Making a series string Kay 703C safer

[Diverted]

Hi,

A friend has a small little 1960s Kay 703C, which is a series string. He's worried about getting shocked and asked if I could make it safer. I have never added an isolation transformer to an amp, but added one to a 1940s series string (AA5) radio years ago.
I'm looking at the schematic though, and it appears that at least the filaments are isolated?
I'm wondering what the experts here could tell me about what to do to make the amp safer. Thank you!

Ted

[sluckey]

I would do this...

[Diverted]

Oh, that's easy! Thanks Sluckey for the great help.

Ted

[Diverted]

Quick followup-should I remove C5, the line to ground cap? Thanks.

[sluckey]

Quick followup-should I remove C5, the line to ground cap? Thanks.

Why? It's no longer connected to line thanks to the added ISO PT..

If you want to make this even safer, move the power switch to the line side of the added ISO PT. Adding a fuse would be a good idea too.

[PRR]

Quick followup-should I remove C5, the line to ground cap? Thanks.

No! You want to bypass R7, the dunsel resistor inserted to limit shock in the original plan.

You can either do a TOTAL re-thinking (and perhaps lose the treasured "tone") or rig it eXactly as Sluckey shows. He remembers the shocks from these things. He won't steer you wrong.

[Diverted]

Thanks again for the help. I wired in the isolation transformer. Nice little amp.

I did not change anything else, and left R7 (filter cap negative to ground) in place. I can remove it if recommended and wire the electrolytics and everything else on that node hard to ground.

I appreciate the assistance!

[Diverted]

quick question on this old topic.

Referring to the isolation schematic above, it has the switch on the ISO secondary. Seems safer to put it on the primary and fuse it, like this. Any reason why I shouldn't be doing this? Thanks (fuse not shown in photo)

[sluckey]

Have you forgotten about this...

If you want to make this even safer, move the power switch to the line side of the added ISO PT. Adding a fuse would be a good idea too.

[Diverted]

Thanks. I must gave missed that post. My apologies, I’ll do it that way.

[Diverted]

Would this be correct? The filter caps in this amp (different one, not the Kay mentioned in most of this post) are elevated above ground and go to "common neutral," not chassis. THanks! I would put a fuse on the other leg of the isolation primary, but I didn't draw it in.
Note chassis ground just under the input transformer in the second photo.

[sluckey]

I would connect the "common return" to chassis. As for wiring the primary of the iso transformer, I would connect the power cord black wire to the tip of the fuse holder, side of the fuse holder to switch, other side of the switch to the iso primary, other side of the iso primary to the power cord white, and power cord green to chassis.

[Diverted]

Thanks.
Just a question on that common return, once again confused. Currently the common return is above ground through a 220K resistor and .047 cap. Wouldn't running common return straight to chassis take the cap and resistor out of the circuit?

[sluckey]

Yes, but since you are now using an iso PT there is no need to float the common return, and the cap/resistor are not needed. It's not mandatory to connect the common return to chassis, but I would.

[Diverted]

Thanks, understood. I was just worried that taking that 220K out of the circuit would affect the secondary winding of the input transformer since it's in series with it under the original schematic.

Thanks for the tip, I'll get rid of the floating ground then.

[sluckey]

Thanks, understood. I was just worried that taking that 220K out of the circuit would affect the secondary winding of the input transformer since it's in series with it under the original schematic.

Has nothing to do with the input transformer. That resistor and cap are simply connected from chassis to "common return". They could have just as easily been drawn beneath the pilot light. Same thing.

[Diverted]

Oh yes, I see that. Every once in a while I forget about nodes  . Thanks for your help on this.

[PRR]

Since the core amplifier was ON the wall-voltage, the grid transformer is the original player isolation. Total isolation is not always best; we want some reference to the ambient AC field in the room. But not a direct connection! Instead the 220K||0.05u is the "legal minimum conductance" which will reference the input jack to the wall and room field without illegally electrocuting the user.

If that were all, once the 120VAC isolation was in place, I'd cut all that jank out, jack to vol pot. BUT this is noted to be a "1:3" transformer. It has voltage gain!! It also has a terribly low input impedance which loads-down a modern g-pickup and makes the sound dark. And so cheap it must also thin-out any bass? But on a 'student amp', maybe that is a good thing? (Good for the housemates.)

Yah, what he is saying: isolate the 120VAC, install 3rd-pin bond to chassis, and then short-out the 220K||0.05u. While it doubles the original price of the amplifier ($39 AC-iso in a list $39.95 amp) it is a better solution, and by now the investment pays-off.

With that in place, add a path to bypass the 1:3 audio input transformer.