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Old 14th January 2003, 05:33 AM   #1
Elkaid is offline Elkaid  Canada
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Default 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 !
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Old 14th January 2003, 01:38 PM   #2
corbato is offline corbato  India
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Hi Elkaid,

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

thanks
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Old 14th January 2003, 02:50 PM   #3
Elkaid is offline Elkaid  Canada
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Sure ! , here's the schematic

The list component list will follow
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File Type: zip el84.zip (44.9 KB, 679 views)
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Old 14th January 2003, 02:53 PM   #4
Elkaid is offline Elkaid  Canada
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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
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File Type: zip el84 bom.zip (69.2 KB, 331 views)
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Old 14th January 2003, 03:44 PM   #5
Joel is offline Joel  United States
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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.
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Old 14th January 2003, 03:58 PM   #6
Elkaid is offline Elkaid  Canada
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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 !!!
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Old 14th January 2003, 04:05 PM   #7
Joel is offline Joel  United States
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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.
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Old 14th January 2003, 04:08 PM   #8
Elkaid is offline Elkaid  Canada
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Heheheh oops

I feel like "stupid"

Thanks !

Just to be sure, the previously mentionned transformer should do the job right ?
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Old 14th January 2003, 04:15 PM   #9
Joel is offline Joel  United States
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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.
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Old 14th January 2003, 04:23 PM   #10
Elkaid is offline Elkaid  Canada
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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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