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Variable power supply for supplying valve amp builds

Solidus

Jun 19, 2011
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As the title suggests, I want to build a variable power supply that allows me to supply valve amplifier prototypes with a B+ supply on the fly.

It beats having to have a hefty (and damn expensive) stock of different power transformers around.

Here's what I've considered to this point:

-Putting a high-wattage potentiometer/variable resistor in series with the primary of say, a 600VAC transformer. Rectification would give me up to 800V easily.
-Making a multitapped transformer, that would allow me to change primary windings to modify the output voltage.
-Variac supply, which I don't want to do, knowing the exorbitant cost of Variacs and that I'm working on a student budget.

I don't want to jimmy down the output using resistors, as that would require I have a large stock of varying resistances around and would also assume I have a pretty well-factored load. With building tube amps, only so much can be garnered from the datasheets and at least to me, nothing beats mocking them up on a breadboard to test that everything operates as it should.

Ideally I'd like to build a constant voltage source, that is, one that would hold the output voltage within a certain degree assuming the load changes a reasonable amount, but how exactly I'd go about this dictates whether indeed that is possible. It doesn't need to go to an extreme degree, say 600V max. The larger RF tube projects I'd still break down and buy a transformer that would give me dedicated output voltage for my needs.

Does anyone have any ideas? I've tried googling this, but information on builds of this type seems pretty scarce.
 

CDRIVE

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May 8, 2012
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It wouldn't be regulated but a Variac feeding the input of your transformer would give you the adjustable output voltage that you want. HV regulation was not a common feature in the day.

Chris
 

Solidus

Jun 19, 2011
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That makes sense. It also wouldn't be very hard to house the HV transformer and rectification/filtering components together so no matter the input voltage, the output is filtered DC ready to use.
 

duke37

Jan 9, 2011
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You could use a switched potential divider across the HT and then feed a cathode follower. If I remember rightly, a 12E1 was used for this but any power valve would do. The heater would need a separate supply because of heater/cathode insulation limits.
 

CDRIVE

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Duke, I like that. good thinking! ;)

Chris
 

Solidus

Jun 19, 2011
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You could use a switched potential divider across the HT and then feed a cathode follower. If I remember rightly, a 12E1 was used for this but any power valve would do. The heater would need a separate supply because of heater/cathode insulation limits.

How would I wire that - would I use a potentiometer to feed the grid?

Heater supply would not be an issue as many HV trafos designed for valve use carry 1-2 6.3V windings.
 

duke37

Jan 9, 2011
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I suggested a switched potential divider instead of a sliding potentiometer since the voltage is so high. You could use a pot if you could find one fat enough.

If you use a tapped resistor chain with a switch, make sure the resistors and switch are capable of the duty.

If you use 10k resistors with 100V across each then each resistor will dissipate 1W (use 2W resistors) then special high voltage resistors will not be needed.
 

Miguel Lopez

Jan 25, 2012
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A valve regulator would do the job. What duke37 says is very important cause the heater-cathode insulation has limits. See the attachment #1.

I once designed a circuit for this purpose for a friend, using transistors. The schematic includes a current protection circuit and a rapid discharge circuit for the caps. See attachment #2.
 

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CDRIVE

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The voltage requirement that Solidus is desirous of is quite a bit higher than your circuit provides. Interesting though but could use some component values. ;)

Chris
 
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Miguel Lopez

Jan 25, 2012
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CDRIVE said:
The voltage requirement the Solidus is desirous of is quite a bit higher than your circuit provides
I know Chris. It was just an example. By adjusting components values, he can get the desired voltage.

('steve) said:
Excellent!
The valve equivalent of a three terminal regulator.
Come on Steve.:)
I was talking about the first image in my post. Althought, thinking better on it....with proper wiring and ECL82 could be considered like that. :eek:

If there were valve operational amplifiers, why there wouldn't be valve monolithic voltage regulators :D. Something like a 78-250 or a 78-300. :D
 

Miguel Lopez

Jan 25, 2012
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Here is the schematic with the values that I used.

To increase the output voltage, you must increase the value of Vnr, the 100k potentiometrer should be increased too (depending on the output needed), the transistors must be selected properly, etc.

Several calculations have to be made. I can do it if needed.
 

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CDRIVE

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Actually, the vacuum tube model interested me. ;)

Chris
 

Miguel Lopez

Jan 25, 2012
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Sorry Chris. I apologize for the misunderstanding. Please, be more especific next time.

I would have to calculate (or simulate) those values. Sorry. It will take some time. Try these examples:
 

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CDRIVE

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No, don't go through that trouble just for me. I'm not the OP. I thought it was a circuit that you designed.

Chris
 

Weezykid

Jul 24, 2013
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If this should be in a new topic, please let me know, but it is along the same subject.

What about using a couple resistors and switch to drop the b+ volt before it powers the ouput tubes? You could cut the power by simple 1/2 and a 1/3.. (Possibly more with more resistors to have multiple dropping steps). Just not sure how to decide what resistor values to use and the type of switch..
 

CDRIVE

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If this should be in a new topic, please let me know, but it is along the same subject.

What about using a couple resistors and switch to drop the b+ volt before it powers the ouput tubes? You could cut the power by simple 1/2 and a 1/3.. (Possibly more with more resistors to have multiple dropping steps). Just not sure how to decide what resistor values to use and the type of switch..

Resistive voltage dividers have limited applications. They're fine for circuits that draw minimal current but that's about it.

Chris
 

Miguel Lopez

Jan 25, 2012
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You can improve that idea, by adding a shunt regulator to the resistor divider circuit.
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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You can improve that idea, by adding a shunt regulator to the resistor divider circuit.

All linear regulators can be viewed as potential dividers.

A shunt regulator has an active "bottom" resistor.

A series regulator has an active "top" resistor.

There's a similar arrangement with buck and boost smps designs, but it's stretching the analogy. It breaks down totally with flyback and other topologies.
 
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