220V power transformerless amp?

[badoumba]

Guys,

In Europe where I am living in, wall plugs deliver 220V AC. Let's say I wanna go for a 25W amp design - considering the price of transformers, is there any reason for not going for a power tranformerless option? Should be enough for2 pushpull output tubes, 1 rectifier, 2 preamp ones and 1 reverb. I know that it is better to have heaters connected in parallel for instance, but is there any real drawback for such amps (aka can it be good quality)?


Thanks

[pdf64]

To comply with electrical standards such an amp would need to be double insulated, which isn't practicable for an instrument amp, especially a tube type.
eg wireless only input, no user access to any metalwork.
Pete

[Phil_S]

You might save some money by using a 1:1 isolation transformer, but that misses the point. Is it really possible? Do you have specific tubes in mind?

Even if it were safe to do, what power tubes would you use to get filament requirements to add up to 220? Something like a 50L6? You'd need 4 of those = 200 + 4x 12AX7 @ 6.3VAC = 225VAC. Unfortunately, Va Max for a 50L6 is 135V, so how are you going to generate the B+ ladder from 220? Rectified with a tube, you might get as low as ~250VDC and solid state, it is going to be more like 300VDC.

If you wanted to do an "All American Five" amp (traditional 110V USA), you'd still need a 2:1 stepdown transformer, and then you can use 2x 50L6 + 3x 12AX7, for a filament total of ~120V and B+ at ~ 150VDC (solid state rectified).

If you have to buy a transformer, why do it? Design options are limited. Considering all the other parts you need, you won't save that much money.

[Tillydog]

...is there any reason for not going for a power tranformerless option?

Yes: Safety.

Don't do it.

[gingertube]

We can not stress strongly enough that that a power transformerless design is DANGEROUS (read "Suicidal").

Over at DIYAudio the Moderators have a policy of deleting any post in which a transformerless design is raised.

As Phil_S referred to above, there was at one time some designs which did this. That equipment must now be used via an isolation transformer.

There are some modern AC to DC Converter ISOLATED Power Supply Front Ends available but if you objected to the price of a power transformer then you will cetainly object to the price of these.

That "ISOLATION" from mains is critcal to keeping you un-fried, not to mention breathing.

Always remember that exposure to Alternating Current (AC) is MUCH more dangerous than exposure to DC. Treat Mains with respect and always have a level of isolation between yourself and that mains supply, for Guitar Amps that means always use a power transformer.

In simplest terms, what that isolation does for you is mean that you must get connected across both leads to get a shock/electrocuted whereas with no isolation then connection between either wire and earth can put you in a grave.

There is nothing scarier than getting connected to mains and realizing that you cannot let go. My workbench has been fitted with a Earth Leakage Breaker/ Residual Current Device. These don't prevent you getting a "boot" but they trip and disconnect you from the mains in less than 1 cycle.

Cheers,
Ian

[rp]

Even if safety wasn't a serious issue unless you have some large varied stash of oddball NOS tubes to pull it off it would still be far far cheaper (and much much faster) to just buy a PT. I can't imagine you have a 100-200W amp in mind, so €60-90 for a Hammond 200 or 300 series from Tube-Town or Uraltone or Ebay isn't very expensive at all. Heck, a chassis is €30-45.

Hunting, waiting on eBay, plus shipping + fact that used/NOS tubes are rather expensive in Europe, would cost IMO 2-3 times more than a PT. Italy seems to have a few affordable transformer makers, then there's eastern EU. Here's some options, can't report on service quality as I never tried except one, and maybe you get what you pay for, maybe check some euro forums. I had Raoli make me a custom husky multi primary PT @ 250mA for a 5C8 Twin, was around €100 shipped - that's pretty honest for a one-off, and delivered in 2 weeks.

http://www.enovaz.it/
http://www.inmadout.com/ -> http://www.uraltone.com/
http://www.raoli.it/
http://www.welter-electronic.de/index.htm
http://www.lisatrafo.com/trasformatori-di-alimentazione
http://www.trm-audio.si/
http://www.valvepower.co.uk/transformers.htm
http://www.tme.eu/html/EN/mains-transfo ... pelny.html

plus these guys:
http://www.musicalpowersupplies.com/

[TUBEDUDE]

It would be an assisted suicide machine. You could call it the Kevorkian.

[Structo]

I was always under the impression that DC was worse than AC to get shocked by.

The DC can lock up your muscles making it hard to break contact.

Where as AC is reversing polarity 60 times a second going to zero each half of the cycle.
You can release your hold easier because of this.

Of course it depends on the voltage and current that hits you.

But getting hit with either one hurts like a SOB!