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B+ voltage drop

Hyakuya

Hyakuya

Active Member
Hello.

I need to drop about 40Vdc from the B+ and reading up has brought up a couple of different methods so I thought I'd ask the Audiokarma for some advice.

I'm starting with a 410-0-410 PT secondary through a GZ34 so using 410x1.41 for a rough idea I get about 575Vdc. I need about 535Vdc so a 40Vdc drop is needed.

I can use a resistor before the first filter cap. I think about 270R will do it and it was the plan until I read about using zener diodes on the PT CT. As I understand, a series of 5 x 10v diodes between the CT and ground should drop about 40v but I've never heard of this method so I'm unsure about it.

Does anyone have any thoughts or advice on zener diodes or the best method to drop voltage generally.

Many thanks
 
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I don't think you want to put the resistor before the first cap. I would add a CRC filtering section further down the chain, which gives the benefit of also creating more filtering. The other option is to use a different rectifier tube to drop the voltage.
 
If your power supply transformer 5 volt can handle 5U4 heaters current, (3A instead of 1.9A) you can use a 5U4GB instead of a GZ34, you can get more vdrop from a 5U4GB , 50V , vs 17V

Tube# - Base - Fvolt - Famp - Vdrop - MaxPmA - MaxPv - notes
5AR4/GZ34-5DA - 5.0 - 1.9 - 17 - 250 - 450 - low Vdrop 5U4/5Y3
5U4-G - 5T - 5.0 - 3.0 - 44 - 225 - 450 - octal 5Z3
5U4-GA - 5T - 5.0 - 3.0 - 44 - 250 - 450
5U4-GB - 5T - 5.0 - 3.0 - 50 - 275 - 450
 
Look up the 6087. It's a 5y3 with slow heater startup like the 5AR4. I've used it to drop extra B+, just check your heater ratings.
 
the problem with resistors is they make the regulation lousy. If this is a class A amp where the current demand is constant, no problem. If this is an AB amp, then it can be a serious issue. Either way I'd go with @StepheK 's suggestion of the RC filter rather than just a resistor.


The zener diodes on the center tap will work. Big benefit to zener diodes vs a resistor, the voltage drop constant. I would probably use a high wattage resistor to carry most of the load, and strap the zeners across it to fix the voltage drop, maybe with a 10 ohm resistor between the zener string and ground just to limit max current through the diode string.
 
Hyakuya said:
Hello.

I need to drop about 40Vdc from the B+ and reading up has brought up a couple of different methods so I thought I'd ask the Audiokarma for some advice.

I'm starting with a 410-0-410 PT secondary through a GZ34 so using 410x1.41 for a rough idea I get about 575Vdc. I need about 535Vdc so a 40Vdc drop is needed.

I can use a resistor before the first filter cap. I think about 270R will do it and it was the plan until I read about using zener diodes on the PT CT. As I understand, a series of 5 x 10v diodes between the CT and ground should drop about 40v but I've never heard of this method so I'm unsure about it.

Does anyone have any thoughts or advice on zener diodes or the best method to drop voltage generally.

Many thanks
Click to expand...

How much current will the circuit draw? In a typical PP amp a 5AR4 is going to drop 40 volts, if not a bit more. Is that not enough?
 
more like 20, but yeah if these numbers are un-loaded its going to be way high. A spitball estimate of current load, and ideally a couple of ohm measurements on the transformer would allow for a useful model in PSUD2.
 
If you want the vdrop of a 5u4, but it draw too much heater current for your PT, you can try a Weber Copper Cap WU4GB, solid state drop in replacement for a 5U4GB

You may notice a difference in sound versus a tube rectifier, you may like it or not.
 
Wow, thanks for all the responses. Lots of great ideas.

The 5v windings on the PT are rated at 3A. The amp was using an Edcor XPWR033 which I'm replacing but I measured the current draw with that at about 150mA idle.

So, I could look into just changing the rectifier but I hadn't thought about adding a RC filte stage and that does appeal.

I can use PSUD to work out the resistor value but what are the rules for capacitance.
 
Is it a SE amp or a PP amp?

Might as well do the easiest solution and not complicate things more than they need to be. If it's an SE amp, I'd use a resistor dropper and be done with it. If it's a PP amp it needs to be thought out more carefully as to the best solution.
 
Hyakuya said:
Hello.

I need to drop about 40Vdc from the B+ and reading up has brought up a couple of different methods so I thought I'd ask the Audiokarma for some advice.

I'm starting with a 410-0-410 PT secondary through a GZ34 so using 410x1.41 for a rough idea I get about 575Vdc. I need about 535Vdc so a 40Vdc drop is needed.

I can use a resistor before the first filter cap. I think about 270R will do it and it was the plan until I read about using zener diodes on the PT CT. As I understand, a series of 5 x 10v diodes between the CT and ground should drop about 40v but I've never heard of this method so I'm unsure about it.

Does anyone have any thoughts or advice on zener diodes or the best method to drop voltage generally.

Many thanks
Click to expand...
You did not answer how much power / current is involved.

But one solution that is within reach today is :
a cap after the rectifier ( 575Volt)
A FET something transistor with cooling adjusted to your 535 Volt ( there is lots of suggestion about how to create a stabilized DC using a FET and some regulators)
more caps after the FET.

The FET used as well as the amount of cooling depends on power needed.
 
kward said:
Is it a SE amp or a PP amp?

Might as well do the easiest solution and not complicate things more than they need to be. If it's an SE amp, I'd use a resistor dropper and be done with it. If it's a PP amp it needs to be thought out more carefully as to the best solution.
Click to expand...

It's a Kegger / Blueglow KT88 SE.
 
Ah. Put a resistor in between the rectifier and first filter cap. The size needed is significantly smaller than you might think because a resistor in that location sees both AC and DC current. Model it with PSUD and once you find the resistor size use the reading of current through that resistor to determine its wattage.then quadruple the wattage size for some thermal headroom.

I did this exact thing just recently. But it’s not the only solution of course.
 
kward said:
Ah. Put a resistor in between the rectifier and first filter cap. The size needed is significantly smaller than you might think because a resistor in that location sees both AC and DC current. Model it with PSUD and once you find the resistor size use the reading of current through that resistor to determine its wattage.then quadruple the wattage size of that resistor for some thermal headroom.

I did this exact thing just recently. But it’s not the only solution of course.
Click to expand...

I worked out the resistance needed as 270R and about 6W.... does that sound about right
 
Resistance seems a little large. Did you account for AC current? Did you model the conditions in a circuit simulator?
 
This is where I get confused....

I modeled the PSU in the Blueglow schematic with measured resistance values for the PT and Choke. This gives 433Vdc at the second filter cap but I saw 460Vdc on my build.

1.png

Am I doing something wrong here because I can't model a dropping resister or addional RC filter stage if the model is wrong in the first place.
 
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