Ford Amp

[Weldaar]

Hi, I am looking for information on an Ford guitar amp. Not sure of the model # as the tag is missing. It has an 8" speaker, 35Z5, 50L6 and 12AX7 tubes, no tranny. The speaker is an RSC speaker. People tell me that this amp can be dangerous because of no tranny and two prong cord. How can I check it out if it's dangerous to plug it in?

[Enzo]

It is a small "hot chassis" amp. That means it is dangerous, because the inner circuits connect directly to the mains. See the big resistor heading from the fuse holder right to the first tube socket? This is old school where you plugged it into the wall and lightly touched it to see if you got shocks, and if you did, you turned the plug over in the wall socket. You cannot simply wire a grounded cord to this, to be safe you'd need to add an isolation transformer. Many have done exactly that.

It puts out a few watts, and will be louder than you expect from a low number of watts.

I can just about see well enough in your second photo to draw up a schematic from the circuit. You could do that yourself. It will closely resemble this one, though there will be differences in resistor values and such.

http://www.webphix.com/schematic%20h...valco_6143.pdf

Note: if you want this drawing, download and SAVE it immediately, because the source web site is closing down in a day or two.

[R.G.]

All "no transformer" (and therefore "hot chassis") amps are as deadly as a pet king cobra. Keeping one can be done safely, but only (1) if you really, really understand what you're doing, and (2) never, ever get tired, inattentive, or careless. What's at stake if you mess up or get sloppy is your life.

I've worked with AC mains designs on electronic equipment for over 35 years, and I get chills running up my spine at what I see people casually thinking they'll do with AC mains connected stuff. Low voltage (i.e. 120-240V) electrocution is indeed scary stuff.

[Jazz P Bass]

And to think of all those 'All American Five' radios that where sitting next to the kitchen sink while mom did the dishes.

Ah, the good old days.

[Weldaar]

I downloaded the schematic, Thank you. OK, I just want to see if it works. Can this be safely done? It's not my amp and I don't want to do any repairs to it. I'm guessing by the replies that I should not even plug it in and play through it. Am I correct in my assumption?
In these pics I am showing the power cord attached to the volume control pot and the fuse. I know where to wire the three prong cord, I think, but I don't know what an isolation trans is.

[Jazz P Bass]

I know where to wire the three prong cord, I think

STOP.

You cannot wire up a three conductor cord to this amp.

[Weldaar]

OK Stopping. Care to explain why?

[nashvillebill]

You can get an isolation transformer from Antique Electronic Supply: Search | Antique Electronic Supply

You'll need to figure out which model to get based on the power supply of your amp. I doubt the $21 model would be adequate but maybe...

[Weldaar]

Thank you all for your input.

[R.G.]

This is one of those places it's worth explaining a bit to people new to AC mains issues.

The way the electrical mains work in the USA is that the transformer out on the power-line pole makes 240Vac, with a center tap. The three wires, 120A, 120B, and the center tap are brought into the house. The center tap is connected to a copper wire that runs down the pole and is spiraled on the bottom of the pole itself, and buried a bout 6-10 ft deep when the pole is set up.

The house breaker panel uses 120A and 120B as "hot" or "line" power wires, and the center tap wire is "neutral", and carries all the return current from the two opposite 120Vac phases. The breaker panel also has a bare copper wire running from the ground bus inside it to a metal rod buried in the ground right at the house. This ground bus is then connected to all outlets and electrical supplies as "safety ground". The "hot" or "line" wire carries 120Vac compared to the "neutral" wire. If no current flows, the safety ground and neutral wires are at the same voltage. But if current flows, there is a voltage on the "neutral" equal to the ohm's law voltage V = I*R, where R is the resistance of the wires back to the breaker panel and back too the power pole center tap.

This was all pretty much the same before third wire ground, and two-wire audio equipment counted on one of the incoming AC wires to be a "neutral" and at nominally 0V compared to earth ground. Death ("line reverse") switches usually connected the chassis ground to one or the other incoming AC line, the low-hum position being when it was connected to neutral.

The problem with the two-wire setup is that HALF THE TIME THE TWO-WIRE PLUG CONNECTS THE CHASSIS TO THE "HOT" MAINS WIRE. This made for bad hum, but also left the chassis floating at 120Vac. Flipping to the low-hum position cut this to a few volts AC, but often enough to get a shock.

The danger is that when the plug happens to put the chassis on the "hot" side of the line, if you contact the chassis, you're fine until you also contact something with continuity to real, true earth ground. Then you get shocked and maybe die. Unless the power switch happens to break both hot and neutral incoming line - and they almost never did - then you die. You don't even have to be stupid or malicious to die, you just have to have bad luck. That's why the safety standards made this unacceptable. A manufacturer releasing a product like this today would be in court within a very short time defending a "wrongful death" action.

The only safe way to run one of these is with an isolation transformer. The isolation transformer is a 1:1 transformer that takes in 120Vac and puts out 120Vac, but with a special difference: the output 120Vac has NEITHER side connected to earth ground. So if you happen to contact the chassis and it's connected to the hot side of the line... well, there is no hot side of the line. Both sides are insulated from earth ground, and so you don't die if you touch something that is well grounded.

I can see people thinking "Well, if it's the grounded side of the AC line that makes it dangerous, why ground either side?" That's because the modern practice of safety ground connecting to all "accessible metal parts" as it says in the standards protects against failures that let the hot side of the line short to chassis.

The bottom line is that with these relics of the two-wire days, you have to use an isolation transformer to be safe.

[bluto]

And to think of all those 'All American Five' radios that where sitting next to the kitchen sink while mom did the dishes.

Ah, the good old days.

Those were actually (relatively) much safer as the case and the knobs were generally made of non conductive material...wood, plastic, etc...no direct connection from the user to the 50% likely to be hot chassis like with a guitar via the cord. FWIW I built a couple of series filament amps from old non guitar gear I got for free but eventually went with other designs as there are low $$$ PT setups that only cost slightly more than an isolation transformer. (but would restore a series filament guitar amp with an isolation xfmr if one comes my way of course...and it looks like the OP's amp has had lots of work already, the resistors are much newer than the amp)