Proton BASIC Community

Ken and All,
I have dabbled with LT Spice but being an old dog I prefer to stick with what I have become accustomed to over the decades.
Re the feedback - I would consider that as plate to grid - effectively local feedback around the output stage but it will load the driver down because it lowers the output device input impedance.  From what I can make out it only amounts to about 6dB feedback anyway so quite modest and equivalent to about 33k from the output transformer back to the 1st stage cathode.
I didn't have 6V6 valves in my library so used a 6L6GC but had to fiddle the bias.   I see there's quite a bit of low frequency roll off due to the coupling capacitor values used. 
Hands up who still has valve base chassis punches?

Cheers,
David

Quote from: david on Jun 29, 2023, 01:06 PMKen and All,
I have dabbled with LT Spice but being an old dog I prefer to stick with what I have become accustomed to over the decades.
Re the feedback - I would consider that as plate to grid - effectively local feedback around the output stage but it will load the driver down because it lowers the output device input impedance.  From what I can make out it only amounts to about 6dB feedback anyway so quite modest and equivalent to about 33k from the output transformer back to the 1st stage cathode.
I didn't have 6V6 valves in my library so used a 6L6GC but had to fiddle the bias.  I see there's quite a bit of low frequency roll off due to the coupling capacitor values used. 
Hands up who still has valve base chassis punches?

Cheers,
David

You are correct re plate to grid, plate to plate saves a capacitor that would be required if the plate was connected straight back to grid. The Schade feedback is normally used when the output transformer is not all that good, lowering the Z at the anode of the output tube helps a less then perfect transformer. The coupling capacitors would normally be a bit larger depending on application and the output transformer in use.
Another feedback method used with less than perfect transformers (single ended class A) is plate to the cathode of the input valve.
Regarding chassis punches I have not used them since I started using Step Drill Bits, they centre perfectly from a small pilot hole. I normally have a block of wood under the work that I have drilled to the required depth with the step drill, a smaller diameter hole is drilled though the wooden block to let the swarf out.  Much easier and less dangerous than conventional drills, if you go slow the next step will deburr the hole.
All the round holes in the 807A amp were drilled using a battery powered drill. My drill press is on the unheated garage.

https://www.youtube.com/watch?v=rX163qRsl84

BTW this is how I fold the steel sheet.
https://redfoxmachinery.com.au/product/sheet-metal-folder-metex-300mm-vice-held-sheetmetal-bender-press-pan-brake/
https://www.youtube.com/watch?v=SPobz3zjpsk

Regards
Ken

G'day all,
Re the hole punches, I used Dad's home workshop and he had an adjustable cutter. It consisted of a central rod that maintained the centre position through a pre-drilled 1/4 inch hole and an L shaped arm with a carbide tipped cutter that was adjustable in radius and held by a wing nut. Usually I made a slightly smaller hole in the aluminium sheet for larger valve sockets and filed to fit. I probably still have that tool as I inherited many of his tools. Like Les I owe much of my interest in making things from Dad.

Aluminium was bought from a place on Port Rd just down from the West End Brewery, Ken. I doubt it's still there. Even the brewery has gone - sacrilege!

I'd scribe the bends for the chassis then using lengths of L shaped steel to extend the jaws of Dad's vice a piece of wood enabled me to bend up the chassis by hand with the scribe enabling a clean bend. The last two bends were stinkers to find suitable steel pieces for the vice.

By the way Les I had that AM transmitter grey hammertoned. It looked pretty good.

I've still got the crystal mike in a cupboard. Can't bring myself to throw it out.

Ah, reminiscences. How come I forget where I put that screwdriver 5 minutes ago but remember all that from when I was 18 in 1958?
Best wishes to all,
Joh
VK5DJ