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I would like to build a stereo tube amp. I am going to modify the Eico HF-12 circuit to become Stereo instead of mono. I am only going to use the last 12AX7 and the EL84's. Unlike the HF-12 my project will not be integrated just the power amp.Total tubes used
4 EL-84's
2 12AX7
1 5U4GBOutput transformers are a pair from a Scott 222C
Does this power transformer look to be a good match (from one of our sponsors at Audio Asylum)
TRANSFORMER, POWER, HAMMOND, 300-0-300 V, 250 mA
Primary 115 VAC, 60 Hz.
Secondary DC current measured with cap. input filter, full wave (two diode) C.T., rectifier circuit.
Enclosed, 4 hole chassis mount.
Minimum 6" long leads.
Class A insulation (105 degrees C).
Hi-Pot test of 2,000 V RMS.
Conservative designs - CSA certified (# LR3902).
Secondary: 300-0-300 V, 250 mA
Filament winding 1: 5 V, 4 A
Filament winding 2: 6.3 V, 8 A
Mounting centers: 3" x 2.81" vertical mount
Weight: 7.6 lbs.
Follow Ups:
calculate the total current draw of the four EL84's and the two 12AX7's. Don't worry about the 5U4 because the 300-0-300 at 250 ma comes FROM the 5U4.After you calculate the total current the circuit will require, then add a minimum of 25%. If that total is less than 250 ma then you're "good to go". If it is more than 250 ma, the transformer will run at max capacity and as a result will become quite hot.
I surmise that you will be fine running those six tubes without going over the 250 ma.
keep us posted on your progress,
cheers
roN
and add that I generally like to derate the whole thing by at least 10% to account for components that drift, spec-wise, and losses in the system. If you're really worried, make that 25% and sleep better.Cheers,
Here is a scan from Jack Darr's classic Guitar Amplifier Handbook. On page 75 he details how to calculate the specs for a power transformer.BTW Chapters 1-6 are posted at this link and a great resource.
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