Brainstorming a push-pull 6L6GC stereo amp

My thought was to install an additional octal socket and use the 12v flavor of this guy: http://www.parts-express.com/pedocs/specs/kest-kir2p-series-specifications.pdf

To use this, it looks to me like I'd attach the 12v control feed to pins 2 and 7, the power transformer primary to pins 8 and 1, and the mains hot and neutral to pins 3 and 6, respectively. I would probably also wire the front panel power switch in parallel to the relay so that I could turn it on without using the control loop.

Has anyone here ever added a power control loop like this to one of their projects? Any gotchas to watch out for?

I've never done this, but I did spec out a delayed B+ circuit for my last amp that would not apply B+ for 30 seconds if the power got cut. (It would however apply filament and bias voltage immediately). The whole idea with that circuit was to build in a fail-safe mechanism for the amp because the amp was going into an environment with toddlers running around who like to flip, pull, push, and turn any switches they can find.

I never actually built that circuit because I ran out of room on the chassis, but I'm pretty sure it would have worked. So I was switching the power transformer secondary, whereas in your situation, it looks like you want to switch the primary. I decided to switch the secondary so it didn't require a separate 12V source to run the relay
 
Oh, cutting an already fairly short screw is such a pain in the a** :) Actually, I think I might have a set of wire strippers with a screw cutter on them, come to think of it. But I need some shorter screws for the tube sockets and tag strips anyway.
 
I've never done this, but I did spec out a delayed B+ circuit for my last amp that would not apply B+ for 30 seconds if the power got cut. (It would however apply filament and bias voltage immediately). The whole idea with that circuit was to build in a fail-safe mechanism for the amp because the amp was going into an environment with toddlers running around who like to flip, pull, push, and turn any switches they can find.

I never actually built that circuit because I ran out of room on the chassis, but I'm pretty sure it would have worked. So I was switching the power transformer secondary, whereas in your situation, it looks like you want to switch the primary. I decided to switch the secondary so it didn't require a separate 12V source to run the relay

I've been thinking of a delayed B+ circuit, too, but I'm likely to not bother. In my case with the remote power-on circuit, something else is supplying the 12V. Here's the snippet of the schematic that shows how I'm planning to wire the power switch / relay.
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Ignoring how ridiculously unsafe this photo looks, the voltage quadrupler is working perfectly. I had conservatively estimated 417V.
 

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Are you loading down the power supply to simulate the power amp current draw?

Yah, no, that's a fair point. This is unloaded. But it's still a bit better than I was expecting. The power transformer has a very low secondary DCR (I'll measure again this evening, but as I recall it was around 6 ohms) and is capable of delivering a continuous 3A @ 67VAC[*], so I'm expecting it to have very good regulation. At full power, I'm expecting to pull a bit shy of 2A.

I don't have beefy enough power resistors to put the full idle load onto the transformer (I have a 13K 10W resistor, but 425V across 13K is ~14W), so I'll have to stack up a couple of 10K 10W resistors instead. That's about 20mA, which isn't really even close (about 1/10 of the idle draw of just the output stage), but it should give me some sort of idea, I suppose. I'll do that tonight.

([*] I'm using the 100V primary for Japanese mains in order to get 78VAC out of the secondary.)
 
That's about a 238 ohm effective power supply impedance.

420v into 20K is about 21ma.

A 5v drop over 21ma, would indicate 238 ohms (5/0.021).

That means it would drop about 47v, if you loaded the power supply with 200ma of load. That's a lot of sag, unless you're trying to make a "high-sustain", crunchy, guitar amp.

How big of caps are you using in the quadrupler? If they're not big enough, and don't have low enough DCR, to store enough charge per cycle, you will get lots of sag. I'd try doubling the cap values, and see if it sags less. If so, you have a good indication you need a lot more cap...

Regards,
Gordon.
 
Caps in the quadrupler are 820uF / 800uF. The 820uF 200V Nichicons have a measured ESR of 0.04 and the 800uF 385V JJs have an ESR of 0.17.
 
Ok, I just went ahead and ordered a 2K 100W power resistor from Mouser. That'll let me put a 212mA load on the power supply. Might as well eliminate the guesswork!
 
Spent some more time thinking about it last night, and whipped up an alternative power supply using a 320VAC toroidal PT. Didn't require any changes beyond the PT, rectifier, and reservoir cap. And it let's me eliminate the separate filament PT. The thought exercise of using the MusicMan PT was certainly worth while, but I am willing to use another option if that is determined to be necessary.
 
Reworked the chassis layout for an alternative power supply scheme using a toroidal PT. The layout works out a lot better, too. This may be the option I end up going with. It gives me some headroom on the 6.3V supply to also add a DC fan.

I listed my usual Vishay bridge rectifier on the schematic, but I can easily adapt the 10A07s I have soldered onto tag strips for the quadrupler into a bridge configuration.
 

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I've had good luck with toroid power transformers generally. I have read that since they have such high bandwidth they tend to let a lot of the mains noise through, but I have never noticed it. They do get hot--hotter than an equivalent EI type with same ratings. Also you might need an inrush current limiter (thermistor) on the primary, because the toroid can pull a lot of current building up its magnetic field on turn-on.
 
I've had good luck with toroid power transformers generally. I have read that since they have such high bandwidth they tend to let a lot of the mains noise through, but I have never noticed it. They do get hot--hotter than an equivalent EI type with same ratings. Also you might need an inrush current limiter (thermistor) on the primary, because the toroid can pull a lot of current building up its magnetic field on turn-on.

If I have a problem with noise, that should be as simple as some safety caps to shunt that off to ground, yah? I've never bothered with these before, so not sure what values are generally used. Yah, inrush limiter... I may end up adding one of those after all. We'll see if I have trouble with it. A buddy of mine uses toriodal PTs in all the guitar amps he builds, and he just finished monster quad-KT120 high-gain build ... I'll ask him which transformer he used and if he needed an inrush limiter on that amp (he also uses these AnTeks).
 
I've had good luck with toroid power transformers generally. I have read that since they have such high bandwidth they tend to let a lot of the mains noise through, but I have never noticed it. They do get hot--hotter than an equivalent EI type with same ratings. Also you might need an inrush current limiter (thermistor) on the primary, because the toroid can pull a lot of current building up its magnetic field on turn-on.
I understand they don't tolerate any DC polarization at the input side.
 
I'm planning to put a 12VDC computer fan in the cage to provide some active air circulation. Here's the snippet of the schematic that shows the 6VDC supply for it (tapped from the heater supply) ... it's a quiet fan (as far as computer fans go) even at 12VDC, but is barely audible at the slower speed that 6VDC affords. The fan draws 350mA @ 12VDC. I plan to have it blow gently downwards on the power transformer, mounted to the cage directly above.

I'd prefer to use a nice looking 3.5mm phone jack rather than poking the mini-molex style connector the fan currently has up through a grommet... but I'll be darned if insulated chassis-mount DC power jacks or phone jacks are hard to find... but considering that the heater supply is hovering at +35V, I absolutely need to float the fan supply. Perhaps it doesn't really matter how it looks, though... it's going to be tucked back in a cabinet and no one will ever see.

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