Kay 503A questions...

[52 Bill]

I have this same amp and it obviously needs new tubes and caps. Although I've never rebuilt an amp, I dealt with electronics in the Navy for 10 years. Would this be a suitable first project amp?

Thanks for the guidance,
Jay

Welcome Jay!
This amp is simple enough that it would make a reasonable first project amp, as long as you understand the dangers of the design.

The filaments are series wired and are then connected directly to the 120 ac wall outlet. This design makes this amp a little more dangerous to service than one with a full power transformer, but if proper care is taken, you should be reasonably safe.

I would suggest that you start a new thread for your amp, as this one already has two amps covered in it.

[otto pärt]

I have a Kay 803, which is very similar to the 503. I put a 12aw6 tube in place of the 12au6, and that gave it a wee bit more oomph. FWIW. (Pinouts on the tubes are the same, except pins 2 and 7 are switched, but those are tied together anyway.)

[jkallday]

kay 503

Kay 503, 503a and early 703 (with the 50l6 tube) are nearly identical and all have isolation transformers. The important thing is that the chassis, including the input jack ring, is isolated. If you add a grounded plug then these are 100% safe from electrocution hazard. The ground connection also quiets the hum a lot.

The later 703C (with the weird tubes) is pseudo isolated; they isolate the input stage with a small isolation transformer (its cheaper) and connect the ground to the output stage thru an "rf ground" which is a capacitor and high value resistor in parallel. So if you plug the non polarized plug in the wrong way you will get a shock but the current is limited to a few mA thru the "rf ground". Add a polarized plug with the ground side of the circuit connected to the white, neutral wire and you minimize the risk. Better yet, use the cord from a scrap hair drier that has a built in GFCI. Of course best way is to add full isolation transformer. My 703C sounds great, more punch, volume and highs than the 503. Maybe its the speaker.

I have done three of these and they all needed new electrolytics. (None of my vintage Silvertones have!). Check the resistors, especially the 1 watt one that goes between the big caps and the plate load resistor. Might as well replace the coupling caps while your in there. Also, on the 503, swap out the 22K ohm resistor that goes to ground in the input combining network with a larger one, like the 220k in the 703. It will up the output.