Neil Young's "Magic" 5E3

[Mike K]

Yes, resonant circuit. But perhaps what I was referring to was simply measuring the inductance, not the total impedance.

[Helmholtz]

Yes, resonant circuit. But perhaps what I was referring to was simply measuring the inductance, not the total impedance.

There's no relation between OT primary inductance and loaded impedance.

[Mike K]

There's no relation between OT primary inductance and loaded impedance.

What changes the AC impedance vs the reflected impedance based on turn ratio?

[Helmholtz]

What changes the AC impedance vs the reflected impedance based on turn ratio?

AC resistance is reflected secondary load resistance + primary and reflected secondary windings DCR.

It helps to study the equivalent circuit of a transformer.

[Mike K]

AC resistance is reflected secondary load resistance + primary and reflected secondary windings DCR.

It helps to study the equivalent circuit of a transformer.

But if that's all resistive load i.e. no reactive load like a speaker, isn't it constant as a function of frequency?

[Helmholtz]

But if that's all resistive load i.e. no reactive load like a speaker, isn't it constant as a function of frequency?

Yes, it's resistive (i.e. zero phase angle) and constant except maybe for frequency extremes.
Nevertheless it can't be measured with DC because a transformer only works with AC.

[Mike K]

Yes, it's resistive (i.e. zero phase angle) and constant except maybe for frequency extremes.
Nevertheless it can't be measured with DC because a transformer only works with AC.

I get that. But couldn't we just calculate it based on primary primary DCR + (turn ratio)^2*(secondary DCR + Load) and simply calculating the turn ratio by measuring the voltage ratio between primary and secondary? So no need for a LCR meter.

[Helmholtz]

I get that. But couldn't we just calculate it based on primary primary DCR + (turn ratio)^2*(secondary DCR + Load) and simply calculating the turn ratio by measuring the voltage ratio between primary and secondary? So no need for a LCR meter.

Sure and I've done.
You'll need a good DCR meter to accurately measure secondary resistance.

[Mike K]

Sure and I've done.
You'll need a good DCR meter to accurately measure secondary resistance.

OK - this is the same process I do for every PT. I've never done it for an OT, but it makes sense. Generally the primary DCR, even if high, is a low proportion of the reflected impedance. I have no idea about the secondary and if that was high, I assume could make a difference.

Sorry we got here in such a circuitous route, but when you mentioned AC impedance, I thought you were measuring something reactive.

[Helmholtz]

OK - this is the same process I do for every PT. I've never done it for an OT, but it makes sense. Generally the primary DCR, even if high, is a low proportion of the reflected impedance. I have no idea about the secondary and if that was high, I assume could make a difference.

Sorry we got here in such a circuitous route, but when you mentioned AC impedance, I thought you were measuring something reactive.

I tried to always use the term AC resistance (not AC impedance).

With a small OT having a high impedance, primary resistance may be higher than 500R. And 1R secondary resistance would reflect to 1250R with a 10k/8 OT.
I only raised the topic because a "10K OT" could actually be a "12K OT" depending on measuring method.

[mozz]

The LCR @1khz is the method i use anymore, since it's easy to switch 4,8, 16 and get a direct reading. As to 5E3's, I've probably built at least half a dozen (stuffed in a room upstairs)and have another i started a while ago on the bench, trying to clean up unfinished projects. I don't remember what the power output and choke were from , but taken from a amp or phono or reel. I've toyed with different caps, .033uf/.047uf but this one has 0.1 mustards and 0.1 quality caps from a Tektronix scope. As to the choke, i read that if you leave the 0.1's in place, and add a choke, a lot of the flubbiness will be gone. I shall find out. 22UF for all 3 power supply caps.

Most common complaint is high voltage on the output tubes when buying transformers. I try to shoot for 350-360. This one i have no idea, power trans is a bit bigger than a stock one and i didn't write down the loaded voltage.

[Mike K]

I tried to always use the term AC resistance (not AC impedance).

With a small OT having a high impedance, primary resistance may be higher than 500R. And 1R secondary resistance would reflect to 1250R with a 10k/8 OT.
I only raised the topic because a "10K OT" could actually be a "12K OT" depending on measuring method.

I'm quite interested in the one I have in the "10W PP" thread. That one has a very high DCR, it might be a significant deviation. I'll measure it when I get a chance, and maybe change the design a bit...

[Mike K]

Regarding (dynamic) plate dissipation, looking at the loadline in a chart showing the plate limit parabola easily shows that dissipation increases at lower load impedance.

Not a parabola, inverse relationship i.e. I = (constant)/V but mentioning that seems obvious to me now. Excessively low load will damage the plates.

[Helmholtz]

Not a parabola, inverse relationship i.e. I = (constant)/V but mentioning that seems obvious to me now. Excessively low load will damage the plates.

You're right. Should read hyperbola.

[Mike K]

Curiously enough when I measure 5E3 OT I have (Mercury FTDO-59), I get the exact same turn ratio as the 18W (n=34:1 @ 8 ohm). Accounting for the primary and reflected secondary DCR, I get a 10.3k effective primary for 8R (this OT only has 8R).

Primary DCR is 655R and secondary is 0.7R.