  Maybe to address some of the saftey concerns here, modify the schemo to include an isolation transformer or something.

But anyway, I was curious, since you're running AC to the plates, do you get any humm or noise?  I'm wondering if DC would provide a cleaner output.

-tR

[soulsonic]

The plates aren't running on AC - there's a half-wave rectifier that makes a simple DC supply for the plates.

This isolation transformer should work, and it's less than $25.
http://www.tubesandmore.com/scripts/foxweb.dll/moreinfo@d:/dfs/elevclients/cemirror/ELEVATOR.FXP?item=P-TN-51X

I agree with what Gus said - you never know who might attempt a project. I can imagine someone thinking it would be cool to build one as a guitar amp, and that's just asking for trouble. With copious use of plastic knobs and a plastic case (and a plastic shaft pot!), this thing could be reasonably safe as a simple little hi-fi, but it would be impossible to do safely as a guitar amp without some kind of isolation. I've got a couple of those old transformerless guitar amps, and they honestly don't sound good enough to even bother fooling with the potential electrical hazards.

[DougH]

My first thought was, what if your house safety ground is not connected, or someone defeats it with a 3->2 prong adapter? I third what Gus and Martin have already stated. You don't want the safety of the device in the hands of an unknown wiring condition in someone's house.

But the worst part of all of this is the posting of the schematic. Please take it down or add an iso xfrmr to it. Some kid out there who doesn't know any better may be building it as we speak, and doesn't even know its improper use can kill him.

As for old radios- before they are used/refurbished/whatever, they should be modded with an iso xfmr. No excuses, period. I don't care if they are all plastic, have guitars plugged in or not. Anything less is just bad form.

[soulsonic]

I've been shocked so many times by those dumb old 2-prong Fender and Silvertone amps that it's not even funny - I don't even want to think about how nasty that is without isolation.

As a side note, one interesting thing about the transformerless amps is the fact that they can be run directly from batteries. In some older texts these are sometimes referred to as "AC/DC" circuits, because of this ability to work either plugged into the wall or from batteries. Granted, 120 volt batteries may not be that common, but high voltage battery setups are certainly possible. Antique Electronic Supply sells 67 1/2 volt batteries especially for powering old radios that run on these kind of circuits. So maybe with batteries it's a bit safer?