Ready to go SMPS for 15W tube amps!

[ChrisM]

With all the new legislation re power factor correction, zero stand by power etc linear power supplies may soon be a thing of the past. I think we should at least investigate SMPS and perhaps look at novel ways of using them in guitar amps.

Agreed

Tube amps in general are an ancient technology so I guess our big, expensive inefficient transformers kinda fit right in.

[sparrott]

The Peavey VB-3 300watt tube bass amp is already using a switch mode power supply. I think it weighs in at 38 pounds versus 85 pounds for a Ampeg SVT head. It doesn't go toe to toe with the SVT, but the power supply is not at fault there.

[Cliff Schecht]

I'm curious to see what topology they used in this supply (first guess is a discontinuous forward converter at these power levels) and how they handled the feedback loop in the supply. One of the issues that power supply designers run into with a discontinuous boost is a right-half plane zero in the pole-zero plot. This zero must be compensated for and by nature slows down the transient response of the power supply. If it can't react quickly enough to transients, you will not only have poor regulation but risk running into subharmonic oscillation issues and/or maxing out the PWM and cooking the FET's.

While this supply is surely protected in some form (built into the controller IC), until we know more about the design it's hard to assume that it's a sound design. Looking at the gutshot reveals it's a pretty simple circuit but I'd have to sit down with the schematic (or draw one out) to figure out what they did where. I'm curious what controller they used too.

[sparrott]

It's hard to make judgement by looking at the simplicity of the circuit, the SOC designers are integrating so many more components these days. I have Panasonic SMPS from a microwave oven on my bench and it doesn't look much more complicated than the supply in question. It is able to maintain stability/regulation when switching a 800W 4000v magnetron load on and off.

[ampgeek]

Do we believe that the 6.3 V outs are DC as well? That could be a "hidden" advantage to consider.

TIA,
Dave O.

[diagrammatiks]

Probably want a higher quality supply for a beefier amp.

Redundancy is good.

[sparrott]

Probably want a higher quality supply for a beefier amp.

Redundancy is good.

Agreed. My thoughts here are a low noise single ended wreck style for practice/recording or a good low gain amp to use with pedals. Single ended 6v6 etc. Or possibly a rack mount multi-channel preamp would be a good application where there would an available 6.3v supply for channel switching logic.

[LeftyStrat]

Agreed. My thoughts here are a low noise single ended wreck style for practice/recording or a good low gain amp to use with pedals. Single ended 6v6 etc. Or possibly a rack mount multi-channel preamp would be a good application where there would an available 6.3v supply for channel switching logic.

Yeah I just checked the dimensions, that would fit in a 1U. Hmm, I might spring for one of those for my Fender/Marshall preamp.

[roberto]

I bought a 48V SMPS to do the same thing, but his way it's easier.
They work, this one is ok for a AC30 like amp.

Do not forget that it's a SMPS, so you can change the output voltage as you prefer! (take care of output capacitor's limitations!)

You just have a voltage divider on the output, with a feedback control: the voltage control chip modulates the step-up output to obtain constantly 5V on its feedback pin.

So raising the upper resistor of the voltage dividing will raise the output voltage (usually you have a trimmer on the bottom part of the voltage divider for fine adjustement).

Another interesting aspect of SMPS a tube amp is that you can control SAG (just add a dual 16ohm 50W lin pot in series used to dampen tweeters), and you can do London Power like attenuators (it's feedback controlled, so you can lower both B+ and bias voltages with a small, simple, easy to be found potentiometer).

[Cliff Schecht]

The reference you are dividing down the output voltage by is usually 1/2 the supply, IME anywhere from 1.25V-2.5V for your typical controller.

Also WRT to getting sag out of an SMPS, there are better ways to skin that cat. The way you propose is wasteful and IMO sort of defeats the purpose of using a lighter weight, high efficiency power supply. Plus using a resistor will cause things to sag linearly which is not how tube rectifiers respond. Like all thermionic devices that we use tube rectifiers follow a 3/2nd square law. Since we are using much more advanced circuits than a single transformer and a few diodes (silicon or tube) we should use clever circuit design to at least emulate the type of sag that we are accustomed to. I'd look to adjusting the response of the control loop to modify the behavior of the power supply itself, this way everything stays efficient, cooler (literally and figuratively ) and less stressed in the end.

My dad had some great insight into this:

When one wants to introduce "droop" into a switching supply's output voltage, a very simple technique exists for incorporating a controlled output impedance as the means to accomplish this. Normally, this is the last thing you'd want to do to a regulated voltage supply, since it amounts to a sanctioning of sloppy regulation. However, Intel used to actually specify droop for use with some of its processors as a means of minimizing the number of bulk power supply caps required, thereby reducing cost. Audio circuits that sonically benefit from the incorporation of rail sag can use the same technique. To implement voltage droop, all that needs to be done is to measure the output current of the supply and feed that information back as a means of modulating the reference voltage for the control IC. This action has the effect of establishing a load line for the output of the power supply, in effect establishing an output resistance that can be adjusted as desired. A non-regulated power supply has a fixed output resistance but this cannot be changed and can be quite variable over the operating current range of the supply. In a tube-type rectifier circuit, the space charge at the cathode more-or-less sets the output resistance of a supply (in combination with the output caps, the transformer and the output choke if one is present). The law of the thermionic diode is a 3/2 power law, relating a-k potential to conducted current. So, dynamic output resistance of a thermionic diode is quite non-linear. This sort of resistive non-linearity could be incorporated into a "droopy" switching supply with a bit of clever circuit design.

[roberto]

I'm not taling about power tube sag, I don't like it.
I'm talking about the sag effect of the supply.

[roberto]

Also, with those kind of smps for guitar tube amps, it's useful to apply some tricks to "tubefying" the power amp, like the 5k6 resistor instead of the choke plus 33µF on screens for a pair of EL84 (2k8 and 50-70µF for a quad), and/or add a bit of depth to "soften" the attack.

[John_P_WI]

@ Cliff,

Check out KOC's approach to sag here:

http://www.londonpower.com/catalog/abou ... n_kits.php

I believe this is one of the methods Roberto was referring to.

The "Standard" that Ian, Gingertube built uses a similar "sag" technology.

John

[Cliff Schecht]

I'm not taling about power tube sag, I don't like it.
I'm talking about the sag effect of the supply.

It still wouldn't be hard to get an output voltage based on whatever relationship you want to the output current using a modified current control scheme (i.e. watching output current and using this to help "program" the sag you want). You can also modify the transient response of the control loop if the supply is too quick to supply transients (i.e. very stiff).

[roberto]

There are many ays to skin that cat, screen control is effective also.

Mike just replied me that the module has 90W power output and works for a 30W 4xEL84 AC30 like, and can be bought custom for a maximum of 350V B+. They are developing bigger units for 100W amps (and I hope more).