OT: amp chassis floating at 60V, and associated oddities

[d_rom]

Hi everyone.
I just brought an old Kay Model 703 33 watt tube amp back to life, and now the chassis, and all associated ground points, are reading 60V.  When the chassis is connected to ground via a 3-prong cable, a terrible hum ensues.  The strangeness is:
 - It works fine with a guitar.
 - when I plug my wurlitzer (which is chassis grounded via 3-prong) in to it, the chassis reads 0V and it works fine.
 - It hasn't shocked me yet, although it was invited to on multiple occasions.
 -  No shorts to the chassis could be found.

Any ideas what's going on?  I'd really like to put a 3-prong cord on to this little thing.

-d

[smoguzbenjamin]

So you touched the 60V chassis and it didn't shock you. :shock: Be careful!

[d_rom]

Yes.  Before I realized it was charged.
I'm not sure if this was going on before I restored the amp or not.

[smoguzbenjamin]

Hmmmm. Maybe it's a positive ground amp. Could that be it? Oh and is the 60v AC or DC?

[d_rom]

AC.
I don't really understand the concept of positive ground.  Does this just  mean that the ground rail voltage is more positive than true ground, or that normal "ground" is more positive than the rest of the circuit?
thanks.

[smoguzbenjamin]

Well you see ground is usually negative. Like having a ckt with the red battery wire going to V (V = any place with +9v going to it) and the black one going to ground, but now its the other way around. Anyway what it means is that the circuits V is 0 and ground is 9v, or if you measure it as usual, the circuits V is -9v and ground is 0.

It's just another way of designing a circuit (not very practical in my idea but people do it for one reason or another).

[Jim Jones]

Be careful with that amp...those old series-string tube amps are potentially dangerous.  They don't have a PT so your wall voltage is going directly to the tube heaters.

Which model of 703 do you have? (apparently there are three different ones.)

Oh, as far as wattage goes, that amp will only put out four or so watts...

Jim

[d_rom]

I'm not sure what you're abbreviating "PT" (Power transformer, maybe?), but yes, the wall power goes directly from the wall -> halfwave rectifier heater tap -> 56 ohm resistor -> power tube heater then to the isolation transformer and an RC network.

As for wattage, it says 33, but I know mannufacturers used all sorts of arcane methods to come up with those numbers.  I haven't tested the actual output yet, though.  The output transformer is permanantly affixed to the speaker, so it would be tough to use a typical dummy-load wattage test.  It sounds HUGE when it's cranked, but it is not very loud.  Probably great if you have a good mic live,  or for recording.

I'm not sure how to differentiate between the various models.  I've found it very hard to find any good info on the Kay and Western Electric amps.

[smoguzbenjamin]

Maybe it's stamped in the chassis somewhere...

[d_rom]

Didn't see it.

[d_rom]

Didn't see it.

[d_rom]

Didn't see it.

[smoguzbenjamin]

Triple post, wow.

Anyway, maybe it's supposed to be like that :? Doesn't seem logical but you ain't dead yet are ya?

[R.G.]

I just brought an old Kay Model 703 33 watt tube amp back to life, and now the chassis, and all associated ground points, are reading 60V. When the chassis is connected to ground via a 3-prong cable, a terrible hum ensues.The strangeness is:
- It works fine with a guitar.
- when I plug my wurlitzer (which is chassis grounded via 3-prong) in to it, the chassis reads 0V and it works fine.
- It hasn't shocked me yet, although it was invited to on multiple occasions.
- No shorts to the chassis could be found.

Any ideas what's going on? I'd really like to put a 3-prong cord on to this little thing.

That's happening because the input circuit of that amp is floated on top of the filament power for the input tube by that isolation transformer. The isolation transformer works off part of the AC line voltage to put heater power on the input tube, and the input tube is isolated from the power line ground by that 47K resistor. When you hook up the safety ground, you're forcing the input circuit to see the filament voltage as an input.

You really ought to deal with that thing as though it's a pet rattlesnake - enjoy it if you really want to deal with the responsibilities of having to remember that it is potentially deadly if you ever get careless. One side of the AC power line is permanently connected to the power supply.

This amp is not a candidate for three wire conversion because of that. To convert it to three wire, you will need to use a 120vac:120vac isolation transformer in front of the whole mess. This will break the connection of AC line to the power supply, and you can then connect third wire to chassis/signal ground.

I don't really understand the concept of positive ground. Does this just mean that the ground rail voltage is more positive than true ground, or that normal "ground" is more positive than the rest of the circuit?
thanks.

It only has meaning for DC power supplies. Whenever there is a single DC power supply voltage, "positive ground" means that the more positive of the two power supply leads is connected to the ground point, which then implies that the power supplies are all more negative than the ground reference. "Negative ground" is shorthand for the reverse: the more negative of the power supply or battery voltages are connected to the ground or reference point, and the power supply voltage is always positive with respect to the ground point.

[d_rom]

Thanks, R.G.  
YOu may not know it, but you've been really helpful over the last year as I have been teaching myself this electronics game.

P.S.  The Leslie switching problem you helped me with a couple of months ago was solved by simple RC filtering.  I'll probably get around to building a relay circuit soon enough, though.

Jim Jones- in teenie weenie print on the bottom of the schematic the Kay says "703-c"

[Tubebass]

I acquired an old Danelectro amp with hot chassis....the cabinet was made of cardboard!!!! My solution to the hot chassis was to scrounge a power transformer with two seperate 120 volt primary windings, which I mounted in the bottom of the amp cabinet. One winding to power line, one to the amp. Now the chassis can be connected to the power line ground wire and all should be safe. I think the transformer I used came out of an old data terminal.
   Hope this helps!
        -Dave