How To drop B+ voltage in transformer without center Tap

[whoops]

Hi,

I have a PT without center Tap and would like To drop the B+ voltage by 50 or 40vdc,
Im usind a solid state bridge rectifier.

After readind a lot on the subject I found 2 efective ways of doing that with a 40/60v Zener 25 To 50 watts like the Webber V dump or using a 75 watts Mosfet ( links underneath) the problem is that the solutions presented are applied on the center Tap of the PT, could the Zener or the Mosfet Be used for the same purpose in a PT with no Center Tap?
How could I connect them?

Thanks

V Dump Zener solution:
https://taweber.powweb.com/store/vdump.htm

Mosfet solution, scrol To the end of page:
http://www.geofex.com/Article_Folders/mosfet_folly/mosfetfolly.htm

[sluckey]

You can put the zener between the bridge negative terminal and ground. I don't advise this if you have a negative bias supply that gets it's AC input from the PT HT winding.

You can also connect the zener in the B+ line. Connect the cathode to the bridge positive and connect the anode to the filter caps. Using this approach you must insulate the diode from any heat sink that connects to chassis ground.

[sluckey]

Is this the Hoffman AB763 amp with the arcing power tube problem and funky PI voltage readings? Did you get that stuff fixed?

[whoops]

Hi Sluckey,
Thank you so much, I Will use you recommendations for the Zenner Diode.
Im getting the negative bias supply from a different secondary, its a 50v secondary that its being only used for the bias supply.

Would it Be possible To use the Mosfet solution also without the center Tap in the PT?
The v dump its out of stock, and also the Mosfet is cheaper.

This is for the same hoffman AB763 amp, its still not fixed but we are getting there, the arcing is gone now, I Will Post the developments in the thread as soon as I have more info.

Thanks for your kind help and knowledge sharing!

[John]

Speaking from my own hard experience (though very little) I strongly advise you to get the amp working flawlessly before messing with anything else.

[whoops]

Speaking from my own hard experience (though very little) I strongly advise you to get the amp working flawlessly before messing with anything else.

Hi John, thanks mate.
Lowering the B+ voltage is the next step to make the amp work flawlessly.
Im just trying to find the best ways to make that happen, the Zener diode is a good solution,
maybe the Mosfet also, Sluckey nicely explained how to implement the Zener in a no center tap bridge rectifier scenario,
I dont know how to implement the Mosfet in this scenario yet.

Thanks

[John]

Lowering the B+ voltage is the next step to make the amp work flawlessly.

Ah! Now I see.

I've suffered from the "well I've got it open already so I'll change this at the same time" syndrome... it's rarely gone well. 

[jjasilli]

IMHO, a power resistor is the simplest solution to drop up to 50VDC, in the B+ line after the diode bridge.  However, this tends to emulate a tube rectifier and cause sag.  In a PP amp, under signal conditions in class A/B operation, the power tubes draw more more current through the resistor and cause a voltage drop.  To enhance this effect use an INductive power resistor.

[whoops]

IMHO, a power resistor is the simplest solution to drop up to 50VDC, in the B+ line after the diode bridge.  However, this tends to emulate a tube rectifier and cause sag.  In a PP amp, under signal conditions in class A/B operation, the power tubes draw more more current through the resistor and cause a voltage drop.  To enhance this effect use an INductive power resistor.

Hi jjasilli, dropping 50vdc with power resistors would produce too much sag and make the amp a bit goofy,
I will use or the Zener or the Mosfet to drop voltage, thanks.

[sluckey]

I've never tried it, but, the MOSFET should work just like the zener. Even though the circuit has 4 components, you connect it to the bridge just as you would the zener. Look at the link you posted.

[kagliostro]

In the link you posted pay attention where is ground

the first time I looked the schematic I've seen the ground on the wrong side of the circuit

(fault of my brain)

K

[whoops]

So in this picture the circuit on the left is to be applied in a transformer with center tap, the circuit in the right is to be used in the B+ Line not needing a center tap, is that right?

Thanks

[sluckey]

Theoretically, this MOSFET circuit can be used ANYWHERE you would use the simple high power zener.

[whoops]

Thanks,
Im seaching in farnell for possible Mosfets,
do you have any advices on what Mosfet specs to search for?

[sluckey]

in the meanwhile I found this schematic also:

You don't want to use this circuit. It's a voltage regulator. The output voltage will be equal to the zener voltage.

[SILVERGUN]

sluckey,,,how about the bucking xfmr idea for this application?

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

[whoops]

in the meanwhile I found this schematic also:

You don't want to use this circuit. It's a voltage regulator. The output voltage will be equal to the zener voltage.

thanks
I Edited the Post and removed that circuit to avoid confusion.

[whoops]

I've seen that some people used the IRF820 Mosfet in this aplication,

http://pt.farnell.com/vishay-formerly-i-r/irf820apbf/mosfet-n-500v-2-5a-to-220/dp/8648506

I'm a bit concerned that this Mosfet wouldnt deal with a 40vdc or 50vdc drop very well, because of it has a 30V Voltage Vgs Max.

I'm sure there's a lot of Mosfets that could be used efficiently to achieve this, I just dont know what specs to search in Farnell.

Should I be searching for a Mosfet with an Voltage Vgs Max above 50vdc , and whats the difference between "Power Dissipation Pd" and "Power Dissipation Ptot Max" ?

Thanks

[jjasilli]

sluckey,,,how about the bucking xfmr idea for this application?

http://www.geofex.com/article_folders/vintvolt/vintvolt.htm

Probably too much iron and space for a small voltage drop.  Also the small tranny may also cause sag which Whoops does not want.  I have used a bucking tranny + power resistor to drop about 100V in a Traynor EL84 Bassmate with stock 410V on the plates. 

[sluckey]

I'm a bit concerned that this Mosfet wouldnt deal with a 40vdc or 50vdc drop very well, because of it has a 30V Voltage Vgs Max.

The 40 or 50 volts you want to drop is felt across the Drain and Source, not the Gate and Source.

I really don't know why you even want to drop the voltage though. Your B+ was only 500V with the tubes pulled out. When you fix the real problems with that amp and put the tubes in, the B+ is going to drop to a normal value for an AB763 amp that's loaded with 6L6s.

[whoops]

I'm a bit concerned that this Mosfet wouldnt deal with a 40vdc or 50vdc drop very well, because of it has a 30V Voltage Vgs Max.

The 40 or 50 volts you want to drop is felt across the Drain and Source, not the Gate and Source.

I really don't know why you even want to drop the voltage though. Your B+ was only 500V with the tubes pulled out. When you fix the real problems with that amp and put the tubes in, the B+ is going to drop to a normal value for an AB763 amp that's loaded with 6L6s.

Thanks for explaining that, I was not getting what was Vgs and Vds. So it seems that the IRF820 or IRF830 would be good candidates for this circuit.

I want to reduce B+ because it's too high, I would like to have it more like 440vdc-450vdc. Most of the problems in the amp are fixed now, and the amp is already working, Power amp it's fine, there's still some work to do on the Pre amp. The B+ voltage with all the tubes plugin in is 497vdc, and even if it works that way, its pretty in the limit on the power tubes and the filter cap and I don't want that.

Thanks

[sluckey]

The B+ voltage with all the tubes plugin in is 497vdc, and even if it works that way, its pretty in the limit on the power tubes and the filter cap and I don't want that.

I'm surprised it's that high. You said in your other thread that the voltage was 425 with tubes in.

Then your last word on voltages was...

Hi Brad,
the voltages are 501v (plates) and 498v (screens) because it was measured without the 6L6 in the sockets. With the 6L6 in place those voltages are lower.

Glad to hear you got it fixed. Would be nice if you would post the solution in your other thread, particularly what you did to fix the sparking inside the tube and those crazy cathode voltages on the PI tube. A lot of people were trying to help you and I'm sure they would be interested to hear your solution.

[whoops]

Glad to hear you got it fixed. Would be nice if you would post the solution in your other thread, particularly what you did to fix the sparking inside the tube and those crazy cathode voltages on the PI tube. A lot of people were trying to help you and I'm sure they would be interested to hear your solution.

Yes, That will be done and I will also post the current status of the amp and the new found issues , I'm glad people were so nice.