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Elkaid 14th January 2003 05:33 AM

Double EL84 Push-Pull supply
 
Hello everyone !

I found a great Double EL84 Push-Pull amplifier schematic (28 Watts) but my problem is that I can't find adquate power transformer. I need the following :

Primary : 120 Volts / 60Hz (Canada)
Secondaries : 2 X 220 Volts and enough current for 4 EL84 tubes + 12AX7

As for the 6.3V, I can cope with the idea of an extra transformer

Anyone know where I could find this ? Hammond seems to have one with correct voltages but it doesn't provide enough current....

Also, would it be possible to use a switching suppply instead ?
Any information would be greatly appreciated !!!
Thanks !

corbato 14th January 2003 01:38 PM

Hi Elkaid,

can you kindly post the schematic of this 4 x EL84 amplifier??

thanks

Elkaid 14th January 2003 02:50 PM

2 Attachment(s)
Sure ! , here's the schematic

The list component list will follow

Elkaid 14th January 2003 02:53 PM

2 Attachment(s)
And there it is !

Hope this helps !

By the way I'm still looking for a Power Transformer for this amp

Primary : 120 Volts / 60Hz (Canada)
Secondaries : 2 X 220 Volts and enough current for 4 EL84 tubes + 12AX7


Oh, by the way, don't forget, the output tranformer has a primary impedance of 4kOhms

Joel 14th January 2003 03:44 PM

Elkaid,

Why two secondaries? This is more than enough transformer for you for an EL84 amp...
270HX 176VA, sec. 275-0-275, DC ma 200, Fil.#1(rct) 5.0v @ 3a ct, Fil.#2(htr) 6.3v @ 6.0a ct

220V is too low, IMO. I'd run EL84's with at least 250V on the plates. And I'd run them UL too.

Elkaid 14th January 2003 03:58 PM

2 Attachment(s)
Thanks you very much :)

I've attached the reason why I'm looking for a 2 X 220 Volts :) .

Also :
Quote:

270HX 176VA, sec. 275-0-275, DC ma 200, Fil.#1(rct) 5.0v @ 3a ct, Fil.#2(htr) 6.3v @ 6.0a ct
Should I connect the suggested transformer exactly the same way as on the schematic ? Will the voltage be too high once the two secondaries connected to the two diodes ?

Also, correct me if I'm wrong but I thought that EL84 were drawing around 60mA each. So a 200mA transformer is a bit tight right ?

Thank you !!!

Joel 14th January 2003 04:05 PM

Ok, I see why you are confused. That's not "two secondaries", it's 220V, center-tapped.

You only have one secondary.

And the current draw depends on the op point you chose, but if you like, go for the 250mA model that Hammond makes.

Elkaid 14th January 2003 04:08 PM

Heheheh oops

I feel like "stupid" :)

Thanks !

Just to be sure, the previously mentionned transformer should do the job right ?

Joel 14th January 2003 04:15 PM

You're not stupid. That's how you learn.

Anyway, I think this amp is biased very far into class A, with 82 ohm cathode resistors? So, I would say definitely use the bigger 250mA model.

Elkaid 14th January 2003 04:23 PM

Well, there's some ways to learn without looking so stupid :)

According to the original Power Supply schematic, we're supposed to get around 300Volts DC if I'm correct.

Using a higher voltage transformer implies that I'll get a higher voltage at the output of the power supply. Is there any risk to blow up something ? (my experience taught me that TDA7294 doesn't like little overvoltage).

Also, I guess that I'll need to increase capacitor voltage (previously rated @ 400Volts)


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