E/PCF802 2W amplifier

[vigilante397]

So.... have you built it yet? Math is fun, but breadboarding/prototyping is more fun

[rankot]

For DC core saturation, if you're winding an push pull transformer you should arrange the lams alternately (one E, one I, one E, one I...). The DC signal of one tube will cancel with the other, and you'll have no DC saturation. But in a single ended transformer, you'll have only one DC signal with no "other tube" signal to cancel, so the core will be under DC saturation. To avoid this, you need to put all the E lams at one side and all the I lams at the other side (check the last image of the calculator screenshot you posted). Between the E pack and the I pack, you need to put an "air gap", which is basically a thin piece of paper.

Also, when you're winding the output transformer, you should do one layer of wire, then one layer of insulation, then one layer of wire... Basically, a layer of insulation after each layer of wire.

If I have to order new OTF, it will definitely have a gap, but I was asking in case I decide to use this power transformer mentioned above (Hahn 220:6V, 3VA). I really have no idea what is it like inside, since it is inside the plastic case and there is some kind of epoxy fill on the bottom (it is PCB mount type). So I am basically not sure if using that kind of TF could cause any damage to TF itself or to the circuit. Or core saturation is not so important in such a low power amp?

[rankot]

So.... have you built it yet? Math is fun, but breadboarding/prototyping is more fun

I will, in few days, just to decide which OTF to use. 

[anotherjim]

Low VA PCB transformers are supposed to be self-protecting? That is, in designed use, overload of the secondary won't burn it out and no additional fuse is necessary. I'm not sure how they achieve that - either the primary winding has such high resistance that (given that the input volts is in spec') it can survive it, or there is a self-resetting fuse inside?

[Marcos - Munky]

If I have to order new OTF, it will definitely have a gap, but I was asking in case I decide to use this power transformer mentioned above (Hahn 220:6V, 3VA). I really have no idea what is it like inside, since it is inside the plastic case and there is some kind of epoxy fill on the bottom (it is PCB mount type). So I am basically not sure if using that kind of TF could cause any damage to TF itself or to the circuit. Or core saturation is not so important in such a low power amp?

Power transformers have the lams arranged alternately. What people usually do is remove all the lams and rearrange them to add the gap. Yeah, lots of work, but you need to remove the lams anyway when you're reusing an old transformer.

Not sure of the consequences of DC saturation.

[thomasha]

You could also try a 100v line transformer. I have one where the laminations already have a gap and are correctly stacked.

Look for one that has 10W and a lot of taps. With the 0.625W you can almost achieve 30k if you use a 8 ohm speaker and the 4 ohm tap. Over wattkins (or was it diyaudio?) they tested one of these and it produced really good results for a line transformer.

[PRR]

> Low VA PCB transformers are supposed to be self-protecting? .... not sure how they achieve that - either the primary winding has such high resistance that (given that the input volts is in spec') it can survive it, or there is a self-resetting fuse inside?

Many are. Either/or.

Very small parts have high surface/volume ratio. Enough surface to cool the heat that can fit inside. Also for similar geometric reasons a small transformer does have high resistance; and for economic reasons they are designed gleefully greedy. (The electric bill waste is hardly noticed, while first-cost is always noticed.)

And at larger sizes, especially when regulation matters, they have internal cut-outs.

[rankot]

But what happens with DC core saturation in case of my small TF (220:6V, 3VA)? I am pretty sure it doesn't have a gap (or how can I test it anyway, since it is hidden inside the plastic). Is it safe to use it as OTF?

[anotherjim]

I "think" you can make SE work in a plain PT IF the core is big enough so it can handle the DC+AC without saturating. I've seen that toroids don't handle this well so it should only be done with EI cores unless push-pull.

I don't know how dangerous the asymmetric core saturation would be, apart from sounding crap. I imagine the traffy suddenly stops being much of an inductor and generation in the secondary stops until polarity reversal cancels the DC and pulls it out of saturation. Can that generate a destructive flyback voltage? I don't know. I would expect you would get whatever dissipation happens according to the primary winding DC resistance during saturation which might only do damage if sustained.

I would attempt to manage it by sizing B+ to get plate idle current as low as possible while getting decent symmetry and volume. You may have to go down towards 150v. If you only get about 1W, provided its used with a guitar speaker, it will be loud enough for home/studio use. 1W is, after all, only 3dB below 2W.

[Marcos - Munky]

Asked about a saturated core transformer on brazilian forum Handmades and got this answer:



The yellow part is the saturated core response. The amp feeds a variable current to the primary, that results in a variable voltage on the secondary. In the saturated zone, we can see a big current variation translates in a small voltage variation, so the speaker won't produce too much dynamics.

Also, a saturated core temperature is higher than a non saturated core. A hot transformer probably can have it's dangers.

[rankot]

OK, so it seems it's best to order special SE transformer from someone. Also, Hammond 125ASE or 125BSE will fit exactly, but they're a little bit too expensive - I am trying to build this using what I have at hand. 

[Marcos - Munky]

You can remove all the lamination from a power transformer and put them again forming an air gap.

[rankot]

Yes, I know that, but those I have are unfortunately sealed