Marshall JCM 800 Lead series 2205, help please!

[Chuck H]

Thanks. When I said "Isn't a volt a volt the diode world over?" I didn't mean that I thought all diodes drop one volt. I know there are very different specs for different diodes. I meant that since in this application the diodes primary function is to set a DC bias point why should one diode be any better than another as far as noise or non linearity. FWIW The circuits I've drawn up all use a bypass cap for reasons mentioned. Though I wonder how significant noise and non linearity are in this kind of circuit I figured that a bypass cap would avoid problems by removing AC from the circuit.

[jmaf]

Thanks. When I said "Isn't a volt a volt the diode world over?" I didn't mean that I thought all diodes drop one volt. I know there are very different specs for different diodes. I meant that since in this application the diodes primary function is to set a DC bias point why should one diode be any better than another as far as noise or non linearity. FWIW The circuits I've drawn up all use a bypass cap for reasons mentioned. Though I wonder how significant noise and non linearity are in this kind of circuit I figured that a bypass cap would avoid problems by removing AC from the circuit.

Hi Chuck, sorry for the trivial explanation above, having english as a second language I missed your analogy.

I don't have a technical explanation as to why the power diodes are noisier and non linear at that region, it's something that someone like RG could probably explain in minor detail. I think it has something to do with the inter-NP-region spacing and amount of doping necessary to get it to block high reverse voltages and allow great current through in forward bias. When you apply something more delicate, like high impedance signals, you don't escape that initial region for some reason, and the transfer from N to P doesn't happen linearly, signal gets "caught up in the silicon" if you will, I don't know what the technical term is. Much like the power mosfets are nothing delicate(except for their inherent ESD vulnerability), some require several volts to turn on and when they do, the channel current and voltage better be high.

In that cathode I'd expect to see a germanium diode, or a small signal bjt wired up as a diode, etc, something cleaner and more delicate. Adding a 1n4007 there really puzzles me.