Kay 703 head scratcher

[bradb]

Hey guys,

I recently got a free Kay 703C...

http://www.freeinfosociety.com/electron ... ay703c.pdf

I recapped the multi-section capacitor and replaced the non ceramics with orange drops. I also added a grounded plug and a fuse. The fuse and power switch are coming off the black wire in my black-white-green plug. The green wire is soldered to my chassis (scraped off paint and used the monster iron)

When plugged into one of the outlets in my house, the amp sounds quiet and insanely great when playing guitar.

When plugged into other outlets it hums INSANELY loudly, no audio can come through. I mean, this is CRAZY HUM, its unplayable.

I've since figured out that that outlet on which the amp works in NOT grounded... and since I've grounded the chassis, I'm also grounding that power transformer since the output has one side running through the chassis.



How do I tackle this so that the amp works safely in grounded outlets?

[Structo]

It looks like the heaters are grounded to the chassis.
In the old two prong days, this was a fairly common practice.

I would get the heaters up off of the chassis and run discrete wires to them.

[rfgordon]

I concur with Structo. I've reworked a few Kalamazoo Model IIs, and the best thing to quiet them is to redo the heaters. Depending on how the amp is built, it may only take a few changes.

[bradb]

as the amp is now, it doesn't work in a grounded outlet because the chassis is grounded.

I could discretely wire the heaters, but still that wouldn't fix that problem.

[jjman]

I’m not understanding the schematic. Why would the “isolation transformer” be isolating the heater on only V1 ? It shows only V2 and V3 in series regarding the heaters. Shouldn’t all 3 heaters be in series to achieve the correct total of 114volts ? Is it really merely an isolation tranny or is it also a voltage tranny to feed 18volts to the V1 heater?

Now I see that there is a resistor in line between the heaters on V2 and V3 so I guess the “isolation” tranny is also converting the line voltage to ~18volts for the heater of V1.

Looks like the inputs do not short when nothing is plugged in…..

With the new 3 prong cord, is the hot leg or the neutral going to the rectifier’s heater?

[rfgordon]

My bad on the heater issue. I failed to look at the schematic. These amps, like a lot of cheap tube radios, put all the heaters in series (the filament voltages add up to 114). Barring completely rebuilding the amp using a new HT/filament transformer, you can't operate this amp safely.

For goodness sakes, never plug into this amp and step up to a microphone!

[bradb]

I'm into safety.

I don't see the danger in the heaters. I can use a 1:1 transformer there.

I think the problem is that one side of the HT transformer is going thru ground. but I could isolate that but i'd need some kind of isolated 1/4" jacks, right?


kinda confusing!

[rfgordon]

OK, here's the basic problem: depending on how you plug the amp into the wall socket, you have either neutral or hot on the chassis, as well as across R7 and C5. Regardless of how you turn the plug, if the filament of V1 shorts to grid, it puts wall voltage, with, as far as your body is concerned, infinite current onto the input cable of your guitar, and, thus, its strings. That, as they say, is a bad bad thing.

A safe amp has chassis at true earth ground potential. The chassis can never be the neutral side of wall AC voltage and be safe.

Isolating the input jack doesn't mitigate the problem, since the R to ground there goes to the chassis--which might be hot 120 VAC!

All of the amp's operating voltages (filaments, HT) have simply got to come off the secondary of a transformer, and they must have some means of protective failure. Look again at the amp: the schematic doesn't even show a fuse!

[bradb]

true,
I wired in a fuse and put a 3 prong plug on the amp. Knowing full well that an improperly wired outlet could screw me....

I could take that leg of the transformer off the chassis and and wire it discretely, but then what is ground?

someone else told me to put a 1:1 transformer, and that would be for safety for the heaters... but its the HT that is the real quandary...

thanks for your continued patience!

[ThePRNDL]

I just finished upgrading a Kay 703c.

I put in an isolation transformer and 3-prong power cord specifically to avoid the issue you mention.

The chassis is connected to AC ground and one leg of the isolation transformer output.

I also replaced the electrolytics.

Here's a link to photos
http://www.naturdoctor.com/Chapters/Amp ... 3/Kay.html

[bradb]

Thanks for the info.

I don't see how this makes things safer.

can someone explain?

and what model isolation tranny did you get?

thanks everyone!

[bradb]

Just wanted to post a little bump.

To recap (no pun intended) this amp has the heaters all run in series and connected directly to the power plug. one side of this circuit runs through the chassis. When you ground the chassis to earth, the amp doesn't work! you've shorted the heater circuit to ground.

If I put an isolation transformer on the input power, before its sent to the heaters and before it is sent to the HT transformer, can I THEN ground the chassis and have it work?

There's something small I'm missing here because these solutions aren't making sense to me.

[Andy Le Blanc]

The scheme looks like, the power side has no ground. The pre, the opt iron,
and the spk are on a floating chassis ground.
good thing about it, if you flip the old two prong plug over the hum will go away.

[Structo]

It seems strange that they went to the trouble of installing an isolation transformer but then used the chassis for the other half of the ac circuit.

It would pretty much take rewiring the whole amp to fix that.

You would need to take the heater circuit off of the chassis and wire the heaters separately.

The 60FX5 has a heater voltage of 60v, the 36AM3 is 36v.

I cannot find any data on the 18GO6A tube but it must have a heater voltage of 18v to total 114v.

So maybe another 1:1 isolation transformer is the answer and wire the heaters discretely away from the other voltage.

[bradb]

I don't think they did.

They have a step down for the speaker and a high tension for the tubes.

There's no isolation transformer.

What i'm curious about is if i throw an isolation transformer right where power comes in, and i leave everything wired the same, can i still earth-ground the chassis?

I think the answer is yes, the isolation transformer will allow me to ground the secondary side with no ill effects.