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Dropping B+ problem!

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2/10/2000 12:02 PM

Dropping B+ problem!

I tried to wire up the B+ dropping trick from Randall Aikens site – the one with a power transistor and a low watt zener between the PT CT and ground.

I used a 30V zener – but instead of a 30V drop the voltage increased from 430V to 475V - maybe more!!! I shut it down quickly when I saw the voltage go up.

Given the simplicity, I’m allmost positive I had it wired up right – (writting this I realize that I could have done some measurings - maybe on the 2N3055 – that could have been helpfull trying to pinpoint the problem/my mistake!) – but I ripped it out, being afraid to exceed max ratings of my filter cap ect.

So, before I give it another try – what do you think!
Any thoughts will be appriciated

Regards
Lion

2/10/2000 5:04 PM
Ken Gilbert	lion, are you telling me you got MORE voltage out of your power supply with the x-istor and zener in place??? that would be a neat trick, i must say... ken

2/11/2000 7:10 AM

Lion
I used this B+ dropping method in a silver face deluxe. It lowered the voltage about 35 volts but it took a 70 volt zener to do it. I was able to use a 5ar4 rectifier instead of the specified 5u4gb. I think your problem may be in the bias supply. I had to increase the supply side resistor in the bias supply, if I remember correctly. Try the mod without the power tubes. check the voltages on the plates and bias supply before and after.

Tracy

2/11/2000 7:14 PM
D.P.Coyle	Trying to drop the b+ of a power stage with a voltage divider is useless because of the too high source impedance it presents. I have had sucess puting an inductor (say 5 henry) after the diode and before the first capacitor of the power supply. this causes the voltage to be 0.707 of what it would be in a convedntional pi circuit.

2/13/2000 4:25 PM

Thanks for the replies!

Ken!
>lion, are you telling me you got MORE voltage out of your power supply with the x-istor and zener in place???
that would be a neat trick, i must say...<

Your are right that would really be something!
But I’ll have to disapoint you ;-)

;-)

I made a mistake (again!)

The thing is, that it turned out that the PT in this amp has 2 (two) taps grounded???
I haven’t noticed that before wiring in the zener and the transistor.
Moving the zener and the transistor to the other tap did the trick – the voltage dropped.
So, just to make sure that I wasn’t kidding myself in the first place, I moved back to the first tap again , and – the voltage increased, approx. 50V!

Is it normal to have two grounded taps on a PT?

Tracy!
I use a 30V zener a the drop is excactly 30V. Re-biased after the drop, of course!

Regards
Lion

2/13/2000 9:12 PM
Don Symes	What would happen if you had wired a 120/240V primary correctly for 240VAC in, and then zenered the 120V tap to gnd? Might this be the answer?

2/14/2000 8:29 AM
Bruce	You might have accidentaly grounded the bias supply tap thinking it was a center tap. There cannot be two center taps... by difinition! Think of it... Bruce

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