Bridge Diodes and sag/noise

[Cygnus X1]

I know this must have been addressed more than a few times, but I could not find an answer by searching.

I have been building my single ended lead amps with a Weber kitchen sink type PT that does not have a center tap. So I went with a full bridge rectifier (4 diodes).
I love the sound of this amp-with all the distortion it provides it is very tight and very quiet.
I'm thinking part of the sound is a result of that full bridge diode setup.

Now here is the rub: Weber no longer sells that power transformer.
I can find ones that provide similar voltages with the required current rating, but they are center tapped.
I could use two diodes.

But, here is my quesion-is the 4 diode setup better?
Could I just ignore the CT and do a full bridge setup?

Or is there really no difference between the two concerning sag and noise?
Somehow I am thinking it achieves better filtering with the 4 diodes. As well as less sag-which is what I want to maintain for this amp.

[paulster]

A centre-tapped tranny with two diodes in each leg achieves electrically exactly the same as a full-wave bridge rectifier, save for the fact that it's not as efficient a transformer and therefore has to be correspondingly larger because you are using one winding at a time rather than the entire winding all the time.

In both methods of wiring the current has to pass through two diodes at a time, so you'll have achieved the same.

[Andy Le Blanc]

I dont have any noise figures for a full wave vs. full wave bridge.

But the full wave ( with the center tap) does better with current than a bridge.
It gives a peak V = .71 x sec. V A.C. and I D.C. = 1 x sec. I A.C.

A full wave bridge does better with volts.
it give a peak V = 1.41 x sec. V A.C. and a I D.C. = .62 x sec. I A.C.

Run the numbers with the tranny you have and then source the next part
It doesnt matter really, as long as you get the numbers you need for your design criteria.

with a bridge youll need a tranny with around half the volts but 40% more
rated current compared to a full wave .....

I like a hot cathode bias, so I tend to go with a full wave and get more juice
at the cost of B+.

[Cygnus X1]

<snip>
With a bridge youll need a tranny with around half the volts but 40% more
rated current compared to a full wave .....

I like a hot cathode bias, so I tend to go with a full wave and get more juice
at the cost of B+.

Thank you all.
I get weak around power supplies so I trust your opinions.


So Andy, I'm running a higher than spec voltage at the outset (340V, which somehow comes out dead on at 301V after the rectifiers).
Current transformer spec is non bridged 125ma, the kitchen sink tranny I am using is rated at 150ma.

So just go ahead and go a tad higher on the B+ in order to end up with the 300V I need?

Oh well, I'm overthinking this.
The specified transformer is a Hammond 270EX.
275-0-275 @ 125ma.
Which is confusing enough because the AX84 drawing spec's that transformer and shows 301V prior to the full wave diodes.
I selected 340V on the Weber and it came in dead-on at 300V after the bridge config.

The Hammond is around 60 dollars most places.

That Weber was only about 45.00 IIRC.
Given what you said, then the Weber was under rated?
I thought it would be over at 150ma.
It seems to run fine.

[Andy Le Blanc]

The numbers information is from Hammond.

The specified transformer is a Hammond 270EX.
275-0-275 @ 125ma.

fullwave should be around 390v peak , 247v avg. and 125ma.

bridge ...... 775v peak, 495v avg ..... but..... 77.5 ma.

your weber seems right on, but id look at its current rating compared to the
tube type your running. make sure theres enough with a bridge rectifier.

[Cygnus X1]

The numbers information is from Hammond.

The specified transformer is a Hammond 270EX.
275-0-275 @ 125ma.

fullwave should be around 390v peak , 247v avg. and 125ma.

bridge ...... 775v peak, 495v avg ..... but..... 77.5 ma.

your weber seems right on, but id look at its current rating compared to the
tube type your running. make sure theres enough with a bridge rectifier.

2x12AX7, single 6V6, 6L6, or 6550.
Cathode biased.
No effects loops, single channel.
Cathode follower TMB tone stack.

[Andy Le Blanc]

so with the weber @ 300v...

a 6l6 to get a conservative 19w max plate dissipation..... 64ma.

you see what I'm getting after...

6v6 14w max plate @ 300v ...... will need around 47ma.

6550 35w max plate @ 300v ....... 117ma.

there are benefits to running a higher plate voltage, watts, linearity, better
distortion figures etc..... but current requirement should come first.

The 6550 is a different class of tube, Id design for its requirement, but you
wont necessarily have a amp thats flexible enough to meet the conditions
for the other tubes, and vice versa.

and this is just one way to ball park the numbers, I know that others do
it in other ways..... In my mind I look at current first.

For example

I have a KT 90 pair on a generic twin PT thats 690v @ 310ma and a full wave.
The PS numbers say I should be getting an avg. 310v and peak 490v and 310 ma.
The reality is a plate voltage of 475v and 66ma static 32w per tube. 132ma static
current draw on a 310ma rated tranny doesn't look bad, but I cant operate
the unit at a lower bias to take advantage of the KT90 with out encountering
excessive heat in the PT. As it stands its a great amp. But one can look
at the numbers and summize that one needs a rated current thats 2.5 x
the expected total static current draw of the power tubes.

[dehughes]

You guys are great....this is just the sort of info I need to cut and paste...

[Cygnus X1]

You guys are great....this is just the sort of info I need to cut and paste...

Damn straight!
Sometimes they even kick me in the butt enough to just go out and learn for myself.

Which I have learned, plenty.
Still I get nervous around power supplies and ask at will.
And they still put up with my nonsense.


Best forum there is, right here.