Another old death amp, Bardon 303-C

[RLBJR65]

One more old amp rediscovered! The Bardon 303-C, thanks to idlechatterbox who sent me some photocopies of the factory schematic
I redrew it and put it in my gallery. http://aronnelson.com/gallery/albums/album132/Bardon_303C.gif

Before anyone asks, yes the on / off switch really is on the tone cotroll pot

Check out who's name the patents were licensed under!

These old amps were referred to as death amps because there is no protection from the mains voltage so they can give you a jolt if you don't have the plug in the right way They can be made relatively safe however by adding an isolation transformer. Check this thread for a wonderfull (as usual) explanation from R.G. http://www.diystompboxes.com/smfforum/index.php?topic=33010.0

I have noticed many of these "death amps" on Ebay lately. Harmony, Kay, Kent, Valco and a few others made them. There are quite a few of these schematics in the Bargain bin section at www.schematicheaven.com/

[Meanderthal]

 I recently added a ground to chassis to our harp player's Supro. No more shockies...

[JasonG]

Where do find tubes for thoes things. They dont look like the usual type. There class A right?

[Paul Perry (Frostwave)]

The problem withteh tubes is this: you have no transformer, but you need to heat the filaments....
Well, back in THE DAY, you could get high voltage filaments, voltages like 55V (put two in series for 110!). Ordinary tube but high v filament. Some TVs had really weird strings of filaments - remember the filaments must have the same current ratings to go in series - who says they werer the Good Old Days?

[Ronsonic]

That is the craziest heater wiring scheme I've ever seen. The usual thing is to have them all add up to line voltage or just under and use a lamp or resistor ballast. This is not like that. Very strange.

ATT was once a part of the same company that spawned Bell. They invented a lot of the tubes and the circuits that used them. We all know about the "Western Electric" patents on elements of the tweed Fender designs. This is the same type of deal. Mostly, as tube makers they didn't restrict the use of the patents since they wanted to sell tubes. They did insist on credit.

Ron

[RLBJR65]

The problem withteh tubes is this: you have no transformer, but you need to heat the filaments....
Well, back in THE DAY, you could get high voltage filaments, voltages like 55V (put two in series for 110!). Ordinary tube but high v filament. Some TVs had really weird strings of filaments - remember the filaments must have the same current ratings to go in series - who says they werer the Good Old Days?

Great explanation Paul! Must be why the mains goes into the filaments center tap on the 36AM3.

Fred Nachgaur also has this little write up. http://www.dogstar.dantimax.dk/tubestuf/4x4intro.htm

I recently added a ground to chassis to our harp player's Supro. No more shockies...

Thats a good idea with any old amp that does not have a ground. Also replace those plugs that people cut the ground lug off of.
However this type of amps needs an isolation transformer to be safe, they have no power transformer so you can't just put a ground lug on them as the chassis can have line voltage running through it

I should have said something like this:
There were a few of these models without a power transformer made by Harmony, Kay, Kent, Valco and others.
Because not all of their amps were hot chassis. Sorry if I confused anyone.

This is my Harmony http://aronnelson.com/gallery/albums/album132/H303B.gif I posted it in the other thread but moved it since then.
As you can see it uses the heater filiments of the 50C5, 35W4 and a small resistor in series. The factory labled "Isolation Transfomer" is actually a step down. The heater voltages are less than ideal but the original tubes all tested okay.

[Paul Perry (Frostwave)]

I can't remember much of my childhood, but I still remember the first time I poked around inside a hot chassis radio... yes, i did say "the FIRST time" - I wa a slow learner

If you think "hot chassis" is crazy, at 110v, they were even crazier in Australia when they were modified for our 240V by putting a resistor in series with the mains. Naturally this generated quite a bit of heat... don't worry, just use a large resistor and put it in the mains lead, outside the box! Why does that mains lead look pregnant? Why is is HOT???

[brett]

Speaking of bio-conduction, my first taste of electronics was reconnecting a speaker wire in a valve radio. 
After connecting the speaker wire and checking for sound coming out  , I thought I'd give the connections on the other side of the OT a push.   
That put me off electronics for the next 10 years (ages five to fifteen), and by then the world had gone solid state.