2A3-S

About the Emission Labs tubes and how to use them.

2A3-S

Postby Thierry » Tue Jan 08, 2013 12:56 pm

Dear All,

I want to build the Sunnaudio 2A3 but with the EML 2A3-S
Must I only increasing the plate voltage or also the voltage on the 6SN7?

Best regards,
Thierry
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Re: 2A3-S

Postby Admin19 » Wed Jan 09, 2013 5:25 pm

Hello Thierry,

first, there is no "must" to change anything when using the EML2A3-S. It's a normal biasing 2A3, which the nice "S"pecial option, to run it at higher dissipation if you want. In order to get that higher dissipation, you need to find the best strategy. So can the SE do the higher plate current? If not, you can only increase the voltage, so stay at the regular 60mA. In case the SE transformer CAN do higher current, you can go to 70...85mA, and increase the plate voltage accordingly, and get higher plate dissipation, like 18...20 Watt for instance. (The EML 2A3S can even do 28Watt, but it is not sure the ampifier can do that much, since it's almost twice)

Limitations as how far you can go, are probably the amplifier's power supply, or the SE transformer. Not the 2A3-S.

If you increase the whole amplfier voltage, also the 6SN7 driver gets higher voltage. There are a few ways to deal with that. You can change the resistors around the 6SN7, simply to adapt the circuit for the desired voltage. That is not a problem to do. A less complicated way is to increase the series resistor from the power supply to the 6SN7 stage. That is if the circuit has one. If not, consider to change the resistors around the 6SN7, or you might simply add this series resistor + the additional capacitors it needs.

Ideally, please post the circuit diagram here. A picture tells more than a thousand words.

Jac
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Re: 2A3-S

Postby Thierry » Fri Jan 11, 2013 8:42 am

Dear Jac,

In attachment the circuit

Best regards,
Thierry
Attachments
2A3.jpg
2A3.jpg (69.91 KiB) Viewed 8954 times
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Re: 2A3-S

Postby Admin19 » Fri Jan 11, 2013 7:56 pm

Hello Thierry, to boost a 2A3 amplifier, you need to run the output stage at higher dissipation. So lets see with this schematic what can be done.

You need to check the transformers and the 6SN7 stage. In many cases there is some hidden reserve in transformers, because they're always wound on standard cores. However we don't know that here. Finding that out is not so hard, but beyond the scope of this short story. So let's go this way: we boost the supply voltage only. The amplifier seems kind of made for that. The capacitors are 500V and the rectifier can do it also.

At first, measure the voltage across the 2k7 resistor (which feeds the 6SN7s) and across the 750 Ohms resistor. From this, derive the current of each. This number we try to maintain.

To increase the power supply voltage we a need small additional transformer, it has two windings of 55Volt, 150mA. So it's really a very small one. These windings come in series with the connections "G" at the mains transformer. This additional transformer should be wound such that half phase load of the windings is possible. This is so in many cases, but not in all cases (like when working with TWO small transformers each 55V, that is impossible). So without explaining this transformer requirement here, you do need to remember this for later. Take care of the windings polarity. Check each AC end against ground, to see if you did that right, and then you will have 55 Volts more on each end that is called "G" in the schematic.

Now, you need to do a quick measurement, and after that turn the amp off again.

Put in the 2A3-S and start up the amplifier. Measure the voltage across the 2k7 resistor, and switch the amp off again. Now calculate the current, and calculate what NEW value of that resistor you need, in order to get the original current. Or just try it out with some values. Each time with a short measurement only. After this is done, comes the 2A3-S

You need to change the 750Ohms resistor, such that the current is the same as before. Actually it's best to use a variable resistor, like a big ceramic pot meter that you buy on Ebay. With that you just set the 2A3 for the required current. When it works, you switch it off and measure the resistor.

Re-check the 2k7 resistor after you re-biased the 2A3-S since the 6SN7 and 2A3 bias influent each other a little bit. When you did it right, the 280V of the 6SN7 will be the same. The current through the SE transformer is also the same as before, and so is the load of the mains transformer and the choke.

Note, this output power boosting would damage a standard 2A3 tube, it can only be done with the Emission Labs 2A3-S.

Disclaimer..... you do this on your own risk. Let's say this is just how I would do it myself here.

Have fun!

Jac
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Re: 2A3-S

Postby kathi » Fri Nov 14, 2014 12:31 pm

As I use an autotransformer (variac) as to obtain exactly the 2,5 V required by my push-pull of 2A3, I wonder if raising gently the voltage from 0 to 220 when the amp gets under tension, would not perform the same target as a temporisator or a Shalter on the HT. Has somebody experimented this ? Would it be dangerous for the electronic or beneficial ?.



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