Power Transformer size/rating

[pb47]

I built a tube amp and wanted to simulate the effect of using a smaller PT. So I put a 300ohm resistor before the rectifier and was able to drop the b+ voltage by 28v.  This PT was originally rated 710v @ 100 mA unloaded. My question is regarding the 100mA current rating. Will this PT's current   also be affected by the resistor that I used? How much/ in what way will it be affected?

[shooter]

use Mr. ohm, measure the VDC on the left of the R, measure the VDC on the right of the R, (|Eleft-Eright|)/300 then subtract that from your 100mA, leaves you with what's left for the Amp - I think

[pb47]

it was 371v before the resistor, 346 after. so thats a difference of 25v. / by 300 = .08333

100mA - .083 = 99.17 mA

This does'nt seem right.

[PRR]

> Will this PT's current   also be affected by the resistor that I used?

No. Current is in series. You are just losing voltage to your amplifier.

My house has 125V power at the street. Good for 100A. Glenn put the house way back in the woods, and didn't spend many-thousands for the power wire. I can still pull 100A, if I wish. Voltage at my house will be under 90V, lamps would be dim, but neither the transformer nor my wire would mind.

>> 300ohm resistor before the rectifier
> use Mr. ohm, measure the VDC on the left of the R

Beware. The wave-form *before* the rectifier is very ugly. Not DC! Not simple AC! The way your meter reads it is un-certain (unless you know intimate details of the meter guts).

[pb47]

Would the resistor that I put into the circuit have the same effect on the sound as using a smaller transformer with a lower voltage and current rating? or would i need to actually put a different transformer in there?

[Paul1453]

Would the resistor that I put into the circuit have the same effect of creating "sag", as if I were to use a smaller PT with lower current rating?

I'm guessing, No.

From PRR's house example, I would not expect an increased voltage sag unless we attempted to draw more than the 100A available.
At which point I would expect all our lights to get dimmer too.

I could be completely wrong, but that is how I apply PRR's analogy to your question.   

[shooter]

 

Current is in series

my brain was modeling a PA tube, the screen was the 300ohm R

[sluckey]

Putting a resistor in series between the PT and rectifier will cause a voltage drop as you already have seen. If the amp presents a fairly constant load on the PT then the voltage drop will be fairly constant. If the amp load changes much as the volume changes, then the voltage drop across that resistor will change also. This produces a changing sag in the amp that can be heard.

That series resistor also limits the maximum current that can be pulled from the PT in the same way a light bulb limiter operates.

[silverfox]

Would the resistor that I put into the circuit have the same effect on the sound as using a smaller transformer with a lower voltage and current rating? or would i need to actually put a different transformer in there?

I don't think so. The resistor would not react like an inductor for one. I don't think you can simulate a smaller transformer this way. If it were me I'd look for some of the power supply simulation software online and use that, short of actually replacing the PT. Not sure but I don't think you would be able to simulate the output sections cranked sound by simply substitution of a load resistor.

silverfox.

[HotBluePlates]

I built a tube amp and wanted to simulate the effect of using a smaller PT. So I put a 300ohm resistor before the rectifier ...

How exact do you want to get in simulating a small transformer?

A resistor will be a linear drop in voltage with increased current. Depending on the class of operation of your output stage, this may/may not translate to a linear drop in voltage/power as you crank up.

A transformer has mutliple factors influencing its own voltage sag and saturation. But simply put, the transfer curve of a transformer core's magnetism-handling ability is S-shaped. So it will act like a very low resistance with a small transfer of energy, and transition over some range to act like a much higher resistance at some higher transfer of energy.

Core saturation is not the only factor that determines transformer voltage sag, though good (costly) design will try to eliminate some of the others as factors. If you have to know exactly how an amp design will perform with a smaller PT, the easiest/accurate (but perhaps most expensive) route is to use the smaller transformer.