Back to Back transformers - which is the max "safe" mismatch ??

[kagliostro]

Often I see that as to have it on the cheap, many people use as PT a couple of 2 transformers back to back

so they can easy obtain a 220v HV (AC) for plates using two cheap 220v/12v transformers

many people connect it in 220v-12v / 12v-220v and obtain the wanted AC HV from usual lamp transformers

In more than a case I've also seen to be used 220v-15v / 12v/220v as to increase the HV

(220v-15v / 12v/220v) = 275.10v

in the opposite side I can decrease the HV, so

(220v-12v / 15v/220v) = 176,04v

the question is

as told 15v / 12v seems to be used more than one time, so seems safe

which is the max safe mismatch between the voltages ?

Thanks

Kagliostro

[RicharD]

That's a tough one to call.  It all depends on the insulation.  It's been discussed here that exceeding the insulation rating of plain ordinary wire is fine as long as it isn't pinched or in an excessively hot location.  Well... transformers are hot, and the more load you put on them, the hotter they get.  I think the key to success here really lies with not so much with the increase in voltage, but in not exceeding the VA rating.  Remember, as you increase voltage in a transformer, the available current decreases to keep the VA the same.  My best guess is that if the transformer is listed for 250V, you could probably go a little over 300V w/o much worry as long as you're not over-heating the transformer.

[kagliostro]

Thanks Richard

So the problem is in how much current we want from the windings

in the range of the VA of the transformer the situation is safe

assuming that the voltages are the same

using an over dimensioned VA rate transformer will give more safe condition

than the voltage limit is a condition depending "only" by the type of the paint used for the isolation

Kagliostro

[Leevi]

I just wonder how cheap this solution finally is? You'll need two PTs for B+ and one extra FT for filaments.
I suppose the transformers can not be the cheapest ones since the power amp and heaters need a certain current.
I think the money you win in the PT price will certainly be spent for the time you need for installation of this.
And you can never be sure about the result.
/Leevi

[kagliostro]

Yes Leevi you are right

only, sometime, I can have those lamp transformer for free or near

Kagliostro

[danhei]

I just wonder how cheap this solution finally is? You'll need two PTs for B+ and one extra FT for filaments.

You don't ordinarily need an extra filament transformer. Your first transformer is a 120/240:12V, so you just run the filaments off the 12V. Make sure the 12V has enough current for the filaments and the second B+ transformer and you're good to go.

[kagliostro]

Yes also this can be done

but here we are going in a difficult territory (or so, for me)

If I've two 220v-12v 60VA transformers

(so I've 5A in the 12v winding and 272mA in the 220v winding)

and I put it 220v-12v / 12v-220v I can have both 220v and 12v

220v at the end and 12v in the middle

if I use 3A (or 36VA) for the filaments from the middle of the connected transformers I rest with 2A (or 24VA) for the HV (I think)

so now I can have "only" 109mA from the HV winding (24VA/220v = 0.109A)

the question is

is this reasoning correct ?

if correct, can I use a bigger first transformer and a smaller second transformer ?

Thanks

Kagliostro

[RicharD]

Your math look correct to me.  There's no problem having a larger 1st transformer.

[PRR]

> exceeding the insulation rating

That's never the issue. "120V" windings use multiple layers of 500V insulation.

> the more load you put on them, the hotter they get
 
True, but still not the issue.

Vacuum (or air) carries magnetic flux with a factor of "1". Good transformer iron carries magnetic flux with a factor near 1,000..... up to a point. (Actually its permeability changes with applied magnetic force.)

So as you raise voltage on a "120V" winding from 10V to 120V, 99.8% to 99.9% of the flux passes through the iron, causes self-inductance, opposes the applied voltage. The no-load current is small.

This is true up to about 5,000 Gauss in the iron. But at higher Gauss, "all" the iron molecules are magnetized and lined-up. The iron is "filled up", it "can't hold any more".

Now when you raise voltage more, past 120V or 130V or whatever safety-margin the transformer winder put in, the excess voltage and excess flux "spills out". Self-inductance falls very low. The no-load current rises very high.

If you ask around your Electrician friends you may find one who saw a 240V winding on a 440V feed (oops!). At best, the fuse kept blowing right after switch-on. But in bigger systems it can heat and burn.

If you have a fire-proof bench and lots of excess transformers, you can try it. Plug a 120V winding to a 120V outlet. It idles nice, slightly warm. Now plug the 120V winding into a full-power 240V outlet. It will pull a BIG current, hum loudly, blow fuses or smoke and burn.

If you have a high-power Variac, and a way to measure AC Amps, you can plot it. No-load current is small 10V to 120V, rising at 130V, and rising fast at 150V.

That excess no-load current causes I^2R losses and heat in the winding. So for a "little" over-voltage, and a little excess heat, you can de-rate the transformer for lower VA output and less load-loss heat. But at some over-voltage the no-load current alone will quickly over-heat the winding.

> can I use a bigger first transformer and a smaller second transformer ?

Yes. In fact since in most tube amps the heater power is as-big or bigger than the plate(s) power, the second transformer can usually be half the size.

[PRR]

Test on a 100V winding:


http://www.bookcracker.com/transformer/

Note the rise 130V-140V, and the sky-rocket rise above 140V.

[kagliostro]

MANY THANKS PRR and Richard

I forgot only to say one thing

The most lamp transformer I can have are toroidal transformers

but I think that the explanation and consideration by PRR can be applied equally

isn't it ?

I think I'll experiment around that

I have a Variac and also I've an electronic industrial thermometer (you know, at flea market you can find lot of interesting things)

so my idea is to put the variac in the input, connect the back to back transformers and look to the temperature and current rise

as to do those experiments is better to have a load or not ?

If the idea for a load is correct, I salvaged some big industrial resistors, which can be a good load ? How many ohms to put in the output of the second transformer ?


Franco

[darryl]

Below are a couple of real-world examples...



This is a 2-watt amplifier, using a push-pull pair of 6DX8's and a 12AX7. The B+ is provided by a pair of 28VA, 240 to 7 volt transformers wired back-to-back. The initial stepdown to 7VAC supplies the valve heaters ( through a .47 ohm dropping resistor ) and the 7 to 240 volt stepup provides a nominal 240VAC to be rectified and filtered for the B+. The stepdown transformer becomes warm, but can be held without discomfort, the stepup transformer remains just barely above ambient temperature.

If you want sag, this is the power supply to do it! When first switched on, with cold valves, the B+ quickly rises to 294VDC. Once the valves have warmed up, the B+ is 251VDC. At maximum undistorted sine wave output, the B+ is 241VDC, and at maximum square wave output it's 232VDC. This power supply arrangement works very successfully in such a low-powered amplifier, but its inefficiency becomes a problem with larger amps.

This is the smoldering ruin remaining after an attempt to push the concept further...



I've over dramatised the situation a bit, but it certainly was a failure. This amplifier was an attempt to use a larger 60VA stepdown transformer with a 28VA stepup transformer. The stepdown transformer had two identical windings 0-9-12 volts. One of the 12-volt windings was used to power the valve heaters. The 9VAC tap on the second winding was connected to the 7VAC "primary" of the stepup transformer.

After running about 20 minutes, even without any valves fitted, both transformers were stinking hot. A thermocouple probe on the stepup was reading 80 degrees Celsius, and still rising. Obviously the stepup transformer was behaving as PRR's graph above indicated it would, and causing some distress to the stepdown transformer as well.

A more efficient way to obtain useful B+ voltages from low voltage transformers is to use a voltage multiplier. A voltage quadrupler or even octupler will have better regulation than the back-to-back transformer arrangement. This was the method used in the 5-watt amplifier I described here a few weeks ago. 

[kagliostro]

An interesting example with good real world explanations

Yes also my idea is about little amps (may be a little more than that in the example) and/or preamplifier

but for preamplifier I don't know if a multiplier is a good chosen

Thanks

Kagliostro