Safe Transformerless Amps Possible?

[Trout]

OK,

Being semi active on a couple of other forums (Guitar Gear Related) I have seen a common complaint by many musicians.

Weight!

Amplifiers are heavy to tote around and though not exactly an audiophile issue, Shipping heavy amplifiers is also very expensive.

Now, my million dollar question.

How do you build a "SAFE" and reliable transformerless tube amp?

Removing 25lbs of iron really interests me.

Trout

[PRNDL]

Assuming that you use a three-prong cord ...

The main problem is when the AC hot and return are swapped (mis-wired), which occurs fairly often even in modern houses and clubs.

You could put in a circuit that tests for this and disables the power and lights an LED.

[Trout]

I can sort of invision the ground issue possibility, But, How would you eliminate the output transformer in an all tube amp?

I found an interesting looking new production ALL TUBE amp making these claims.

All tube no transformer design
Variable power of 0-100 watts
Weighs only 18 pounds (in head form)
Independent pre amp and power amp tone adjustments
MIG Fighter Jet 6C33 Power Section

Sounds like a great idea, But the million dollar question is, Can It really be done Safely & properly?

And If so, how would it effect the color of an instrument amp? Many guys love the sag/compression of the old vintage tube rectifier designs.

Where would one find a Mig Fighter Jet power section schematic to review?

Trout

[Eli Duttman]

Dude,

You can "ditch" the B+ power trafo under a VERY restricted set of circumstances. However, you may never get to the point of being legal. Forget about OTL, as bipolar PSUs without "iron" are not in the cards.

B+ rail voltage is limited to approx. 300 V. EL84 and 6V6 designs are fine, but EL34 and KT88 circuits need to be tweaked into a "hot and low" configuration.

Before use, the wall outlet that will supply the power MUST be tested for correct wiring. Plenty of outlets are miswired.

A 3 wire power cord, with the safety ground securely connected to the metal chassis is essential. Fuse both the "hot" and "neutral" lines in front of a DPST switch. Use a terminal strip mounted on a NYLON standoff with NYLON hardware as the substrate for the PSU. Connect the "neutral" line to the anode of a UF5408 and the cathode of a 2nd UF5408. Connect the "free" cathode to the + end of a series connected pair of 250 WVDC 'lytics. Connect the "free" anode to the - end of the cap. stack. Connect the "hot" line to a CL150 inrush current limiter. Connect the "free" end of the CL150 to the center of the cap. stack. The circuitry described is a "full wave" voltage doubler that isolates the "hot" line from both B+ and signal ground. Use the highest value caps. you can get in the doubler stack. Follow the doubler stack with a RF choke and a final reservoir 'lytic.

An "all in 1" is appropriate. ABSOLUTELY no metal can be exposed, where the player can touch it. Safety DEMANDS the I/P jack be isolated by a trafo from the rest of the circuitry. Mount the I/P jack on insulating material.

Please observe that some weight is saved, but cost savings are not realized.

[Trout]

OK,

Here is what inspired me to ask these silly questions.
http://www.vikingamps.com/

Pictures here are not detailed enough to determine what mechanical isolation was implemented, But These are some pretty big claims.

Trout

Edit, this is not spam, I am not in any way affilated with these guys nor do I endorse them.

I just hoped that it was someting I could try without to much investment.

[Conrad Hoffman]

IMO, a truly transformerless amp that connects to anything outside it's insulated box (like microphones, guitars, and speakers) is not possible. OTOH, all you really need is a switching power supply, using a high frequency transformer, that weighs next to nothing. Done correctly with optoisolaters, it should be as safe as anything.

[phase_accurate]

Yes they are possible: If you supply them from batteries.

For mains powered amps even the discussion of the subject is a taboo on this forum due to the serios dangers involved.

I bet it takes less than 24 hours until this thread is closed by a mod.

Regards

Charles

[SY]

Eli, "legal" is less the question than "homicidally insane." In an amp without a power or isolation transformer, there are many failures which can kill the innocent. I'm not familiar with the Viking amp, but I'll bet it uses an isolating switching supply. If not, it should be renamed "The Les Harvey Memorial Edition."

diyers shouldn't even THINK about direct mains connections.