2 G-Pultecs in 1 BOX

Hi,

I'm building 2 Gyraf Pultecs in one enclusre with one set of knobs...

I had some problems with hum and hf noise in the last tests...
so I changed the location of the boards and put the big power transformer out of the box...and it's really better now...

but I'm still unsure how to connect the both pcbs with 2 transformers

actually I have a 2*12V 120VA toroid to drive both pcbs
and a 2*12V toroid 30VA to get the 220 V for the tubes


I still remember some interesting statements I read in the last days of the old board...

"you have to connect both windings of the trafo, otherwise you get hum"

"never connect more than one rectifier to one trafo"

what should I do??

first of all:
is a 50va "main transformer" enough to drive both pcbs ??
I have to replace the 120va because the second winding delivers only 10V , the other 17V so it is broken...

should I connect only one lead of the 2*main trafo to the first pcb and then grab the power for the 2nd after the rectifier of the 1st pcb ??

or should I use both "windings" of the trafo and each set of rectifiers on every board ?? (use the 2*trafos as I would do with 4 single trafos)


so any advice is really welcome
thank you


here are some preview pics I shot at my last test setting...









more pics if I have finished...


cheers,
matthias

Nice knobs. Very nice face.

ya, you can even see the reflection of you taking the picture =)

ju

UnbelieveAble!

Awesome!
Build me one!



:guinness:

super!
hope to have this quality of boxes when i start to build one
nice color :green:

tdB

Matthias,

first of all:
is a 50va "main transformer" enough to drive both pcbs ??
I have to replace the 120va because the second winding delivers only 10V , the other 17V so it is broken...

Click to expand...

It's not necessarily broken. The load on the two windings is very different, so you'll get different voltages "under load". If you're unsure - measure the transformer with no load.

50VA should be enough for two boards.

The simple way to run two boards from one psu would be to have only one psu - mount the psu components on only one board, and then take the two DC voltages and the ground to the other board.

Maybe have the 3k3 HT PSU resistor and the 220uF/350V on both boards to keep them separated.

Jakob E.

thank you jakob !!

when I use the 50VA 2*12 transformer and use one winding to supply the first pcb, does it delivers 50VA or only the half of ... 25 VA... ??
so do I have to connect both wind. together ??

hmmm... ???

you see...

I'm very confused :? :shock: :roll: :?: :wink:

Transformer power ratings are not easily split up like that. But as a rough estimate, you can draw half the VA rating off each secondary.

If you use one winding to make the 6.3V heater voltage for both boards,, that should consume 2x365mA = 730mA in total. At a 12V secondary winding this is 8.7VA

The HT consumes some 5mA per board - 10mA in total - at 250V this is only 2.5VA on the other winding - plus all the loss in transformers etc..

So you should have plenty of power in a 50VA first transformer

Jakob E.

thank you jakob!!

should I do the same for the 220 V from the second trafo...
then I would need these big caps (220µ , 400V) only on one board ??

(that will be a "soldering night" ... tonight

matthias

I would use both 220uF/350V caps on the one board that has the actual PSU, and on the other one only the 3K3 resistor and the last 220uF/350V. Connect from the junction of first board's 220uF/350V and 3K3 resistor to the same point on the other board.

This way HT will be individually filtered in the two channels.

And remember - TAKE CARE when working with these high voltages..!

Jakob E.