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Shaving voltage off HV supply?

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deicide67

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Joined 2008
#1
Is there any reasonable way to shave 100V off a HV supply?

I have an amp with a doubler circuit that has a B+ of around 680VDC, and I'd like to see it come down a bit to about 580-600V

It's an old Bogen, and the PS is rated to put out some pretty high current as well, so I am inquiring as to a method to shave the voltage while maintaining the high current output.

Thanks!

Blair
 
#2
What sort of current are you calling High Current. a 700V PSU is only going to be in the order of a few tens or hundreds of mA, This can be dropped fairly easilly with a resistor.

Best way is to go for a CRCRC (get the jist) power supply and drop a few volts at a time between the caps over the R's.
 
#7
deicide67 said:
Is there any reasonable way to shave 100V off a HV supply?

I have an amp with a doubler circuit that has a B+ of around 680VDC, and I'd like to see it come down a bit to about 580-600V

It's an old Bogen, and the PS is rated to put out some pretty high current as well, so I am inquiring as to a method to shave the voltage while maintaining the high current output.

Thanks!

Blair
Click to expand...

What is the power of your PSU ? If its < 100W or so you could just add a string of high power zener diodes(better run them below their power rating, lets say 3W for a 5W zener) to drop as much voltage as you want. If you know what's the max. current of the PSU then zener ratings and count could be calculated.
Resistors in series are a bad choice; voltage drop will highly depend on load. And reducing caps will greatly increase ripple voltage, also voltage drop will highly depend on load.
 
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#9
deicide67 said:
Hi,

It is an old Bogen MO-200, so I want to try running Kt88s in pentode off their spec'd ~100W PP off 560-580V.

Wouldn't reducing the caps significantly, then feeding a beefy choke eliminate the ripple issue?

Thanks!
Click to expand...

I don't think so. The problem here that without any load, you will arrive at your ~680V again. And depending on load there will be a serious drop in voltage, but, more importantly, ripple may become high. You could take an LTSpice and simulate if I am wrong.

By the way, why not go a proper way and use a high voltage reguator IC? There are available which can drop > 100V , like LM723.I am not sure if this particular unit can be used a a floating regulator but I don't see a reason why it could not. A floating regulator is not connected to ground; that way the regulator only sees the voltage across it and does not need to be rated at full voltage ouput.

Edit : I have entered wrong regulator IC. Should be TL783
 
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#11
deicide67 said:
Hi,

That's interesting. Why not just use 2-4 LM317s in series at max voltage?
Click to expand...

Heat! Usually, high value caps. are appropriate in a Greinacher, AKA "full wave", doubler's cap. stack. However, this is an unusual case. Remember, there's nothing especially innovative in employing a smallish cap. at the I/P of a CLC filter to hold the net rail voltage down and all voltage multipliers are cap. I/P filters. Ripple voltage is high and a large choke and a high value reservoir cap. are in order.
 
#12
deicide67 said:
Hi,

That's interesting. Why not just use 2-4 LM317s in series at max voltage?
Click to expand...

I see no reason why you couldn't use several LM317 in series in a floating configuration. However, there are some downsides of doing so:

1) The circuit will be quite complex, as each of 317's will have to work independent so each will need its parts(resistors , etc.).
However, there are also LM317HV available and you could probably use 2 of these to drop close to 100V.

2) If it happens you short the output ALL chips will fry without protection of some kind. This is because when output is shorted they will see the full output voltage of PSU. Protection can be incorporated by using a high power zener diode across input and output of regulator(or all of them) and a fuse before.

So, we can return back : just a simple string of zeners has its advantages 😉.

By the way : maybe the transformer has some other windings that are not used? If so , you could wire them in series but out of phase with the winding you are using; it would cause the voltage to drop by the same voltage that out-of-phase winding has. This only works with windings within the same transformer!
 
#14
What makes you say that a winding used out of phase to reduce voltage needs to be in the same transformer? It doesn't have to be. Anyway its a good way for the OP to reduce his final voltage. If there is room for a small transformer under the chassis it could be used to buck the voltage from the main tranny. Plus I think zeners, regulators or dropper resistors will generate too much heat and be wasteful. 100V at say 700mA is 70W after all.

Cheers Matt.
 
#15
Matt.B.H. said:
What makes you say that a winding used out of phase to reduce voltage needs to be in the same transformer? It doesn't have to be. Anyway its a good way for the OP to reduce his final voltage. If there is room for a small transformer under the chassis it could be used to buck the voltage from the main tranny. Plus I think zeners, regulators or dropper resistors will generate too much heat and be wasteful. 100V at say 700mA is 70W after all.

Cheers Matt.
Click to expand...

But if we say 100V@150mA is 15W, and that is reasonable with few zeners. That's why I asked what is the power rating of PSU in previous post.
About windings out of phase : you are correct as long as neither transformer is overloaded. So the second transformer should not be too small.

Regards,
Lukas.
 
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#16
It ran 8x8417 so I reckon somewhere around 5-700mA as the OP stated. Taking 700mA worst case its 70W. That is a bit too much to waste. I guess a bucking transformer of somewhere near 35-40V and 50VA would do the trick, may be too big to squeeze in.

Eli's idea of small cap-choke-larger cap is also a good plan and it works well. Sort of semi choke input supply. You can tune the size of the first cap to get the output voltage you require.

Cheers Matt.
 
#17
Bucking the transformer using one of the 6.3v windings gets me 241VAC unloaded on my secondary. So, roughly 680v out of the doubler unloaded.

I have no idea what my voltage drop will be with 200mA on my tubes at idle, but my experience is usually between 70-80% of the unloaded voltage. If it were 80% or 545V, then I would be extremely happy, but I doubt I'll get that lucky.

Thank you for all the suggestions guys! I will also add a CL-70 or something on the primary line which can drop me a touch also.

Regarding running modern KT88s on 500+V, I've done it on several amps at 510-520V with no issues. I hope my luck does not run short here.

Blair
 
#18
Looking at the tube data, it appears that each 8417 is probably running at about 65mA making total current less than 550mA. How about converting the power supply from doubler to full wave bridge? You should be able to get about 460V using a SS bridge (based on the 335V screen supply).
 
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