Kay 703 grouding scheme and crazy squealing

[choosebronze]

I had most of the parts to build a Kay 703, including an old 6" combo amp willing to donate its cabinet and speaker. So I figured I'd build one for my daughter to have a little practice amp. And boy has it been a learning experience...

The original schematic shows T1 but I gather from prior posts that T1 wouldn't have been a true isolation transformer. I replaced that with a Triad N68X in my build. Other than that, adding a fuse, and deleting two of the input jacks, I built the amp just like the schematic. I read another post where Sluckey recommended adding an isolation tranny to a widowmaker and he showed what I would've basically expected, just dropping an isolation tranny between the hot and neutral wires with no other circuit changes.

But I'm getting stuck. I've built a bunch of amps and I feel like I understand that power scheme well, but the isolation transformer is really throwing me for a loop. The N68X has two secondary wires and a shield wire. Shield goes to the chassis/safety ground. The other secondary wires confuse me more. I originally had them wired the way I'd wire any other PT (they were wired as a series heater string and rectifier pin 5), but that gave me very weird voltages and no audio. So instead I wired a secondary wire to pin 5 of the rectifier. The other secondary wire sent to chassis ground, and redid the heater wires, just like it's drawn on the schematic. I don't.... exactly understand. It seems like that would've electrified the chassis? But in any event, that worked and the amp now plays. So something about that must be right even if I don't understand what it is.

Now I do have guitar audio, but I also have an incredibly loud squeal. Not a little hum. Not a "noisy" amp. Full on screeching that changes with the volume and tone controls. Happy to upload audio if you need a laugh. I went to ask AI about that (I know, sorry sorry) and it confused me even more!

It said that I don't want any part of my circuit touching the chassis anywhere - true "isolation," which I guess makes sense. That all "grounds" - my PT secondary return, input jack, all ground points in the circuit, everything, should go to one floating "ground" point that never touches the chassis. And the only things that should go to the chassis are the earth safety ground and the PT shield wire.

I'm now very confused, and very much not wanting to accidentally shock myself. Like I said, I'm comfortable working on a standard amp but I feel like I have to quadruple check everything with thxis thing because the electricity isn't necessarily where I expect it to be.

I am incredibly grateful for any help or advice you guys have.

[choosebronze]

Okay I reread Rob Robinette's article on widowmakers for like the 4th time. And I read a couple more articles about converting antique radios to isolation transformers. I'm actually thinking I wired the PT the correct way.

I did some chopsticking and moving the wires around definitely changes the squealing, and turning the pots does too, big time. So I think a wiring problem is more likely. But anyway, here's what this craziness sounds like: https://limewire.com/d/oPfwo#T0VhQu8Isf

[AlNewman]

I don't think you want your heater transformer to connect with your B+ at all.  I do think you need a ground reference for your heaters. 
Do you have a schematic as you have it wired currently?

[sluckey]

The attached file shows how to connect the Triad isolation transformer to provide complete isolation. Notice the two primary windings are connected in parallel.

There is no need to change any wiring on the original schematic.

[choosebronze]

Ooh, okay. Thank you Steve, that makes sense. So then 2 questions.

1. If I'm building the stock circuit with the Triad in front of it, do you have a suggestion for what I should use as T1? Thus far I'd been using the Triad in place of T1, not in front of it.

2. When built as you've shown, I assume the chassis remains my ground, and all of the floating ground stuff AI mentioned is bunk.

[sluckey]

That original amp is mostly safe to play just as is. Just add a 3-prong power cord with the green connected to ground. The only issue is the filament circuit is still connected to AC line. It's really only an issue if the filament circuit becomes shorted to chassis, but that's not dangerous to the player. Just might trip the house breaker. So, it would be a good idea to isolate the filament circuit from the AC line. You could do it as shown in my schematic. Or you could just use the second iso tranny to only power the filaments. Connect both red wires to the series filament string and also connect one of the red wires to chassis. Now you have everything safely isolated from AC line and the filament circuit is referenced to chassis ground which may help with any filament hum.

Still requires two iso trannies. I don't know what AI told you, but there is no reason to float all your grounds.

Your sound clip indicates a problem in the amp. Most likely a wiring error. We need to see a schematic of your actual circuit. Hi-rez pics of the actual amp may be useful too.