Don’t be held ransom by rogue trafo winders!
El Cheapo Power Supply
An overseas DIYer called me up an hour ago and enquired about the Simple 5687. He’s very keen to build it, and this is a GOOD thing. But I was shocked to hear that to get his power trans custom wound, it would cost him USD80! This is daylight robbery! Since I could get one here for less than USD20… Yeah, I know, I should wind the trans and send them everywhere but shipping charges for heavy trans aren’t cheap either. And what a mickey it would be if the shipping charges is more than the trans?!
But DIY Paradise is always here to help you. Share your pain and Agony Aunt (or Uncle) here will help you. Yeah right.
So I gave this some thought and figured out ways to avoid custom make those trans. Aha! Easy as ABC and cheap to boot!
OK, OK, I have modified the power supply circuitry a little, but what’s the big deal? Well go to your local electronics store and ask for 240V in – 240V out with center tapping power trans. In other words, an isolation transformer. For heaters, get those 3-0-3V trans. Am I speaking Greek to you? Get a piece of cardboard and write this down. Show this side to the electronic store.
Primary 240V. Secondary 120-0-120V. At least 100mA.
Primary 240V. Secondary 3-0-3V. At least 3A for 5687 and 2A for 6X4.
On the reverse side, write “Go to hell you stupid t@#!%$” but only show this side if the electronic store guy doesn’t understand you. Seriously, the first trans is meant for B+. For heaters, you can use one for 5687 and another for 6X4 or use one for both.
But why are these trans cheaper? They are mass manufactured. To custom make a one-off is definitely an expensive exercise as the power trafo winder is not well rewarded for his effort. You could help him and yourself if you pool a couple of orders but if among your circle of friends, you are the only cool dude, then my suggestion is, as always, a better suggestion.
The 1.5uF is meant to “bump” the B+ a little as without this critical first cap, the B+ is around 190V. Nah, too low. So in essence, it’s no longer a choke input but it’s neither a cap input. Some folks call it “pseudo-choke input”. Semantics aside, I assure you, it works just as well. The 1.5uF is a simulated value. Try 1uF, 1.5uF and 2uF (or anything between 1 and 2uF. parallel the caps if neccessary) in this order to see which cap will give you around 260V B+. But slightly higher or lower shouldn’t matter. Your tubes will love you just the same. But don’t go more than 4uF! The 6X4 won’t be pleased. Neither would I.
Err, voltage rating of this cap is important! Go for at least 600VDC. More is fine. No need for boutique part here and no triode/audio police is going to arrest you if you use 20 cents mylar here. I like no-name polypropylene but use whatever’s available.
Of course, this is just an example. Use the excellent Power Supply Designer above and simulate your power supply circuitry. So next time you see an amp you intend to build, don’t let that power trans trouble you. With a little creativity, we can save big moolah. Also, a preamp/amp with lots of iron looks sooooo cooooool.