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OTL update - tube power supplies Done!

Posted 4th November 2012 at 03:14 PM by wlowes

Over the past few weeks I continued to build out and test the power supplies. I really am pleased with the decision to make each supply as a module that can be fully developed and tested outside the box.

After the success with the first supply for small tubes, I moved on to the power tubes. After abandoning the large Sangamo can caps, I ordered in 8 2200uf snap in caps.. 2 per rail, 4 per monoblock. After they arrived I realized that these things are pretty big. The question was how to securely mount them. I settled on another wood block so they can be mounted securely under the chokes. As before, I mounted them through the wood using some silicon cement. There are some nice mills wire wound bleeder resistors. The B+ power supply design calls for a single center tapped 230v transformer. I had scored 2 pairs of 120v 500va Toroidal transformers. I must admit it took a lot of research and a couple of helpful posts before I was sure that I really had this figured out and properly grounded. Again that is why this build is helping me learn. So I ultimately settled on a dual power supply with 2 transformers feeding 2 full bridge rectifiers followed by 4700uf Nippon Chemi-con large cans, a 50mh 2A choke and then the pairs of 2200uf caps. I mounted the bridge rectifiers on a nice heat sink that I picked up my local surplus shop. I doubt I need this much, but with 1000va of transformer feeding 9kuf caps there is the potential for some serious current. On that point I designed a simple soft start circuit using a 12r 25W resistor in series with the transformer. After a few seconds I can flip a switch to short out the resistor. It should be a simple but effective way to avoid blowing fuses and relieve stress on the tubes and power supply on start.

I wired it all up on the bench with alligator clips. I was delighted and relieved to see rock solid +170 and -170 on the rails. When I first fired it up, I used much smaller (50va) transformers fed with an isolation transformer. I figure if I get something wrong I don't want 10A on tap. After the low current test run I fired up the big toroids. Wow, very nice. They are almost dead quite. Even without the slow start in place they snap to life without blowing a breaker and no sound of stress. +170v came up in just a couple of seconds. I used PSU II Designer to model the circuit at 6mV ripple. It should be very quiet. When I turn it off it takes fully 5 mins for the bleeders to take the voltages down to safe voltages. Bleeders get hot to the touch, but no issue. Transformers and caps and rectifiers stayed dead cold.


Next up the bias circuit. It is designed to deliver -70 and a variable voltage centered on -240v. Again I used another wood block with the caps mounted through the wood and secured with silicon. This supply is important. I added a 10H choke and another cap stage in the filter over the design. As with the other supplies I modeled it to have negligible ripple voltage (less than 1 mV). I am using an under the chassis choke. I decided to mount it directly to the wood so the power module can be removed and replaced as a unit. I am using a 10 turn pot for the voltage control. After wiring it all up and triple checking my work I fired it up. Perfect -70v. The variable power side came on at about -235v. I was pleasantly surprised to see how precise the pot was at setting the bias voltage. I literally could dial in exactly -240v with decimal point precision and it stayed rock solid. Again the bleeders take a while to drain it back to 0v, so itís clear that once the amp is wired up the rule will be to give it a solid 10-15 minutes before touching anything.

Again I am really happy with the decision to build modular supplies. With almost 600v on some rails and 10A on the B+ I really did not want to be reaching around in cramped quarters to debug a power rail. It also allowed me to constantly adjust the placement of items on the top plate before committing Front Panel Express to mill the plate. I also find myself getting really comfortable with the amp even though it is still just a concept. I think the final assembly will be pretty straight forward (fingers crossed).
On a different note the layout of the top plate has changed many times. I am finally happy with the look. A bit busy but not cramped. There will be 1 toroid, 2 EI and 3 chokes on top. Under the hood will be 3 toroids, another choke and a couple of EI for filaments. I probably should weigh the collection of parts. It will not be an easy lift!

Next step is to make one last double check that I have the right hole sizes for all the bolts and screws, then send the order off to mill the top.

Then time to debug the filament supplies. I picked up some magnet wire. I figure a few turns added to the filament toroids might just dial in the voltage I am seeking.

At any rate, I am really enjoying this project. A few pictures follow of my power supply build. The picture with the large collection of modules represents one monoblock. Obviously down the right side is the power tube supply with the big toroids and lots of big caps. On the upper left is the small tube supply producing the high voltages. On the lower left is the bias supply.

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