120V vs 125V PT primary taps??

[wcrankshaw]

Whats the effect of using one instead of the other. The more I learn about this stuff, the more I start to wonder about things I took for granted. Let's say I have 123V wall voltage. What happens using the 120V tap, or the 125V tap.

[Chuck H]

It won't have a significant affect on the filament winding because that's a step down process. The B+ wind will vary quite a bit though because it's a step up process. The actual amount will depend on the exact ratio of your tranny but for example lets say its a 400-0-400 (or 800 volts end to end). That's going to be about about a 1 to 6.6 ratio for either primary wind (without getting too specific). So the voltage difference of the two primary winds will be multiplied by 6.6 on the secondary. Thats about 31 volts. So let's say you end up with a Vp of 450 using the 120 volt primary but you wanted it closer to 420V. No problem, just swap to the 125 volt primary and your real close. Or vise versa.

This same rule wouldn't apply for the filament secondary with larger primary wind differences because it could throw the filament voltage out of spec.

Point is, transformers, for this application anyway, are all about turns ratio.

[wcrankshaw]

Thanks for the info.

To go further, if I have my PT hooked up to the 120 tap, and I'm getting Vp that is about 50-60 V higher than it's supposed to be, I could probably just switch to the 125 tap and be all good?

[Chuck H]

Consider that the loaded voltage of the PT will be a lot less than the unloaded voltage. Gotta run now but I'll cover more detail later if someone doesn't beat me to it.

[Enzo]

The only voltage readings that matter will be with all the tubes installed and operating. Without the tubes, your B+ readings will be WAY high.

Chuck explained it OK. I look at it a little different, but get to the same place. If my B+ is more or less 480v, that is four times my 120vAC mains. So every volt of change at the mains is a 4v change at the B+ up or down as appropriate. The difference in ratios between the 120v and 125v taps is tiny and you will find most places the mains voltage jumps around all day anyway. But you could think of the 125v tap as more or less the same as lowering the mains voltage to the 120 tap by about 5v. Now that would be maybe 20v at the B+ in my scenario. A 50v difference is not the result of the mains tap.


And if you have 123v mains, then it doesn;t matter which tap you use from the 120/125 choice. Select the tap that nudges the secondary voltage the direction you prefer. The 125v tap will lower the secondary voltages a small percentage.

[wcrankshaw]

Thanks for all the explanation. I think moving it to the 125 will nudge it down just enough to make me a little more comfortable. I'm starting to get the impression that weber PT's are extra hot in general though.

[Enzo]

When a transformer is spec'd the output voltages are at full output current. SO if the thing is spec'd as 1A at 400v, if you draw less than 1A from it, the voltage will rise above the 400v.

[R ski]

If your power transformer does have 120 and 125 primary taps the amplifier not going to exhibite any significant change, that amount of voltage change can happen on any north american power grid about 4%.

No big change will be percieved, only persodo effect bro

[Chuck H]

Sure... True IME also. But it's the difference of the B+ shifting between, say, 425Vp and 455Vp or 455Vp and 485Vp. That's pretty significant, bro.

[Diablo]

Don't forget the effect of the rectifier on B+. Changing from diodes to various tube rectifiers can drop your idling B+ by 50 volts too. Which PT from Weber are you using and with what type of rectifier?