Series Resistance - running the numbers in my EL84 amp

[dubadub]

Greetings Friends. I was browsing valvewizard this morning, the Rectifiers section, and got to the bit about series resistance. I've posted about it before, don't quite get it, decided to run these equations with the little EL84 amp I built back in january.

So. As per the datasheet,
Rlim = Rsec + Rpri × (Vsec/Vpri)^2 + any extra resistance

Where:
Rpri is the DC resistance of the transformer's primary winding;
4.524 ohm

Rsec is the DC resistance of one half of the transformer's secondary winding, i.e measured from one end to centre tap;
76.55 ohm

Vpri is the primary (i.e. mains) voltage;
117 V

Vsec is one half of the secondary voltage, i.e. measured from one end to centre tap.
260 V

(260 / 117)^2 * 4.524 + 76.55 = 98.9

that's my take on it. assuming they use the same PEMDAS order of operations on the other side of the pond.

Now there's no charts on series resistance in any of the EZ81 datasheets I've found, but every sheet has a Rt value and it seems to be 2x150 Ohm for my voltage range. So I need to add 50 ohm resistance to each anode, or one between the cathode and the reservoir cap? Does the choke's resistance count?

thanks for taking a look!

w

 

[jcalvarez]

Sounds right, this datasheet has the explanation: https://datasheetspdf.com/pdf-down/E/Z/8/EZ81-Philips.pdf

 

[e_fortier]

Yep, as you said: add 50 ohm resistance to each anode.

 

[Suncalc]

Now there's no charts on series resistance in any of the EZ81 datasheets I've found, but every sheet has a Rt value and it seems to be 2x150 Ohm for my voltage range.

Keep in mind that this limitation is for the stated configuration. In this case C=40µf (as listed in the Phillips 1970 EZ-81 data sheet). The real limitation on the tube is the steady state repetitive peak plate current, which for the EZ-81 is 500mA. If your first capacitor is smaller than 40µf then the resistance required to limit the peak plate current goes down. As a side benefit, this lengthens the conduction angle lowering losses (and heating) in the power transformer. Since your circuit (as posted) uses a 22µf capacitor the steady state repetitive peak plate current is likely much lower.

A simple way to check this is to place a 1Ω resistor in a rectifier plate lead and use a dual channel oscilloscope to examine the waveform across the resistor. This way you can see the current waveform and measure its peak value. My suspicion is that with the 22µf first capacitor and the ≈100Ω equivalent winding resistance then the waveform is likely well under the 500mA limit. Especially given that the average plate current is only ≈36mA. Even if the peak to average ratio was 10 (highly unlikely; usually between 4 and 7) the peak would only be ≈360mA which is ≈28% margin.

Personally I wouldn't change anything if the amp is working fine. You can use the sense resistor trick if you really want to look at the waveform but personally I wouldn't bother.

Just my thoughts on the matter.

 

[Suncalc]

In this case C=40µf (as listed in the Phillips 1970 EZ-81 data sheet)

Oops. Just realized that the capacitor in the figure is 50µf; even bigger! And that chart maks an assumption about average plate current which is not explicitly stated.

Now I REALLY wouldn't worry about it.

 

[dubadub]

Wow, I was just about to quote from Your article on Power Supplies for Tube Amplifiers and here you are!

I got to the part about Hot Switching, just below Fig 3, and since I'm not using a standby switch, 150ohm is not my limit, it's much lower. So I'm good 2 ways 😸