Shedding Voltage on Tube Rectifier

[Paully]

We are trying to shed some of the voltage coming off of the transformer in the legs going to a GZ33 tube rectifier. The idea is making the voltage lower earlier on in the power supply chain so the downstream components don't see too high a voltage. The GZ33 will soft start the voltage.

Say we want to shed 250V and the circuit current, total, is 100mA. Does that split to 125V off of each leg? Does the current through each leg come to 50mA?

If it is split to 125V off of each leg, and the current on each leg is 50mA then we need 2500 ohms in each ac leg feeding the GZ33 plates, right?

Sorry for the cross-post for those of you who read Tube DIY but often it seems we get some really good responses hear.  Thanks!

[Paul Birkeland]

Hey Paully,

Give us some more details.  When you need to shed that much voltage, a better option to consider may be direct coupling or choke input. 

Note that putting a bunch of series resistance in will generate a ton of heat and introduce poor load regulation.

Do you have a schematic of your power supply? 

[Paully]

I don't have any problem putting the resistors after the rectifier if that makes a difference.  The rectifier is a GZ33 so it should be good for 1000V (500-0-500) and we are using a 375-0-375 transformer.  Just thought that running the rectifier lower would be better.  Need to get around 450 out.  But the power supply is a simple CLC filter that isn't going to drop anything like the voltage we need it to now that we have changed our plans around.  The inductor is 20H 181 ohms.  The power transformer is rated for up to 200ma and we are only using 100ma, if that makes any difference either.

[Paully]

PB,

Grainger here.  We didn't give you all the information.  We are in the assembly stage of a tube rectified, 76 driver, 45 output tube amp with dual PDMPS.  That is, after the rectifier (GZ33) there are separate filters/rails for the two channels with a split (here is the PDMPS) for the driver and 45 voltage.  

We see that we have too much voltage.  So we now just want to shed some of it.  This, even if Paul needs to buy another, lower voltage, power transformer.  

Another option to shed some of this voltage, mentioned on Tube DIY asylum, was to regulate the B+.  Something I'm not up on.  We do have one of Paul's old Paramount C4S/Driver boards which might be used if the voltage drop (250V) is not too great.    Ok, thought about it and realized that the current for the 45s is 70mA, the transistors are not going to last at that current.

[Paully]

Ok, I looked at the current ratings for the parts on the old Paramount boards.  They might work?  I think at this point my options seem to be to use the regulator section of the old Paramount boards and figure out the resistance I need.  Still think there are problems there that I don't see, like can a 5670 stand up to this, are the transistors really rated for 100ma, etc?  I am thinking my easiest and simplest option is to go ahead and pop in the resistors straight into the power supply till I get the voltages I need and then by the right sized transformer and take out the resistors though that is certainly the least interesting.

[Paul Birkeland]

Ah you want to shed voltage from the Paramount transformer/power supply board to run 45's?

I'd recommend the direct coupling with an approach like this, especially since you have a tube rectifier to soften the startup. 

[Paully]

That wasn't our original goal, regulating the B+ in any fashion, it was just mentioned as a possibility to use up the excess voltage.  But it seems to be beyond us and more than we were planning on for this weekend of finishing up the build.  I think I am going to go the simple route and just get a properly sized PT once we get it zeroed in.  Plus we aren't actually sure what direct coupling would be in this context.

[Grainger49]

BTW, there was no excess voltage, not even expected out of the transformer.  Check this thread:

http://www.bottlehead.com/smf/index.php/topic,3108.msg0/topicseen.html#new

[Paully]

Obviously we had some confusion on how a tube rectified power supply works.  But now we don't!  Anyway, we got lucky and after some adjusting we have the voltages we need with the PT I bought.  It is working and sounds marvelous.  Thanks for the help and the Bottlehead parts we bought and used in it certainly make a difference in making it sound good.

[Grainger49]

Ok, the site didn't like the size of the photo so I'm linking it.

The Mongo 45 is in the foreground, back of that are my Paramours (Zebra Wood) and the FP 2 on the left (light wood base) my Eros on the right (dark wood base).

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi244.photobucket.com%2Falbums%2Fgg7%2FGrainger49%2FFor%2520Posts%2520Private%2FIMG_1041.jpg%3Ft%3D1341709563&hash=ec52dca573381151fe9590beae9e2731fd0f83e8)

This is one sweet sounding 45 amp.  It is parafeed to MQ all nickel output transformers, a BPC-15 and 76 drivers.  It uses the FC-1 to filter the 45 heaters.  Those are not the output transformers in the back on the right and left, they are the chokes in the power supplies.

BTW, with a low mu tube, the 76, for the driver and 94dBW speakers there is more than enough volume in my room.  Paully stepped out and returned to Taiko music shaking the walls.  Trust me, it plays loudly! !