Using 5V tap (connect to primary) to lower voltage

[BrownIsound]

I saw this today: How To Use 5V Tap to Convert from 115V Primary to 120V Primary (Mark Fowler) http://www.ws.binghamton.edu/fowler/How ... rimary.pdf

Any potential negatives to doing this?

Sorry if this has been discussed before, I didn’t find it in a search.

[pompeiisneaks]

I don't know if that's the case but that may actually be written by forum member MFowler?

~Phil

[bepone]

I saw this today: How To Use 5V Tap to Convert from 115V Primary to 120V Primary (Mark Fowler) http://www.ws.binghamton.edu/fowler/How ... rimary.pdf

Any potential negatives to doing this?

can be , if the winder thought that 5v winding will be used on low voltage line and made it without proper insulation to other layers, then can be some arcing

[R.G.]

As bepone says, the potential (sorry... ) downside is just that you're moving section of winding that was intended for secondary use over to the AC mains. This can be a safety hazard, but mostly, you don't know what will happen.

It will function, of course. Connecting a winding in series with the primary lowers the volts per turn on every turn. I haven't gone through all the equations, but the idea is solid.

But a 5V heater winding, even one intended for riding on a 500V rectifier output can be under-insulated for the AC mains. Mains windings have to be insulated for a few thousand volts to account for AC mains spikes and transients. It used to be that you could get a safety rating with a mains-to-anything-else withstanding voltage of 2500V, but that went up to 3000, and now is ... um, 4000 if I remember correctly. The 5V winding is probably not wound with multiple insulating layers and 4mm or more edge margins, so it's susceptible to AC mains spikes when used as part of a primary.

So one possible downside is an AC spike punctures the insulation on the 5V section, where it would not on just the primary. Mostly, it'll work fine. It's your amp; you're informed about the risks; proceed as you see fit. I personally would not sell, return, or give an amp with this setup to anyone else, just because I'm liability averse.

[Stevem]

That 5 volt winding is made from a gauge of wire to hold up to supplying a tad over 2 amps at that 5 volt level, so let's call the power it can dissappate 11 watts.

The winding on the primary side is made to hold up to some 30 watts ( .250 amp x 120) so I would not expect that 5 volt winging to hold up for long in a 50 watt Marshall no less if where talking about a 100 watt version .

Now if you where using a PT made to power a two 6V6 amp and had a 5 volt winding that could power a 5U4, then that I might take a chance on.

[Phil_S]

Hmm...the question seems more academic than practical. There is something about this that looks to me like the snake eating it's tail. Why would you use a 5V secondary winding that is powered by the 120V primary winding to buck the primary? If you really want to do this, get a separate 5V transformer that you know is up to the task, details already covered here by others. More to the point, though, what is really achieved by the 5V reduction? I think I understand the why...so you can run a vintage amp at the lower voltage because that was how it was back in the day. Still, I don't see much of a practical benefit. Thanks for allowing me to put in my 2¢.

[R.G.]

That 5 volt winding is made from a gauge of wire to hold up to supplying a tad over 2 amps at that 5 volt level, so let's call the power it can dissappate 11 watts.

The winding on the primary side is made to hold up to some 30 watts ( .250 amp x 120) so I would not expect that 5 volt winging to hold up for long in a 50 watt Marshall no less if where talking about a 100 watt version .

That's a bit of a misconception. The wire in a transformer doesn't dissipate the rated output power, it only supplies it to the load. Transformer wires dissipate on the order of 1-2% of the load power, and then only at full load.

The wire in the 5V winding in your example can withstand 2A flowing through it without overheating in its normal use. The wires don't care where the current comes from as long as it's less than their maximum. So it would work fine in series with the primary and conducting only 0.25A.

The idea is fine for just working, but has issues with safety isolation considerations.

Hmm...the question seems more academic than practical. There is something about this that looks to me like the snake eating it's tail. Why would you use a 5V secondary winding that is powered by the 120V primary winding to buck the primary? If you really want to do this, get a separate 5V transformer that you know is up to the task, details already covered here by others. More to the point, though, what is really achieved by the 5V reduction? I think I understand the why...so you can run a vintage amp at the lower voltage because that was how it was back in the day. Still, I don't see much of a practical benefit. Thanks for allowing me to put in my 2¢.

It does seem funny, doesn't it? Yet it does work. As the article points out, it's not bucking the primary. It's making a new "primary" with more turns included in it. This lowers the volts per turn on all the wires inside, and thus lowers the secondary voltage proportionately.