Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: mat on February 19, 2011, 03:49:04 pm

Title: suitable pwr trafo for the new project
Post by: mat on February 19, 2011, 03:49:04 pm: Would these specs ( Primary 230V, secondary 230V 0,12A, 20V 0,12A and 6.3V 4A.) be enough for amp project that contains 12ax7 and ef86 for the pre, 12ax7 for PI and two el84's for the power tubes ?

Thanks,
Mat
Title: Re: suitable pwr trafo for the new project
Post by: FYL on February 19, 2011, 07:19:29 pm: Just calculate total power consumption using typical values from the datasheets. Heater current is fixed, plate current depends on the actual amp and output level - take the max from the datasheets.

2 x 12AX7 = 300 mA H, 3 mA Pl (two sections) max. => 600 mA H, 6 mA Pl
1 x EF86 = 200 mA H, 6 mA Pl max.
2 x EL84 = 800 mA H, 50 mA typ. => 1.6 A H, 100 mA Pl + Sc

Total = 2,4 A H, 112 mA Pl + Sc

Your PT is OK with it's 4 A 6.3 V and 120 mA B+ ratings. Heater voltages could be on the high side because the 6.3 V winding will be loaded at 60 % only - a couple of dropping resistors could be needed if you fancy tube longevity.
Title: Re: suitable pwr trafo for the new project
Post by: mat on February 19, 2011, 11:13:04 pm: Thanks FYL for the math. I'll further study the numbers.
Title: Re: suitable pwr trafo for the new project
Post by: stingray_65 on February 19, 2011, 11:14:45 pm: you can knock down heater voltages a little at a time by adding a diode or two

(------>|------4,5-----
(
(
(------|<-------9-----

That is if it is a problem.

Ray
Title: Re: suitable pwr trafo for the new project
Post by: mat on February 20, 2011, 03:55:24 pm: Quote from: stingray_65 on February 19, 2011, 11:14:45 pm
you can knock down heater voltages a little at a time by adding a diode or two

(------>|------4,5-----
(
(
(------|<-------9-----

That is if it is a problem.

Ray

Sorry Ray, how do I connect the diodes to the heater wires if I want to drop the voltage ?

So there would be enough juice for one more 12ax7 on the amp with this trafo ? maybe a buffered loop..
Title: Re: suitable pwr trafo for the new project
Post by: FYL on February 20, 2011, 04:37:57 pm: Just add 3 mA for plate and 300 mA for heaters...
Title: Re: suitable pwr trafo for the new project
Post by: J Rindt on February 20, 2011, 06:53:39 pm: Keeping in mind I am new to electronics....
So, you do not need 2 diodes, cathodes reversed, in parallel, for each heater wire to allow for the AC change in polarity.?
Thanks

Quote from: stingray_65 on February 19, 2011, 11:14:45 pm
you can knock down heater voltages a little at a time by adding a diode or two

(------>|------4,5-----
(
(
(------|<-------9-----

That is if it is a problem.

Ray
Title: Re: suitable pwr trafo for the new project
Post by: J Rindt on February 21, 2011, 11:46:02 pm: ^^^^^^^
Can somebody set me straight on this.?
Thank You
Title: Re: suitable pwr trafo for the new project
Post by: sluckey on February 22, 2011, 06:49:55 am: Your thinking is correct. Ray's circuit looks like a half wave rectifier to me and that will knock the voltage down too much.