I am planing to make this 2A3 se amp but i can't select proper power trans
i will make stereo channel with this circuit
will my calculation can operate this amp ?
350V-0V-350V (250ma)
5V-0V (3A) [rectifier heater]
6.3V-0V (3A) [2X12AX7 heater]
2.5V-0V (3A) [2A3 heater]
2.5V-0V (3A) [2A3 heater]
Yes should run a nice little stereo amp with your calc tranny in my view. you have way more then you need to run the 12ax7 heaters 12AX7 heater .300ma@ 6.3v .150ma@12.6v 3A for rectifier is right to as a 5U4GB takes 3A on the heater side so you do have enough for all heaters.
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Hi,
Look at the data sheet for the tubes you want to use and you will see how much current they need, in your case
For example,
12AX7 - heater current is only 300mA @ 6.3V per tube which is tiny so you probably need to put a power resistor in series with the heater to lower the output from the transformer because it will put out more than 6.3V when you are loading it with only 300mA
2A3 - filament current is 2.5A at 2.5V per tube,
GZ34 - heater current is 1.9A at 5V with only one tube in the amp to worry about
For a stereo amplifier you need about 100mA per channel for the B+ (you have 60mA through the output tube and likely <10mA for the rest of the circuit) so you could look at the Hammond 300 series: Hammond Mfg. - Universal Primary - "Classic" Power Transformers
The 373BX just meets the needs but it will run hot, the 373HX is bigger and heavier but will operate without stress. You would also need separate transformers for the 2A3 filaments - each output tube needs a separate filament transformer winding.
Also look at Edcor transformers: https://edcorusa.com For example, the XPWR086. You will also need separate filament transformer for 2A3 such as the Edcor LVP2.5-3-230 (use two filament transformers total for stereo)
Be careful - the B+ voltage on this amplifier is dangerous, work carefully.
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I'm skeptical of the voltages quoted in that schematic.
I am using 500V rectified over a Graetz bridge with two TV damper diodes, choke loaded into 7.5Hy and I get a B+ of about 415V which is ideal.
That would correspond to a 250V-0V-250V mains transformer in your schematic, and without the input cap at that (!) Perhaps you can simulate it in Duncan Amp's PSU designer II and decide for yourself...
I only draw about 45 to 50 mA per 2a3 - this is also better sonically. Also, that schematic won't sound very special for a lot of reasons..... google "darius loftin white" for a better idea of how to implement 12ax7 for a true Loftin-White design.
Yes, the B+ is dangerous on these kind of amps - Bigun is right. I hope this isn't your first build...
Ian
I am using 500V rectified over a Graetz bridge with two TV damper diodes, choke loaded into 7.5Hy and I get a B+ of about 415V which is ideal.
That would correspond to a 250V-0V-250V mains transformer in your schematic, and without the input cap at that (!) Perhaps you can simulate it in Duncan Amp's PSU designer II and decide for yourself...
I only draw about 45 to 50 mA per 2a3 - this is also better sonically. Also, that schematic won't sound very special for a lot of reasons..... google "darius loftin white" for a better idea of how to implement 12ax7 for a true Loftin-White design.
Yes, the B+ is dangerous on these kind of amps - Bigun is right. I hope this isn't your first build...
Ian
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