Safety question: amps w/o PTs

[rfgordon]

So I went to the antique/junk stores today in search of some container to put my stompbox + computer speaker practice amp in, but I was skunked in that quest.

Instead, I got a really cool 1952 Zenith AM/FM radio--all tubes of course.

So now I'm thinking of turning it into a wee tiny geetar amp. However, it's kinda like those amp-in-the-case Silvertones in that it has no PT. All the tube filament voltages add up to 115 or so.

So my question is, to those who have more real engineering knowledge, is there a safe way to isolate the guitar-playing human from the possibility of wall voltage on this thing?

I was thinking I could add a 3 prong cable, grounding the chassis, and put a cap in line with the guitar signal. Ideas?

There's a zillion old radios like this around, and some of them look really cool. They'd make killer little 2-3 watt amps.

[LOUDthud]

The only fool proof thing is an isolation transformer. Hammond sells some torroid's that are pretty small. There is almost always a resistor in series with the heater string. This should be adjusted upward to account for the higher line voltages of today. The one bad thing is that you don't really need the RF tubes to make a guitar amp except to complete the heater string but a 5 or 10W resistor could be used.

[krash]

you're afraid of 120VAC, but you play guitar amps with step-up transformers with 5x or more of that voltage in them all the time?

I don't get it.

As long as there is a fuse on the mains that is the appropriate size, there is no worry.

If you have an electric stove, it runs on mains with no transformer. So does your toaster. All kinds of appliances. This is very normal.

Just make sure the chassis (and therefore jacks & pots etc) are solidly grounded and there is a reliable earth connection from the amp (three-prong power cord).

[leaveitalone84]

you're afraid of 120VAC, but you play guitar amps with step-up transformers with 5x or more of that voltage in them all the time?

I don't get it.

As long as there is a fuse on the mains that is the appropriate size, there is no worry.

If you have an electric stove, it runs on mains with no transformer. So does your toaster. All kinds of appliances. This is very normal.

Just make sure the chassis (and therefore jacks & pots etc) are solidly grounded and there is a reliable earth connection from the amp (three-prong power cord).

I have a 50's Danelectro tube amp with out a PT. Your logic/advice is not correct IMHO.

If the chassis is live, which Gordons may be; just grounding the amp is not sufficient. You'll have 120VAC to ground. Electric Stoves and Toasters do not have live chassis even though they have no PT.

My Danelctro has bitten me already. The power switch was off on the amp and it was unplugged; but my guitar was plugged in. I then plugged in the power chord to my conditioner. The guitar strings were against my arm and I accidently touched the metal chassis on the conditioner. Nice little bite there.

LoudThud was right. You need an isolation transformer at the very least.

[Doug H]

http://ax84.com/bbs/index.php?id=333220

Read and heed...

Proper ground and isolation does two things:

1. Keeps the chassis, guitar pickups, strings etc at earth ground. If any of these are "hot" you don't want to complete the circuit (esp through your heart) to the floor or whatever else you are touching that is at earth ground.

2. Isolates the circuit and hence, potentially *you* from wall/line current. If you do get shocked working on your amp, or by some combination of grounding/B+-shorting when playing, you don't want to be at the mercy of a 20 Amp breaker in the building's breaker box. You'll be long dead by the time it pops - if it pops at all.

This is "Electronics Safety 101" stuff, folks- basically standard safety practice stuff. I don't know about you but my life's not worth saving $30-$50 on a power transformer...

[krash]

you'll be long dead if the guitar strings become live before any fuse blows, mo matter whether you have an isolation transformer or not.

i edited this. just nevermind. put in an iso transformer, you'll feel better. no point in talking about it.

on the link that Doug H posted, Paul Ruby makes the important point. using an iso transformer allows you to earth the signal ground (secondary), one side of the transformer (or the CT etc). earthing the neutral on an AC mains circuit bypasses any GFCI. the neutral is earthed at the source and should be at earth potential unless there is some other problem in the building wiring.

[leaveitalone84]

Josh,

I attached the schematic of my lil Danelectro. Are you saying if I just add a 3 prong cable to the amp I'll be fine? My father who has been working on electronics and avionics (tube and solid state) for over 30 years said "no, and you'll want to isolate the line".

Since I was just bitten last night by 120VAC I think its pretty likely. No fuse blew or anything.

BTW what are the patent numbers you hold? I work at an IP office an I'm always interested in reading new patents.

-Seth

[LOUDthud]

A couple of quick points. On the schematic above there is a 68K in parallel with a .05uF cap between one side of the line and the chassis. Not much different from the old death cap. Still, it's more of a shock that I like to get. I'm not sure if that technique was used on line operated radios where no part of the circuit was exposed to the user. Was the user of one of these amps like the one above supposed to reverse the line plug if a loud hum was heard from the amp?

These amps usually employ half wave rectification and that's not what a torroid transformer wants to see. The solution to that is to place a solid state bridge rectifier (no filter cap) between the transformer and the amp.

[CaseyJones]

True story: I pissed on an electric fence when I wuz a kid. Taught me everything I needed to know about electricity right there, it did!

Yeah, I know. THAT explains it.

Seriously, messin' around with old transformerless junk makes as much sense as pissin' on an electric fence IMHO. If you want electroconvulsive therapy sign up at yer local loony bin, at least they'll administer the juice in a controlled environment.

[jaysg]

I've been shocked by a GE rangetop. Pissed me off big time. I had to replace a burner which did function.

[krash]

Josh,

I attached the schematic of my lil Danelectro. Are you saying if I just add a 3 prong cable to the amp I'll be fine? My father who has been working on electronics and avionics (tube and solid state) for over 30 years said "no, and you'll want to isolate the line".

well as you can see, that's the conventional wisdom.

at this point, I am not going to argue against it. you don't want to be carrying return current on your protective ground wiring, it is there to carry fault current only. if you carry more than a few mA of return current on that line it will trip the GFI in your house anyway if you plug it into a GFI-protected outlet. in theory, the neutral wire should always be at ground potential, so in theory, the amp should function fine and keep the chassis at ground potential as long as it has a polarized plug and the house wiring is the correct (code-compliant) polarity.

of course that's not always the case and you know the safety engineer's motto is to protect against single and double fault conditions ... which in this case necessitates a mains transformer.

BTW what are the patent numbers you hold? I work at an IP office an I'm always interested in reading new patents.

hmm.. let me look them up real quick...

6,873,243
6,600,642

and my non-safety patents:

6,839,383
6,449,171

hmm there's a fifth one I can't find at the moment and frankly I don't remember what it is.

I'll have to wait until I get home and look at the wall plaques

only one of these is actually in use in any product as far as I know (the lightning suppression one).

[Structo]

What are they for?

[benoit]

What are they for?

http://www.google.com/patents

Check it out.

[benoit]

hmm there's a fifth one I can't find at the moment and frankly I don't remember what it is.

6600632

Conductive lock washer. Ain't the google patent lookup great?

[krash]

nope, that's not me!!

that one might reference my patent tho.