High voltage power supply headscratcher

[mlabbee]

Her's what I have sitting on my breadboard:

9 VAC 830 milliamp wall wart
Amveco toroidal transformer - 120, 120, 12, 12:  secondaries in parallel connected to the wall wart, primaries in series connected to a full wave rectifier

Supply filter - 30 uF 250 cap, 2k2 ohm resistor, 30 uf 250 v cap.

Dummy load 30k resistor.

I plug everything in and I get 150 volts across the 30k resistor.  According to my calculations, I should be getting closer to 254 volts.

Upping the dummy load to 220k takes me up to 187 volts.

This is a power supply for a tube circuit.  The same setup in the circuit gets me up to about 110 volts.

Any htoughts on what I may be doing wrong or how to get my voltage up?

[puretube]

12V wallwart!!!

[Aharon]

9V into a 12V will never give you 120V,besides 120*1.41 is 169.20V.
Aharon

[mlabbee]

9VAC on the secondary should give 90 volts on the primary, times 1.41 after rectification should give me 127 volts.  Since there are two primaries, hoodking them up in series should give me 254 volts, minus losses in the rectifier - target is around 240.   The fact that I'm getting 160 on the dummy load suggests that I have the things hooked up right (as going with a single primary or hooking it up in parallel should give me no more than 127 volts).

I think my head is going to explode.  :-)

[puretube]

rather watch out, not to explode the caps...

other people have done the headscratching before...

[Aharon]

Voltage doubler instead?
Aharon

[mlabbee]

Aharon - I've thought about it, but I'd really like to figure this one out.  Results that don't compute usually signify a problem with the design that could cause other problems.  I may just use an EH toroidal and drop the voltage with Zeners.

Puretube - trust me - I've searched and searched but can't find an answer yet - I'll keep looking.  I hope your not p***** at me - I really appreciate your thougts on all this - you've given me a great deal of help on my tube projects so far.

[Aharon]

When I built RGs Vintage Voltage Conditioner I had to reverse the leads to get the voltage up,with the phase the other way the voltage went down.Reverse the leads?
Aharon

[mlabbee]

Jst tried it - I reversed the leads of the primary into the rectifier.  Voltge went up by about 8 volts, but that's it.  Aaargh!!!!

[puretube]

mike: not at all p***... by you, at all.
Quite contrary:

I`ll give some hints tomorrow (or you search yourself for "toroid/torroid/
EH/E-H/transformer meanwhile).

Now I`m just out of time...

ps:
I hope I don`t have to bother you with requesting the help of your generous offer!

hats off, Ton.

[puretube]

...

use a 12V wallwart, which too will provide heater supply (12V) easily;
or use a lower voltaged step-up xfmr low-volt winding;
(like explained earlier, I stopped using "primary & secondary"
in these b-t-b configurations)
you have to live with difficult to calculate AC-losses.

maybe this link (+ sub-links) can help you further:
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=28003&highlight=tube+supply
R.G. has posted on the topic, too:
http://www.diystompboxes.com/sboxforum/viewtopic.php?t=29134&highlight=toroidal

[mlabbee]

I see what's happening.  I guess when you run two windings in series you can get a pretty significant loss (one of RGs posts said he was getting a 45% loss).  That would explain what I'm seeing.  Bummer  

[R.G.]

Yeah, you're losing it in the open circuit voltage ratios and the IR losses in the windings. Little transformers have big losses. They're usually around 30% or more from no load to full load.

Full wave rectifying makes it worse because the pulse current drawn through the winding resistances can easily be ten times the average DC currents, and the bigger the capacitor, the worse that gets.

Just a detail you have to watch. You can measure the winding DC resistances and open circuit voltage ratios and compute what you'll get pretty closely. It's depressing at first, but it's just another design variable to be coped with.

[mlabbee]

Indeed.  I wanted to use the Amvecos because I need around 250 V and no more (I'm using 6021s and 250 is more than they are rated for).  I was afraid the EH toroidal would pump in too much voltage, but it looks like they might be the trick.  I can always dump voltage with zeners if necessary . . .

Damn it!  Why can't the real world obey the math??!?!  

On the bright side, I can still use the Amvecos - I built a little two stage preamp with a Baxandall tone stack that works fine between 100 and 150 V, so I'll be able to package that one up in  a smaller box.

[Aharon]

FWIW I used 2 back to back 12V transformers+voltage doubler for my HotBox and it worked fine if you rather go that way.
Very quiet PS.
Aharon