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Tube full wave rectifier vs tube full wave bridge rectifier

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S

sumotan

Member
Joined 2013
#1
Hi Guys,

A little tutorial pls. My experience has been with a couple of EZ81, 5u4g.
Am researching PS for my next project. I've seen the use of 4 Tv diodes to build a bridge rectifier. Question if I already have a transformer with 45-0-450 acv, can I use it to build a full bridge supply, if so how do I go about connecting it ?
Also is there any advantage to be had aside from much lower cost of Tv Diodes

Thank you
 
#2
There is a major disadvantage to a bridge made from 4X vacuum diodes, a doubled forward drop loss.

Bridge rectifiers are used with a simple, not CT, winding. Assuming a "typo", your power trafo is 450-0-450. To use those low cost TV tubes (probably damper diodes), continue with the FWCT topology you're familiar with, constructed with 2X of the TV bottles. Just tie the cathode connections together.
 
#5
Numerous damper diodes had current ratings over 250 mA. The common 6AX4 would barely make that with a pair; 6AU4s will do 350 mA with 25V drop. There were higher current ones; but not as common. 6DT3 has 20V drop at 680mA...

But these may not be so common outside the US. If you were in the US, I'd send a bunch for the postage...
 
#6
Another "gorilla" among damper diodes is the 6CJ3. Keep in mind the fact that the DC current rating is for a single bottle.

If you're dealing with a simple winding and bridge rectification is a must, go hybrid. Vacuum diodes in the "hot" legs and SS diodes in the "cold" legs yield performance that's dominated by the vacuum diodes, without incurring the doubled forward drop penalty.
 
#10
How many volts B+ do you need?
How many milliamps B+ do you need?

Using a 4 tube bridge rectifier to rectify a 450-0-450 transformer (and the whole 900V secondary), and a capacitor input filter,
you will get about (900V * 1.414) - 2 rectifier voltage drops.
That will be about 1270V - (60 to 120V for two rectifier drops).

Using a 2 tube in full wave mode, using the center tap, and secondary 450-0-450, and a capacitor input filter you will get about (450V * 1.414) - one rectifier drop.
That will be about 636V - (30 to 60V for one rectifier drop).

If you use a choke input filter (with a swinging choke), and the choke has at least critical inductance for the current drawn, then replace the 1.414 numbers above with 0.9.
Recalculate with 0.9, and you will get the voltage output.

In order to power a given milliamp current, the full wave rectification using the center tap will present an easier load on the secondary, versus a bridge rectifier across the whole 900V secondary.

We are back to how many volts and how many milliamps do you need?
And, what is the transformer secondary current rating?
 
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