Why iso transformers for AC/DC amps?

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samgraysound

Well-known member
Joined
Jun 9, 2014
Messages
284
Location
Olympia, WA
Hey All,

I'm working on a Magnatone Starlet 107 for a client. Its an AC/DC amp, with the mains connected directly to heater of the rectifier tube. It already has been given a 3-prong cord, with he hot wire going to the power switch and neutral and ground wires going to chassis. It does not have a fuse or a iso transformer.

I am trying to convince my client to do the safety upgrades and install a fuse and iso, but I find I am unable to explain the point of the isolation transformer.

The fuse in my mind, limits the shock hazard current to the fuse rating (1/2A) instead of the breaker rating (15-20A).

But what does  a1:1 isolation transformer do for safety? The 3 prong plug prevents the mains from being plugged in backwards and putting hot on the chassis. I know my GFCI sockets don't like the amp right now, but I can't articulate why an iso transformer would be better.

 
You definitely need a fuse on the mains input.

Chassis should be connected to safety ground.

If GFCI outlets don't like the amp that suggests it is leaking more than 6-7mA between mains lead and safety ground... not good  :eek: :eek: because that would energize the safety ground if floated.

If GFCI doesn't like it, I don't like it.  8)

JR
 
The fuse is for the safety of the unit ,it really doesnt help save your life in the event of shock ,milliamps across the heart from live mains and your a gonzo'd .

I just dont like two wire mains on anything with exposed metalwork ,never mind a guitar amp which your grounded too , I personally wouldn't take on the job if he refuses to attend to the safety .

As you mentioned the ground and the neutral are one and connected to chassis , the most serious problem with this is if the live and neutral are reversed ,your amp may appear to be working as normal ,but with a live chassis, you might just experience a tingle until you make a proper ground someplace else ,then bang ,your going to be blown half way across the room . So having a proper mains transformer  ,it doesnt make any odds if the neutral and live are reversed as ground is seperate from neutral  .Im not even sure if an isolation transformer makes things 100% safe or if it will work as your amp was built from the ground up for direct mains connection ,sound like a major redesign will be needed to seperate parts of the circuit that belong at neutral potential and the ones connected to ground ,JR might be the best person to explain the dangers of what your trying to do and if theres any realistic way of modding it to be safe .
 
magnatone_107B.png


What could go wrong?
 
JohnRoberts said:
You definitely need a fuse on the mains input.

Chassis should be connected to safety ground.

If GFCI outlets don't like the amp that suggests it is leaking more than 6-7mA between mains lead and safety ground... not good  :eek: :eek: because that would energize the safety ground if floated.

If GFCI doesn't like it, I don't like it.  8)

JR

The only thing connected between Mains lead and safety ground is all the tube heaters. And weirdly it trips the GFCI even with the power switch off.
 
Sure a three pin plug will prevent the chassis becoming live normally ,but imagine if you happen to plug into a socket thats wired backwards ,its just not worth the risk .

I just saw Johns response ,and yes as I imagined he's flashing a hazzard light on it , maybe if you post a schematic we could have a look and see if theres any way to make it safe with a transformer .

In old radios here you sometimes see common ground/neutral and no mains transformer ,in most of the better sets the chassis is completely covered by insulation and you cant even get your fingers onto it ,but of course if your plugging in a guitar your going to be making contact with chassis, its a potentially lethal situation and has led to the death and dissablement of many .You could of course swap out the valves with ones with 6.3volt heaters, ditch the long chain mains powered heaters and get a proper mains multi winding transformer for the job ,not really sure if you could justify the cost of all this with the customer unless its of very sentimental value , Its my least favourite part of the repairs job ,when you end up having to tell someone their baby is not worth repairing ,proceed with the utmost caution .
 
samgraysound said:
I keep trying to upload a schemo with the current mods but the forum is rejecting it :(

Your attachment has failed security checks and cannot be uploaded. Please consult the forum administrator.
 
Thanks for the schem  PRR ,

I see everyting barr the first valve opperates with the chassis neutral ,I dont understand the 270k resistor between chassis/neutral and the grounded end of the first valve .

Id say replace the output tube with something similar with a 6.3 volt heater ,and try and find a transformer with both HT and LT windings ,and rewire. You could afford a bit more Ht volts from the 125 you get directly off mains too ,its all do-able alright but will the owner pay ?  end of day its still only a handful of watts output at best ,not really a gigging amp I would have thought.



 
samgraysound said:
The only thing connected between Mains lead and safety ground is all the tube heaters. And weirdly it trips the GFCI even with the power switch off.

Circuit current should not connect to safety ground

Mains power flows between hot and neutral... safety ground is a third wire bonded to neutral back at the panel, but just for safety.

That is obviously wired wrong.

JR
 
JohnRoberts said:
Circuit current should not connect to safety ground

Mains power flows between hot and neutral... safety ground is a third wire bonded to neutral back at the panel, but just for safety.

That is obviously wired wrong.

JR

This one has been modded so the .05uf and 270k resistor between neutral and chassis are bypassed. So circuit ground is the chassis. Safety ground and neutral are both connected to chassis.  power switch is between hot and the rectifier. Also added cathode caps for the 1st and 3rd stages, and removed it from the 2nd. Changed various resistor and cap values.
 
Looks like someone has added a 1:1 isolation tx across the mains on this Magnatone .
 

Attachments

  • Magnatone mains iso.jpg
    Magnatone mains iso.jpg
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A shock direct from live mains is way way worse than through a transformer . By installing it you seperate ground from neutral and any chance of the unit becoming dangerous due to live chassis even if the live/neutral connections are reversed.
 
samgraysound said:
This one has been modded so the .05uf and 270k resistor between neutral and chassis are bypassed. So circuit ground is the chassis. Safety ground and neutral are both connected to chassis.  power switch is between hot and the rectifier. Also added cathode caps for the 1st and 3rd stages, and removed it from the 2nd. Changed various resistor and cap values.
So unbypass them...

270k won't kill you, a hardwire bypass could.  Neutral should not hard connect to chassis, only safety ground bonds to chassis.

JR
 
Tubetec said:
A shock direct from live mains is way way worse than through a transformer . By installing it you seperate ground from neutral and any chance of the unit becoming dangerous due to live chassis even if the live/neutral connections are reversed.

Thank you.  I now feel clear on how it protects from reversed connections and seperates the neutral and safety ground. Why is it worse  to receive a shock from live mains than through a  transformer?
 
JohnRoberts said:
So unbypass them...

270k won't kill you, a hardwire bypass could.  Neutral should not hard connect to chassis, only safety ground bonds to chassis.

JR

Ok. This is an alternative to an iso transformer or it is preferable to an iso transformer?
 
samgraysound said:
I have them. I know how to install them. The question I originally asked was why they make the amp safer.

Because power hot and neutral are not connected to the audio circuitry. It really is that simple - you can' t get an energized chassis if hot and neutral only go to a transformer primary. That allows you to ground the chassis to the power safety ground (as you should), and also use the chassis as signal ground.
 
Well its a lot to do with source impedence , mains power has very low impedence , once you go through a transformer that raw energy from the mains is very much reduced , the transformer limits the current . This doesnt mean getting electrocuted off a Ht line on an amp cant kill you though. Take the example of a large 1:1 mains isolation tx,  without it you touch the live and have a good ground from another part of your body lots of current will flow. If you interpose a Tx now both live and neutral are completly seperated from ground , indeed with a iso tx you can touch either wire off the secondary and not get any shock at all ,only if you grabbed both live an neutral off the transformer secondary would you get a shock ,and this wouldnt have anywhere near the same shocking abillity as direct live mains due to resistance in the windings of the transformer.
 
> the .05uf and 270k resistor between neutral and chassis are bypassed.

Deathtrap.

Despite 3-pin and GFI, the Player is still connected to one of the wall-outlet Power wires.

Best-case (and maybe least-hum), player is holding the White wire, only a Volt or few away from household grounds (other stage gear, heaters, water-pipes). But very good chance Player is holding the Black wire, 120V away from household grounds.

I place no trust in "3-pin". Every musician has a ground cheater. And many 3-hole outlets do not have an honest ground.

I place no trust in White/Black being correct in the wall outlet. In my last house, 60% were reversed.

I put marginal faith in GFI, but only as a secondary protection. Primary is that the User can NOT touch either power wire (all plastic housing with no direct user connections).

An isolation transformer re-sets the galvanic conductivity. You can touch *either* wire of the iso's secondary *and* the water pipes, and not be shocked. It is a floating supply.

We normally then tie all touchable metal (particularly guitar jack) to green 3rd-pin. At best this is a good ground and forces the chassis and user to same potential as everything around. The likely worst is that it does nothing. (It is possible for the 3rd-pin to really be the black 120V wire; this is fairly rare and the occupants are usually aware that something is wrong.)
 
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