Small Amp With No Power Transformer

[RightLurker]

I have an old, small amp that is an oddity, and at this point completely dead. The circuit has no power transformer. The amp has a single 8" Jensen speaker, a volume/power switch knob, and a tone knob. The chassis is in the bottom of the amp with the controls facing the rear. It says "Rex" on the control panel, along with "Four Tube Dual Input."

The four tubes are one 12AX7, one 117Z3, one 35Z5, and one 50L6GT. I opened it up last night, and discovered there is no power transformer. The wiring and components look remarkably well put together. There is no circuit board, only two terminal strips.

The lack of a power transformer seems bizarre to me, and it sets off alarm bells in my head. I read somewhere that the power supply in circuits like this are connected to the chassis (?), and that the potential for shock is greatly increased.

Does anyone know anything about this type of circuit? I'm not touching the thing further until I find out what I'm dealing with.

Thanks in advance.

[bancika]

yeah, those tubes run their heaters at much higher voltages than tubes we're used to so they are feeding right from the mains. B+ is rectified directly from mains too. I'd add a simple isolation transformer just to be safe. Older fender amps had "death cap" from mains to the chassis. If you have something like that it may be a good idea to remove it.
Cheers

[Andy Le Blanc]

Their based on old ac/dc radio, used to be a common product design in the low end.
The tube heaters are series, They need a 1:1 isolation tranny.
I got couple three of those too, not much of an amp, but they're fun.
They really have safety issues, There are some threads on the forum.
they have come up before, I have a opt to one of those if you need it.

[Structo]

Very dangerous without an isolation xfmr.
Depending on which way the plug is plugged into the wall you have a 50/50 chance of having a "hot" chassis.

I can't believe they ever let those types of devices be sold.

Don't use it while laying in the bath tub.

[RightLurker]

Thanks to all. I've learned that the amp is actually a Valco model 6143, badged as a "Rex." There's a great article with pictures on installing an isolation transformer in amps like this at instructables.com, for those who might be interested.

[Structo]

Here's a schem

http://www.schematicheaven.com/bargainb ... o_6143.pdf

[Cliff Schecht]

Anytime you see a bunch of goofy tube names like 50xBlah, 35Lwhatever and then maybe one or two actual preamp/poweramp tubes it's usually safe to assume that the amplifier is NOT isolated (heaters are just wired in series) and should be considered a safety hazard. Isolation transformers are cheap, trips to the hospital and funerals are not!

[M Fowler]

Here is an All American Five type amp converted for safer operation.

[Cliff Schecht]

I hate that most amplifier schematics omit the output and power transformer information. These are important to know in most designs and do have a big influence on the sound..

[M Fowler]

117v 60hz in 60v 1/2 amp out PT

OT 2k primary 8 ohm seconary 3watts