400v Switch Mode PSU

[defaced]

I've been trying to find a suitable power supply for some three triode high gain preamps I want to build. There don't seem to be any readily available transformers that can get that sort of voltage without being overkill in size/price, so switchers some to mind. I ran across this circuit and was wondering with a few tweaks, would it be suitable for this sort of application?

I'm guessing the following needs to be changed to get a happy 350v to 400v:
UF4004 to a UF4005 (or 06)
IRF740 to maybe an IRF820 (or higher voltage)
adjust the voltage divider feedback as required.

http://www.ledsales.com.au/kits/nixie_supply.pdf

[Enzo]

Use a lower voltage secondary and feed it to a voltage doubler, rather than looking for a low power transformer that will make 400v directly. Your switcher won't run the heaters.

[Steve Conner]

If I understand right, it's a boost converter, so you can't have a voltage doubler. Increasing the voltage rating of the components as suggested will work. The efficiency of all these boost converter circuits will be pretty poor, and the higher the step-up ratio the worse it gets. But if you only want to power three preamp triodes, it hardly matters.

One problem with switchmode power supplies is that they tend to produce audible noise. I'd be especially suspicious of the above one, since it uses a 555 instead of a "proper" switchmode controller. Watch out for the MC34063 though: IME it's almost impossible to get it running without hash in the audible region.

Here's another one, designed by a fellow Tesla coiler Nixie HV Switching PSU

And another interesting one that can run heaters too: Power Supply for small Tube Amplifiers

For some overkill, a 600 volt, 1.7 amp one http://scopeboy.com/xantrex-xkw-600-17/

Finally, surely you know the back-to-back transformer trick. A 120:6 hooked up to a 6:240.

[Enzo]

The voltage doubler idea was aimed at a conventional linear supply, not a switcher. Since his original concern was finding a transformer of suitable rating within his budget. SO right or wrong, I thought it might be easier to find a small, preamp size PT at a lower voltage and then double it up.

[Steve Conner]

Sorry Enzo, I misunderstood. You're quite right, you can take a PT of a lower voltage and slap a doubler onto it.

Also, I think Weber or Hammond or someone makes a little transformer just for tube preamps. It's a squashed looking thing that fits in a 1U rack.

[defaced]

I hadn't thought about rack transformers. I'll take another look and see what I can find. I'd like to avoid back to back transformers if possible because of size and cost. Switch mode PSUs fascinate me so I'm kinda looking for an excuse to learn about them.

But this makes me wonder, could I take a 120:6.3 transformer that had dual secondaries and dual primaries, wire a single primary to the wall, parallel the secondaries, and use the remaining primary as the HV winding into a voltage multiplier? This would kinda be like the back to back idea, but it'd be two sets of windings on the same core.

Or maybe just get a single secondary but dual primaries. I know both of these ideas will screw up the VA rating of the transformer because it's not getting as much current in as it wants.

[Steve Conner]

Switch mode PSUs fascinate me so I'm kinda looking for an excuse to learn about them.

Who needs an excuse! You might want to check out my other favourite forum, 4hv.org. There are quite a few hobbyists there building and modding switchers. Some guy is working on a 2500 watt one for his ham radio gear at the moment.

In theory, you can do what you just suggested, use one primary as a secondary. But in practice, there isn't as much insulation between the two primaries as there is between the primaries and the secondaries. So by using one primary as a secondary, you don't have enough insulation there to meet electrical safety codes. Toroidal transformers are especially bad: they can have the two 120V primaries wound bifilar, which is to say no insulation between them except what's on the magnet wire.

If you're just building gear for yourself, it doesn't have to meet any code requirements, but it makes me a little uneasy anyway.

[defaced]

Thank you for the pointer to 4hv and the info about the insulation. I had never considered the insulation issue, but now that you mention it, that does make sense. Hmm, off to switcher land or find a transformer that does what I want it to.

[tedmich]

Switch mode power supply for car vacuum tube amplifier

a ~360vdc version an it needs a 12vdc SMPS supply to power it, which are dirt cheap and could also drive heaters

have you seen this one? Just a pre PS though...
http://diyaudioprojects.com/Tubes/12...sschematic.jpg

also check these guys, they seem to have something perfect:
SMPS for Tube Amplifiers

[defaced]

You rock. I had seen the DIY Audio Projects one, but I didn't know how it worked to know if I could get it up to 400v. Now that I've read the circuit description in that PDF in my first post, I see what I could tweak to make it a good solution. This is going to be a fun little trip.

[Gregg]

Watch out for the MC34063 though: IME it's almost impossible to get it running without hash in the audible region.

+1. Forget about that chip.

I've played with SMPS a lot and I can tell you that not every design is suitable for tubes. Some of those produce very unpleasant artifacts. Also you should bear in mind that SMSP can make the sound "hard" unless you take care to emulate the behaviour of a regular tube PSU which will sag to a certain degree.
Use your ears and make A/B tests with a regular PSU.
Personally I was interested only in running couple of preamp tubes for my pedals. It's possible to build a SMPS that will power up the whole amp but a well designed SMSP (and that's definately not for everyone) that will provide the necessary voltage and current may cost you more in terms of time and money spent.

[tedmich]

this one is cool:
Silicon Chip Online - Regulated High-Voltage Supply For Valve Amplifiers

they mod an old ATX supply, which are common as cockroaches!

these tiny Vicor front ends take universal AC (85 – 264Vac / 47 – 440Hz) and turn it into >300VDC at 250w unregulated with ~300mV ripple, (scary!)
http://cdn.vicorpower.com/documents/.../ds_vi-aim.pdf
2 for ~$40: http://cgi.ebay.com/VICOR-FRONT-END-...item2eab303a32
maybe voltage double the 120VAC into the Vicor, feed the resultant DC into Gary Pimm's version of the Swenson regulator
http://www.pimmlabs.com/web/regulators.htm
for a tiny light variable DC supply ~400v at 0.5A

add a 300->5v Vicor DCDC converter: http://cgi.ebay.com/VI-260-CW-VICOR-...item45f127aaaa

trim it to 6.3VDC and you have the heaters too!

[defaced]

Excellent. Thank's tedmich.

There's no way I'd want to run an entire amp on a SMPS. For about 50 bucks I can get an 400-0-400 400VA Antek torroid, which is a perfect price/performance ratio for me.

[Enzo]

Um... that would make about 560v B+ and 400 watt capability. Isn't that a trifle large for a three-tube preamp? Includes heaters?

Just a quick look...

Allied 227-0001 250v, would yield about 350vDC and it has an amp of 6v - good for three 12AX7. $16.

Or 227-0005 325-0-325 for about 450vDC, and 2A of 6v. $26. It even has a couple amps of 5v.

[J M Fahey]

Hi guys, thanks a lot for the brainstorm, ideas and links.
Although I wind my own power and output transformers, that makes them cheaper and customisable, but definitely not any lighter, which scratches me the wrong way.
You gave me food for thought, I'll experiment many of these.
I'll re-start making some tube stuff but wanted to stay away from the same old, tired, beaten-to-death paths.
It's irksome to wind tube preamp regular HV transformers, they use lots of turns of very fine wire, I feel like a pickup winder !!!
Russians (Yerasov) and Poles (Mr Hector) are using switchers within preamps and even pedals to make practical tube circuit supplies without ill effects.