Converting 1960s Fender amp for 240V UK use

I live in Israel and have the same problem all the time. First of all, the transformers you buy in the shops in plastic cases are junk. They are rated in VA and sold as if that were Watts, so you never get the size you need.

They almost never carry the ground through.

They are also autotransformers, not real transformers, so they do NOT provide any isolation.

Luckilly we have a guy here who makes transformers. He is not cheap, but he will make one to specification.

What you want is:

1. A True transformer (no connection of any sort to the mains, i.e. isolation).
2. Ground carried through, i.e. the grounding pin of the 120 volt socket is directly connected with a decent sized wire to the grounding pin of the mains plug.
3. A easilly accessable fuse socket in the secondary (120 volt side).
4. At least 50% more than the maximum current draw rating if in real Watts and DOUBLE if VA. Note that maximum is not always the rated value.
5. Since US equipment made in now is designed for 127 volts, not 115 or 120, the ratio should be adjusted for a slightly higher output than 2:1 (240/130 instead of 240/120). 1950's equipment would be better off with a slightly lower ratio (240/115 or 240/110) and heaivly motorized equipment (mentioned for compelteness) should be lower (240/110 or 240/100 for compressors) because of the different power factor.

BTW, if you've never done this type of work before, be careful about the voltages, replace all the capacitors and buy yourself a large bottle of DE-OXIT and a bunch of toothpicks (to apply it).

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

cab,

continuity

disconnect

exist

provide

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120,

All original inside except one resistor. 1964 Twin Reverb. I see US 3 pin outlet auto transformers are available. But owner wants to retain the moulded , sorry molded, 2 pin plug. All I can think of is making a 2 part shroud that fits overe the existing plug and includes a third earthing pin to mate with the 3 pin outlet, also retaining polarisation for added safety (assuming anyone rewiring the UK main plug in the future , wires it correctly). Third wire under heatshrink and bonded to the chassis mounted 110V outlet , removing the PK and replacing with a bolt and lock nut/star washer inside. Some sort of shrouding over the earth /chassis bond and exposed stub of wire. I'll try and convince him to go for isolating rather than auto but the original 2 pin plug plus grounding pin union problem remains.

I really want to be able to condemn the mains lead. But the internal exposed sheathing looks fine, how to non-destructively check for internally mangled insulation or wiring filaments inside the cable strain relief?

You could, of course, tell him to take it elsewhere because you're not going to be involved in him killing himself.

Jeff

--
"Everything from Crackers to Coffins"

I see Jeffrey A offers a good solution, same as what I'd generally tell a friend, even.

If the amp is so valuable to him, he should have it insured and not be using it.. and buy a display case to protect it.

A proper step-down isolating transformer (not auto transformer buck types) would allow 120VAC operation, just the way it was made. Amp and guitar isolated.

As long as the users of the amp don't plug in any line-operated accessories/equipment (patch to mixers etc) the amp should operate flawlessly. Any accessories (pedals etc) added between the guitar and amp should be battery powered only. Battery only. No exceptions.

So.. a 5-6A? capacity (see the amp's fuse rating) 240 to 120 step-down isolation transformer (no 3/third wire BS for the amp) with only 1 single,

2-blade 120VAC outlet. The single 120VAC outlet is a factor because idiots will start plugging more crap into it.

If the user has any problems (likely to be self-induced), he can take it up with the manufacturer of the isolation transformer.. and he should be informed of this, as his only option.

Or.. pull the amp chassis out of the case, and install a properly grounded

240VAC amp chassis in it's place.

-- Cheers, WB .............

A possible face-saving route. Release the tethered 2 part mains cable strain relief, bend the cable back and forth at that strain point enough times and let the owner feel the floppy section. Then 3 core can go in place.

Don't you Brits use a 110 Volt (center tapped) system for construction site power tools? On a lot of '60s vintage transformer powered tube gear, the only connection between either side of the power cord to chassis ground* was a small filter cap. So the amp may be perfectly happy running that way as neither side of the power line was expected to be ground. And, if something goes wrong, at 60 volts or so, it should keep your customer from killing himself.

*Ground came from somewhere else, as the gear was floating at 1/2 the power voltage at high impedance. (Made working on transistor circuits with an old tube 'scope a learning experience. First lesson: always hook up the ground lead first).

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

"Wild Bullshit Artist

** But that is NOT how such amps work in the USA.

Note the switch and the 0.047uF cap in the AC line to the amp - this is the " polarity switch" and it connects the chassis to one or other side of the AC supply via the cap. Users are meant to select the less noisy position, which equates to the neutral conductor.

To duplicate this set up , the isolation stepdown tranny must have one side of its secondary linked to the supply ground.

The only thing left " un-American " then is the frequency of 50Hz - which WILL cause the power tranny to get rather hot.

** It will work fine even with them - you posturing ass.

** Total lunacy.

The amp will always be safer if the chassis is grounded.

** A 300VA isolation stepdown tranny would be plenty.

.... Phil

Returning to plan A. What I think of as a lawnmower connector, 2 part outer shell. Just space to mould in the existing US plug and fix in a third pin. All demountable leaving the original cable and plug untouched, if so required. Then either black nylon spiral wrap or heatshrink over the green wire along the original cable. Satisfying the original brief but would be a lot cheaper for him just having 2 core cable replaced with 3 core and 3 pin plug, retained in the cab for any , unlikely, future USA based user.

Since the owner wants the amp to be at least retrievable then why not un-solder the old cord, coil it uo, and store it inside the amp. Re-wire witha grounded cord. Then if he ever wants to return the thing to stock someone can remove the cord from inside the amp and solder it back in place. Probably my suggestion will not please your customer though, you must have already suggested this. Eric

Why be a prick? Are your Ewes not in heat?

--
Live Fast Die Young, Leave A Pretty Corpse

We know you are.

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Live Fast Die Young, Leave A Pretty Corpse

continuity

exist

Site transformers , tend to be bright yellow and start at 1000VA, so an appreciable weight and industrial size connectors.

"N_Cook" schreef in bericht news:iuhfek$65a$ snipped-for-privacy@dont-email.me...

IMHO the only way to solve all the problems that has been risen, is finding a good, fully insulated 50Hz to 60 Hz converter. That way you can ground the enclosure of the converter as well as one side of the 60Hz output. An American outlet that matches the Fender plug can be build in the converter so you do not need to modify that Fender at all. No need to say that the converter has to provide a clean 60Hz sine wave as well as enough power to satisfy the Fender all of the time.

petrus bitbyter

"petrus bitbyter"

** Massively silly idea.

50Hz to 60Hz sine wave converters capable of 300VA at 120 Volts AC are not " off the shelf " items.

One would have to be custom built and there is simply no need.

Just use a normal 240/120 step down transformer and ground one side of the secondary - perfectly safe and legal.

If there is any concern about how warm the tranny in the Fender Twin is getting, just place a small fan behind the amp.

My god you fuckwits here know how to complicate simple matters.

.... Phil

"Phil Allison" schreef in bericht news: snipped-for-privacy@mid.individual.net...

You may be right, you may be wrong but I got 60Hz trannies fried on 50Hz mains. I'd take no risk with a - at least in the owners eye - precious Fender.

petrus bitbyter

It's not SUPPOSED to happen, but it does. A properly spec'ed 60Hz transformer will happily work on 50Hz, right at the edge of where it was supposed to be run. (20% overload)

However, quite often things sold in the US were not spec'ed properly, they were designed to work at 60Hz, without a big change in frequency or current draw.

I had a wonderful Phase Linear 400 watt per channel amp I had to sell when I left the US because their transformers were well known not to be able to stand 50Hz. :-(

Geoff.

--
Geoffrey S. Mendelson N3OWJ/4X1GM
Making your enemy reliant on software you support is the best revenge.

It's fairly easy to build a 50Hz to 60Hz switching inverter. Start with a 230V 50Hz power supply that produces 330VDC. Then run the

330VDC through a 60Hz chopper to get a 330VDC peak-to-peak square wave. If your xformer is fails to appreciate square waves and converts the harmonic energy into heat, then something more sophisticated, like a step approximated sine wave switcher can replace the chopper.

Well, maybe this is overkill (and inefficient):

Also, I just downloaded the Phase Linear 400 schematic:

and found that the xformer is just a full wave center tapped single winding (other than the winding running the pilot lights). You could possibly have found a 50Hz replacement for something that simple or deviced another +75v/-75v power source. Oh well, too late.

--
Jeff Liebermann     jeffl@cruzio.com
150 Felker St #D    http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann     AE6KS    831-336-2558

Yeah but that's half the fun.

I'm thinking of a nice motor-generator set with a common armature. ;-)

And a nice roll around cabinet. Tolex covering of course with shiny brass fittings.

Jeff

--
"Everything from Crackers to Coffins"