22-09-2018, 07:50 AM
Thanks Trevor. I haven't had a chance to listen to them properly to be honest, even as MK1. I did run them briefly with my workshop speakers and they did sound cracking but I've yet to sit in a comfy chair, bung on an LP and crank em up. TBH I lose interest once I build amplifiers, it's more fun mostly building them especially when things don't work properly, gets the old noggin fizzing.
Simon, the small tfmr powers a 50v PSU and RC timer to SW in the big relay which SW's out two big resistors, one on each half of a 180v secondary (to give us 500v unoladed, 450v loaded) which sit under the relay, after 10 seconds; a soft start circuit in other words. The 12v winding powers a +/- 15v PSU which runs the VU meter driver, clipping indicator and fault circuit. There are 10 ohm sense R's on each EL34 cathode to ground , if a EL34 draws more than 100mA , a thyristor latches switching in a relay which cuts the neutral mains to the primary of the big toroid tfmr. So I don't think that's the issue, but will bear it in mind.
This oscillation on SW off problem started after I changed the front end from a bog standard triode gain stage to one with local NFB (which improved the frequency response no end), like this one here - http://www.valvewizard.co.uk/localfeedback.html This is AC coupled to a LTP. I'll draw the circuit later and post.
Cheers, Andy.
Simon, the small tfmr powers a 50v PSU and RC timer to SW in the big relay which SW's out two big resistors, one on each half of a 180v secondary (to give us 500v unoladed, 450v loaded) which sit under the relay, after 10 seconds; a soft start circuit in other words. The 12v winding powers a +/- 15v PSU which runs the VU meter driver, clipping indicator and fault circuit. There are 10 ohm sense R's on each EL34 cathode to ground , if a EL34 draws more than 100mA , a thyristor latches switching in a relay which cuts the neutral mains to the primary of the big toroid tfmr. So I don't think that's the issue, but will bear it in mind.
This oscillation on SW off problem started after I changed the front end from a bog standard triode gain stage to one with local NFB (which improved the frequency response no end), like this one here - http://www.valvewizard.co.uk/localfeedback.html This is AC coupled to a LTP. I'll draw the circuit later and post.
Cheers, Andy.
