Current limit resistors for GZ34?

[Mark]

I have been wishing to ease the burden on my GZ-34 and have considered adding current limiting resistors from the power transformer secondary to the plates of the GZ-34. In the Dumble Manzamp circuit the resistors are only 5 ohms and would barely effect the amps performance.

I looked at the valve wizard page on rectifiers. At the bottom of the article he mentions current limiting the rectifier valve and an equation for current limiting.

http://www.valvewizard.co.uk/bridge.html

Rlim = Rsec + Rpri x (Vsec/Vpri)^2 + any extra resistance.

Rpri = DC resistance of the Power transformers primary winding.
Rsec = 1/2 of the DC resistance of the Transformer secondary winding, from one end of the secondary to the centre tap.
Vpri = The mains voltage.
Vsec = The voltage from the centre tap to one side of the secondary output.

I'm not sure what Merlin means by extra resistance, I assumed it was anything in series with the load.

I have a Mercury Magnetics Haddon clone transformer and the figures are as follows:

Rpri = 3.3 ohms
Rsec = 28.2 ohms on one side and 29.5 ohmson the other side.
Vpri = 240vac
Vsec = 303.5vac

Here is a typical spec sheet for a GZ-34. The spec sheet gives a different equation for the series resistance.

Rt = Rs + n^2*Rp

http://www.r-type.org/pdfs/gz34.pdf

EDIT:When I look at the GZ-34 datasheet it seems I have the 28.9 DC resistance and I require an additional 50 ohm resistors on each leg. This seems a high resistance to me, as all the circuits I've seen have small values as Rlim.

Any thoughts on this issue?

[martin manning]

It's the same equation, which is the series resistance of the coil feeding each diode, plus the reflected impedance of the primary, plus any added series resistance. This is Merlin's Rlim, the current limiting resistance, which should be at or above the minimum resistance from the data sheet. Note the secondary resistance is half the total, as measured from CT to either end.

[Stevem]

Here's a mod that takes a good portion of the load off the tube recto , yet still allows for tube sag.

With this mod should the recto blow you can just run on the SS recto's once you yank the tube out and replace the fuse.

[Mark]

Thanks for you assistance.

Steve thanks for the circuit diagram. Did you come up with that mod? It's similar to a mod Gerald Weber published (and I had forgotten all about.)

Martin, could you do the equation with my figures?
I'd like to see where I went wrong with my equation.

[martin manning]

Rpri = 3.3 ohms
Rsec = 28.2 ohms on one side and 29.5 ohm son the other side.
Vpri = 240vac
Vsec = 303.5vac

Average Rsec = 28.9
Rt = 28.9 + (303.5/249)^2 * 3.3 = 33.8 Ω

[Stevem]

Yes, that's from Weber.

[Mark]

Martin thanks for doing the calculation. I assume that your result gives me the value of Rt (33.8 ), the data sheet states for Vtr = 2 X 300, and then Rt = 2 X 50 ohm. Thus the series resistor values are 50 - 33.8 = 16.2 ohms per plate.

According to Merlin's equation for the power rating of the resistors.

P = (1.1 × Idc)^2 × R

P = (1.1 x 0.250mA)^2 X 16.2
= (0.275)^2 x 16.2
= 0.075625 x 16.2
= 1.225125W

That seems a low power value compared to other values I've seen which are typically over 10 watts.

Once again, if I have misinterpreted the information could I trouble you for some assistance?

Thanks for your assistance.

[trobbins]

Mark, you can use PSUD2 software to also estimate the peak diode current, and compare that peak value to the datasheet allowed peak (750mA). PSUD2 allows you to determine the influence of the filter capacitance you may be using, and its got a built in calculator for that effective resistance calculation.

If you want to go that way, and are a bit unsure of PSUD2 using GZ34, then there is an example in link:
https://www.dalmura.com.au/static/Power ... 20amps.pdf

[SoulFetish]

According to the formula Merlin references Rsec + Rpri × (Vsec/Vpri)^2,or Rsec + Rpri/(Vpri/Vsec)^2 I get a Rt=34.177Ω.
Which, admittedly, is a sea hair just off from your figure. But I'm wondering what the discrepancy is?

Also, Mark. the datasheet specifies that with a capacitor input, for Vtr = 2 X 300, the Rt min = 2 X 75.
Therefor, the series resistor value for each anode is 75 - 34 = 41Ω.
I think the power rating should reflect the RMS current through each leg of the transformer, which will also pass through these resistors. So, with an RMS current of around 303mA through each resistor during each half cycle, the power dissipated is 40Ω *.303^2 = 3.67W. Figure you want at least 5W resistor to get a minimum amount of clearance. But if you wanted the resistors to survive a Tube recto failure, then you would probably want to double it and use a resistor designed to handle a large surge current long enough to blow a fuse.

[SoulFetish]

Oh yeah! Great point trobbin, With an average DC load of 250mA, you're already stressing the GZ34 to it's maximum limiting current stated in the datasheet. If you model the 250mA DC current as a resistive load across the reservoir capacitor, then even with a capacitor value of under 10µF, I think you still exceed 750mA peak

[Mark]

Please excuse me as I've gotten a little confused. Merlin's formula is:

Rlim = Rsec + Rpri x (Vsec/Vpri)^2 + any extra resistance.

Rlim = 28.9 + 3.3 x (303/240)^2 (unaware of additional resistance.)
Rlim = 34.159 ohms

Looking at Merlin's article each anode requires a 34.159 ohm resistor. I'm thinking this resistor value will have an effect on the regulation of the power supply and thus the tone of the AC-30.

Again any thoughts?

Thank you for all your assistance, I appreciate you guys taking time to assist me.

EDIT: 33.3 typo corrected to 3.3 ohms.

[martin manning]

The 34.159 ohms is not what’s required, it’s whats there.

[SoulFetish]

Please excuse me as I've gotten a little confused. Merlin's formula is:

Rlim = Rsec + Rpri x (Vsec/Vpri)^2 + any extra resistance.

Rlim = 28.9 + 33.3 x (303/240)^2 (unaware of additional resistance.)
Rlim = 34.159 ohms

Looking at Merlin's article each anode requires a 34.159 ohm resistor. I'm thinking this resistor value will have an effect on the regulation of the power supply and thus the tone of the AC-30.

Again any thoughts?

Thank you for all your assistance, I appreciate you guys taking time to assist me.

I'm sorry, I used the primary resistance figure you posted at first stating that it was 3.3Ω. That was a typo? is the actual measurement 33.3?

[SoulFetish]

Looking at Merlin's article each anode requires a 34.159 ohm resistor.

Martin is correct. The manufacturer's datasheet gives you the minimum required resistance. I gave you the heads up in my post above that for a capacitor input filter, which is what you are using, the Rt value for Vtr 2 X 300 = 2 X 75 Ω. Minimum. Like martin said, the 34.159Ω resistance is already there as part of the transformer. So you need to come up with the rest of the needed resistance for each anode. Which is about 40 ohms or so on each anode.
You dig?

https://drtube.com/datasheets/5ar4-amperex1958.pdf

[Mark]

I'm sorry, I used the primary resistance figure you posted at first stating that it was 3.3Ω. That was a typo? is the actual measurement 33.3?

The figure is actually 3.3 ohms. Sorry about the confusion over the typo.

Thanks Martin and S.F. Yes, I got it 40 ohms per leg, I wonder if that will effect tone of the amp?

The desired rail voltage is 320vdc, so with a secondary of 303 - 0 -303 I should have a bit of wiggle room.

The power dissipated should be a follows.

P = (1.1 × Idc)^2 × R
= (1.1 x 0.250)^2 x 40
= 3.025 watts