Hey there, I'm building a variable voltage regulated guitar tube amp (push-pull, 6v6, fixed bias) that consists of a preamp circuit and PI/power amp section.
I'm planning to vary the B+ to the power amp section of the amp (often called power scaling, VVR, etc) to achieve lower volumes while still running the output tubes in their prime range. In this scenario the preamp section will not be variable and will ideally remain the same voltage regardless of how the power amp is scaled in order to maintain the same preamp tone. (a master volume will adjust the preamp signal into the power section to avoid overdriving the output tubes). The problem is when you lower the output section voltage, you unload B+ and the preamp supply voltage, which is supplied by the same HT transformer winding, rises. There must be a simple circuit I can build to keep the preamp node voltage supply from rising (people who have done this voltage scaling have observed preamp supply voltages rising by 50 volts or so). I think my problem is I don't quite know the correct terminology to effectively research the implementation. Clamping, clamper, shunt regulator, nothing has given me much to go off of. I don't want to regulate the voltage but instead just limit it from going above say, 180vdc. I want to maintain the character of the voltage potentially dropping under load as that's part of the amp design. Do any of you have insight into how I should be finding the answer I seek? I feel like I'm missing something very obvious! I look forward to hearing your ideas. Thanks!
I'm planning to vary the B+ to the power amp section of the amp (often called power scaling, VVR, etc) to achieve lower volumes while still running the output tubes in their prime range. In this scenario the preamp section will not be variable and will ideally remain the same voltage regardless of how the power amp is scaled in order to maintain the same preamp tone. (a master volume will adjust the preamp signal into the power section to avoid overdriving the output tubes). The problem is when you lower the output section voltage, you unload B+ and the preamp supply voltage, which is supplied by the same HT transformer winding, rises. There must be a simple circuit I can build to keep the preamp node voltage supply from rising (people who have done this voltage scaling have observed preamp supply voltages rising by 50 volts or so). I think my problem is I don't quite know the correct terminology to effectively research the implementation. Clamping, clamper, shunt regulator, nothing has given me much to go off of. I don't want to regulate the voltage but instead just limit it from going above say, 180vdc. I want to maintain the character of the voltage potentially dropping under load as that's part of the amp design. Do any of you have insight into how I should be finding the answer I seek? I feel like I'm missing something very obvious! I look forward to hearing your ideas. Thanks!
How is your PSU set up?
If using separate PT windings is out of the question, I would look to separating the preamp and power amp PSUs as much as possible - don't have them in series, as is the rule in instrument amps.
So after rectifier, have one cap, and then from that cap separate the two PSUs completely. One wing for the power amp and scaler, and one wing via dropping resistor to preamp PSU.
Then they shouldn't interfere with each other much.
However in the normal series PSU, when the power amp gets a transient, it eats up more current and the preamp B+ lowers. This creates a compression effect. However you wouldn't get that anyway with power scaling, because the current is much much lower, right?
I myself use EF80 pentode mode as outputs for "small power" (0.5W or so) guitar amps. Can't run them full power anyway...
If using separate PT windings is out of the question, I would look to separating the preamp and power amp PSUs as much as possible - don't have them in series, as is the rule in instrument amps.
So after rectifier, have one cap, and then from that cap separate the two PSUs completely. One wing for the power amp and scaler, and one wing via dropping resistor to preamp PSU.
Then they shouldn't interfere with each other much.
However in the normal series PSU, when the power amp gets a transient, it eats up more current and the preamp B+ lowers. This creates a compression effect. However you wouldn't get that anyway with power scaling, because the current is much much lower, right?
I myself use EF80 pentode mode as outputs for "small power" (0.5W or so) guitar amps. Can't run them full power anyway...
That would be a kind of clamp. If you post the complete circuit, someone may be able to come up with a simple circuit that does this.
Maybe the easiest way is to use zener diode shunt regulation. Here's an explanation that might help you out.
Of course, here's the schematic I'm building:
MrCurwen: as you can see, the original supply is (as is common practice) several nodes in series. I'm planning to only vary B and C. I like the idea of creating a separate parallel power node for the preamp section that starts at the unaffected A node. Seeing that the reverb driver tube requires B supply and I'm not sure how well it'll work if I were to vary it, I might even create a separate B supply just for that reverb tube.
mt877: I saw that link but wasn't sure if it was exactly applicable to my situation. I'll think more on that.
I'm thinking I'll build the amp stock and get it working, then incorporate the voltage scaling and see if how much the preamp supply even moves first. Perhaps I'm jumping the gun worrying about it yet.
I'm still learning all I can about these circuits so I really appreciate all your ideas, guys!
MrCurwen: as you can see, the original supply is (as is common practice) several nodes in series. I'm planning to only vary B and C. I like the idea of creating a separate parallel power node for the preamp section that starts at the unaffected A node. Seeing that the reverb driver tube requires B supply and I'm not sure how well it'll work if I were to vary it, I might even create a separate B supply just for that reverb tube.
mt877: I saw that link but wasn't sure if it was exactly applicable to my situation. I'll think more on that.
I'm thinking I'll build the amp stock and get it working, then incorporate the voltage scaling and see if how much the preamp supply even moves first. Perhaps I'm jumping the gun worrying about it yet.
I'm still learning all I can about these circuits so I really appreciate all your ideas, guys!
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Since this is a guitar amp thread moved to the Instrument and Amps forum per forum policy.. Someone here should have some good thoughts on how to do this...
That wouldn't clamp, but would just regulate the B+.
If you add a Zener string equal to the clamp voltage, fed with a good amount of current to make it reasonably stiff
(or it could use a buffer), the Zener string could be used as a reference. Then point D could be diode connected (with a 1N4007)
to the Zener string, so when D exceeds the reference voltage, the diode conducts, and clamping occurs. Below the clamp voltage,
the 1N4007 diode would be reverse biased, and the added circuitry would have no effect.
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in the context of what you intend to do....
i am not sure that raised B+ is a problem, tubes can run higher voltages without serious consequences....
that said, i doubt that a 50 volt increase in B+ will matter, you may even like it.. 😉
but in the end it very easy to lose voltage, it is just ohm's law...
i am not sure that raised B+ is a problem, tubes can run higher voltages without serious consequences....
that said, i doubt that a 50 volt increase in B+ will matter, you may even like it.. 😉
but in the end it very easy to lose voltage, it is just ohm's law...
If you want to do power scaling, look up Kevin O'Connor.
He has a power scaling kit that should get you where you
want to go. Read up on it here:
The Ultimate Tone Tube Amp Books by Kevin O'Connor
Scroll down and click on power scaling.
What ever you do, don't go cheap, please
do it the right way.
Good luck.
He has a power scaling kit that should get you where you
want to go. Read up on it here:
The Ultimate Tone Tube Amp Books by Kevin O'Connor
Scroll down and click on power scaling.
What ever you do, don't go cheap, please
do it the right way.
Good luck.
What you want is a powerful Zener diode from point 'C' to ground. Measure the voltage you currently have at 'C', then find a Zener with a voltage just a little higher. In practice it will be easier to make the value up using several 5W (or more) Zeners in series, so they share the dissipation. For example, if the voltage is currently 350V then you might use three 100V devices, plus one 56V device, which will clamp node 'C' to 356V if it tries to rise. If the voltage remains below 356V then the Zeners will be 'off' so they have no effect on normal operation.
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That guy regulates the power going to the pre sections, which the OP doesn't want to do..
Thank you, Merlin, for that clearly stated idea using the zeners. It actually helped me see exactly what rayma suggested too (I'm still getting familiar with a lot of these terms so forgive me). It sounds like the most straightforward option.
I've checked out several power regulating products and am considering now which one is a good fit for my build.
Fyi, Kevin O'Conner's power scaling can scale the preamp or power section, in addition to the entire amp as well and sounds like a great system.
Thank you all for the insight and answers, it was exactly what I wanted to know!
I've checked out several power regulating products and am considering now which one is a good fit for my build.
Fyi, Kevin O'Conner's power scaling can scale the preamp or power section, in addition to the entire amp as well and sounds like a great system.
Thank you all for the insight and answers, it was exactly what I wanted to know!
The tricky part is making sure that the clamping circuit can sink enough current. Make its idle current at least twice the current
that is drawn by the node D that you want to clamp. You may have to power the Zener string off the same node that the
output stage is powered from.
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