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alexg 7th March 2008 01:49 PM

2a3 maximum plate voltage
I am looking at a 2a3 SE schematic with 450VDC on the plate (plate to ground) and 200V across the cathode resistor.

According to the tube manual, 2a3 has maximum plate voltage of 300V and maximum plate dissipation of 15W.

Is the maximum plate voltage measured from the plate to the cathode?

Sorry for such a newbie question. Thanks.

kmtang 7th March 2008 02:01 PM

The maximum voltage in the data sheet is between the plate and cathode. If the plate voltage is 450V and cathode is 200V, the plate to cathode voltage is only 250V. Therefore, it is well within the safe margin.

I believe it is kind of Loftin White or Free Monkey configuration.


alexg 7th March 2008 02:06 PM

Thanks Johnny.

The circuit is an EL84 driver direct coupled to a 2a3.

jazzbo 7th March 2008 02:12 PM

You are correct. Plate voltage is measured wrt the cathode. You are obviously looking at a DC-coupled design...

alexg 7th March 2008 03:48 PM

Yes it is a DC-coupled design, only three parts on the signal path, the EL84 driver, 2a3 and the OPT (well, more if we consider the RCA jack and the wires). :)

The German designer claims that it sounds excellent, and I am very curious that I might do the circuit to find out.

Here is his work: his page

I was originally thinking of doing a parafeed 2a3, driver tube will be a 12at7 or a two stage driver using 6n1p or 5687 or even the ECC99.

alexg 7th March 2008 03:58 PM

BTW, am trying to simulate the PSU using PSUD2 and with a 5ar4 rectifier with 430-0-430 into the 5ar4 into a 10uf -> 10H 220ma -> 40uf -> 10H 220ma->40uf->190ma load and am getting this error: "The rectifier IFRM of 0.75A has been exceeded with a value of 0.90A, at time 10.152969S"

I checked the tube data of the 5AR4 and my voltage and current draw does not exceed permissible values.

Any ideas? Thanks.

jazzbo 7th March 2008 04:22 PM

Every tube rectifier wants to see a certain series per plate resistance (this includes the resistance in the secondary of the power transformer) depending on the AC voltage it sees and the peak current it has to deliver, especially in a cap input filter in order not to exceed the max. dissipation of the tube. This is often overlooked in the design of tube rectified supplies. Go to Frank Philipse's site and download the G.E. datasheet for the 5AR4.

They have a good explanation and chart for calculating the needed per plate resistance. Hope this helps.

alexg 7th March 2008 10:24 PM

Thanks Jazzbo.

From what I gather, I need to put a series resistor after the rectifier?

When I tried adding this on PSUD2, I am not getting the error anymore.


jazzbo 8th March 2008 04:29 PM

You can place the necessary resistance between the rectifier and first cap, yes, and that is probably the easiest in practice and to simulate in PSUD2. Or you can put a resistor in series with each plate between the secondary winding and the rectifier. If you want to model it that way in PSUD2, you have to measure the DC resistance of each half of the secondary of the power transformer and add the needed resistor value when entering the data into the program. If, for example, your power tx has 50 ohms DC R in each half of the winding and the rectifier needs to see 100 ohms per plate, then you can simply put a 50 ohm resistor between the rectifier and cap, OR you can put a 50 ohm resistor in series with each plate between it and the secondary winding of the PT. They are electrically equivalent. Whether one way sounds better than the other...? Try it both ways...

rdf 8th March 2008 05:12 PM

What load are you using in PSUD, alexg? Constant current loads result in unrealistic startup conditions. No tube pulls 100% design current at zero plate volts.

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