Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: guntherbuffalo on October 13, 2014, 01:11:37 pm

Title: Rectifying 2 uneven HT
Post by: guntherbuffalo on October 13, 2014, 01:11:37 pm: Hi,

If you are using a solidstate bridge rectifier as in the below image, you would often see, for example two equal voltage taps, lets say 300v (adding up to 600v), come from the power transformer to the rectifier, giving an output voltage of the 1.4 times the PT secondaries - in this case - 420v. CT to ground.

If you had a power transformer which instead of two equal 300v taps, had one 250v tap and one 350v tap (still adding up to 600v), could you connect these two taps to the rectifier in the normal way, and still achieve the 420v output voltage?

Thanks,

GB
Title: Re: Rectifying 2 uneven HT
Post by: kagliostro on October 13, 2014, 02:15:38 pm: If the transformer is a 0-250v-350v you can connect the bridge to the 0-350v tap and obtain (after rectify and leveling) ~490v DC

if you connect it 0-250v you obtain ~350v DC

to have ~420v DC you must use an hybrid rectifier to drop voltage (but you need a 5v winding for the filament of the tube)

in one of this way

(http://www.valvewizard.co.uk/bridge3.jpg)

(http://www.valvewizard.co.uk/bridge4.jpg)

or a zener (an amplified zener is better or you must use a big one) connected between the ground and the "ground" of the bridge rectifier

K
Title: Re: Rectifying 2 uneven HT
Post by: jjasilli on October 13, 2014, 02:18:42 pm: See the Hammond Power Transformer Guide attached (and available online). The PT may be center-tapped, or not. In your example there is no CT. It would be described as 0 - 600. The bridge rectifier circuit provides an artificial center tap (CT).

A similar PT with a center tap would be described as 300 - 0 - 300. It would not use a bridge rectifier. Hope this helps.
Title: Re: Rectifying 2 uneven HT
Post by: guntherbuffalo on October 13, 2014, 02:22:02 pm: Thanks,

The transformer I am looking at actually has 0 - 250 - 300 - 350 taps. So if I use the 0 - 300 taps with a solid state rectifier I am guessing this will achieve the 420v DC?

Thanks,

Paul
Title: Re: Rectifying 2 uneven HT
Post by: kagliostro on October 13, 2014, 03:07:56 pm: If you have 0-300v connected then you obtain ~420v DC (now you must verify if there is enough current for your project)

K
Title: Re: Rectifying 2 uneven HT
Post by: guntherbuffalo on October 13, 2014, 03:20:14 pm: There is a 165VA max and a 76VA max version of the transformer for a 6v6 lightning style build. I think either should be fine. :-D
Title: Re: Rectifying 2 uneven HT
Post by: guntherbuffalo on October 13, 2014, 04:30:30 pm: Can you have too high a current for the tube heater supply on a transformer? The amp I am building has 2x 6v6 and 3 or 4 (if I add reverb) 12ax7 tubes. which amounts to about 1.8A current draw. The transformer I am looking at has 3.15 - 0 - 3.15 @ 7A for the heaters. I don't know whether this means the current is too high, or whether it is just a maximum rating.

Thanks

Paul
Title: Re: Rectifying 2 uneven HT
Post by: kagliostro on October 13, 2014, 05:00:50 pm: If you need 1.8A and the PT has an ability of 7A all is fine

7A is the max current you can take from it

with a consumption of 1.8A may be your heater voltage stay a bit high

but tubes can easily afford a +10% voltage

K
Title: Re: Rectifying 2 uneven HT
Post by: guntherbuffalo on October 14, 2014, 02:39:35 am: Why would the heater voltage be high if it is rated at 3.15 - 0 - 3.15?
Title: Re: Rectifying 2 uneven HT
Post by: kagliostro on October 14, 2014, 09:22:25 am: They are rated 3.15v-0-3.15v at 7A, if your consumption is lower you can expect a bit higher voltage

K
Title: Re: Rectifying 2 uneven HT
Post by: guntherbuffalo on October 14, 2014, 10:16:53 am: Ok I see. Maybe I will wire up a 6.3 indicator bulb too then as there is excess current that can be used.