Nick wrote: ↑Thu Dec 13, 2018 10:26 pm
Just FYI, I have used these in the past to disconnect HT. Specifically in the ref phono if someone unplugs the connection between the two boxes CE specifies how log a voltage can remain on the pins. I detect the removal and use these to isolate the caps in the power supply while they discharge. Just letting them discharge on their own would take too long.
Cheers Nick.
I think I've got a pair of big SSRs somewhere one of which could be pressed into service. Phil sent me them years ago but I never used them. I think they were left over from his 833A project.
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Cressy Snr wrote: ↑Thu Dec 13, 2018 9:51 pmThe 6336A regulator tube data sheet specifies a minimum cathode heating time of 30 seconds before HT should be applied.
The 13E1 and 6C33C have massive >30W cathodes just as does the 6336A, so one addition I want to make to the breadboard, is a separate switch to bring in the HT when the heaters have reached full operating temperature. All it needs is a switch in the AC side of the rectifier bridge; a small price for peace of mind.
Steve, I plan to use a couple of K&K Audio's time delay circuits on my build (I used them in the 300B SE-OTL, to switch on the relevant transformer secondaries, and they've worked perfectly). You can find them about three quarters of the way down this page;
One version of the circuit has the facility to slowly ramp up the B+ voltage too but I think that would have to be used after rectification/smoothing because of current limitations.
K&K do some useful bits and pieces like small power supplies, shunt regulators, current sinks, amongst other things and I've always found Kevin to be excellent to deal with. I thought about using their current sink (which is built around a 1000V 8A Mosfet) when I was building the 300Bs but stayed with the TL783s.
Thanks for that Ray. I'll order one of those for the proper in-a-box build next year and in the meantime, I'll fit a switch in the HT transformer secondary to the breadboard.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
The thermal delay relay I have won't switch an amp.
A simple switch in the AC side of the rectifier bridge is doing a sterling job at the moment. It's crude by modern standards but perfectly adequate for a breadboard situation. I just let the power tubes preheat for one minute, then throw the switch.
A soft, gentle thud from the speakers and away we go. No drama and more importantly, peace of mind with no cringing at switch-on.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
When you get to the 'boxing it up nicely' stage, another option for a two-stage start-up is to use one of the boards Nick put together a few years back;
I've used one before very successfully for just the scenario required for this project. If you don't have one I'm pretty sure I have a spare one around. Obviously there's a limit on the total current it has available (don't remember what it is but I know Nick documented it somewhere in the thread) but for my use it was fine for switching a couple of SSRs so it should be OK.
Steve, do you have a link or a product name for the epoxy you're using to bond the TL783s to the Aluminium? The heat-transfer epoxies I've found are all two part and require a temperature of around 100deg centigrade to cure. Cheers
Ray P wrote: ↑Thu Dec 20, 2018 10:58 pm
Steve, do you have a link or a product name for the epoxy you're using to bond the TL783s to the Aluminium? The heat-transfer epoxies I've found are all two part and require a temperature of around 100deg centigrade to cure. Cheers
Well finally I got the heatsink for the CCS chips, so the 'boxing' process can begin in the New Year.
I'm looking at a steampunk style this time around; all polished mahogany, brass and Fender jewel lamps.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
Steve, I've been thinking about the switch on delay for the B+. As prototype PCBs from China are so cheap nowadays I'm going to layout a board based on Nick's start-up board that I linked to a couple of posts back (have cleared it with Nick). I'm planing on using the control part of Nick's board to power three mechanical relays. On pressing the switch the power up starts with the first relay connecting mains power (so filaments can start warming up); at this stage the LED in the switch flashes. After a period of 30-60 secs the other two relays switch on, one for the B+ for the input stage and the other for the B+ for the output stage; the LED turns on fully.
I've ended up listening to the OTL breadboard for most of today. Loads of Christmas classical orchestral and choral music.
I can't listen to any of my other amps, which could be a slight problem when I have to dismantle the breadboard to box it up.
Sgt. Baker started talkin’ with a Bullhorn in his hand.