17-08-2020, 07:36 AM
(This post was last modified: 17-08-2020, 07:54 AM by Diabolical Artificer.)
"Plenty of 2H? Its going to be dual purpose is it? ....Have you plugged in an axe yet?" Took me a while to twig there Amie, combined amplifier & wood chopping device, eh!!?? Then the penny dropped, axe = guitar, no, didn't think of that to be honest. 2H BTW is shorthand for 2nd harmonic just in case anyone was wondering wotIwozonnabout.
Thanks everyone for the kind words. Wired up the protection yesterday using resistor colour coded wires, see pics. The wires go from each OP valves cathode sense resistor, EG 10r, if cathode current rises over 140mA, IE 1.4v across the 10r, this triggers a thyristor, which grounds the ground of a relay coil , thus completing a circuit. This relay sits next to the main trmr, neutral comes in through the IEC socket, through mains SW, neutral goes through said relays AC contacts, when the relay is energised neutral is cut to the mains tfmr, thus cutting the power to the amp.
There are a few other bits of protection to add, namely reversed diodes from OP valves anode to ground to clamp voltage without a load, or when some numpty forgets to put a speaker or dummy load in, which I've obviously done in the furtherance of scientific research and not because I'm a dolly head. Actually the amp is fine without a load, as long as the volume isn't cranked up.
There will also be a "zobel" network across the OPT secondary, something like a 10r and 0.1u cap, this cuts HF gain at about 40khz and helps keeps the amp stable with all loads. To find the right values I've been testing the amp with a capacitor as the load, so far just a 0.1u, then running frequency sweeps looking for resonance ( EG peaks of OP) at extreme high and low frequencies. this is done at various OP levels, EG 0dB which is full whack, -3dB, -6dB etc, etc. This is time consuming and a bit boring but does show what your amp is up to whilst your back is turned. This is done with no NFB applied and then with NFB applied, so far the amp behaves well up to -3dB, at full whack things go pearshaped with massive instability under 100hz but less so at HF. So far all tests have been done form 20hz to 50khz but really should be from a tad above DC up to about 200khz (or more) so I've had to dig out a Marconi boat anchor which is about as big and heavy as a house. The LF is trickier, really need to get a half decent function generator.
Anyhoo, waffled on long enough, more later, Andy.
Andy.
Thanks everyone for the kind words. Wired up the protection yesterday using resistor colour coded wires, see pics. The wires go from each OP valves cathode sense resistor, EG 10r, if cathode current rises over 140mA, IE 1.4v across the 10r, this triggers a thyristor, which grounds the ground of a relay coil , thus completing a circuit. This relay sits next to the main trmr, neutral comes in through the IEC socket, through mains SW, neutral goes through said relays AC contacts, when the relay is energised neutral is cut to the mains tfmr, thus cutting the power to the amp.
There are a few other bits of protection to add, namely reversed diodes from OP valves anode to ground to clamp voltage without a load, or when some numpty forgets to put a speaker or dummy load in, which I've obviously done in the furtherance of scientific research and not because I'm a dolly head. Actually the amp is fine without a load, as long as the volume isn't cranked up.
There will also be a "zobel" network across the OPT secondary, something like a 10r and 0.1u cap, this cuts HF gain at about 40khz and helps keeps the amp stable with all loads. To find the right values I've been testing the amp with a capacitor as the load, so far just a 0.1u, then running frequency sweeps looking for resonance ( EG peaks of OP) at extreme high and low frequencies. this is done at various OP levels, EG 0dB which is full whack, -3dB, -6dB etc, etc. This is time consuming and a bit boring but does show what your amp is up to whilst your back is turned. This is done with no NFB applied and then with NFB applied, so far the amp behaves well up to -3dB, at full whack things go pearshaped with massive instability under 100hz but less so at HF. So far all tests have been done form 20hz to 50khz but really should be from a tad above DC up to about 200khz (or more) so I've had to dig out a Marconi boat anchor which is about as big and heavy as a house. The LF is trickier, really need to get a half decent function generator.
Anyhoo, waffled on long enough, more later, Andy.
Andy.
