Tag: cascode

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Direct coupled SET proposal

I’ve had a few 6CB5As kicking around for a while waiting for a project. The 6CB5A has been documented by Thomas Mayer and Ale Moglia, among others, as a great option for triode strapping. Thing is, I like trying new things when I build and repeating a cap coupled formula for a two stage single-ended amp just wasn’t making it to my short list of projects (which I don’t quite have time for anyways).

A recent discussion reminded me of an idea to use the shunt cascode topology to direct couple to a single-ended output. It required some extra power supply rails, including a fairly large negative rail. These requirements aren’t anything too unusual; you see them with Morgan Jone’s Crystal Palace amplifier or any kind of fixed bias scheme (in a way).

Anyways, the more recent discussion reminded me of a thread discussing a novel way of direct coupling two stages by stacking the power supplies. This is kind of similar to the Free Lunch AKA Monkey on a Stick arrangement. Applying this idea to my original shunt cascode brainstorm lead me to this:

We have a shunt cascode input stage. The output resistor (R2) idles at about 75V across it. This feeds a MOSFET source follower, which will have just a couple volts less on its output. So we have a very low impedance output at around 75V above ground and we want to direct couple that to the next stage. This is where I think it gets exciting.

We raise the cathode of the output stage so that it is positive relative to the grid (at 75V) by floating the output tube power supply (V2). The voltage we float it at is roughly equal to the target bias voltage plus half the target output swing. In other words, we raise it by twice the bias voltage for A1 or twice plus a bit for A2.

The output tube anode is connected normally and the cathode returns to the point where the output supply (V2) floats on the bias supply (V1). Our input stage is powered by another supply floating on the bias supply (V3). Our input and grid driving circuits are all referenced to ground and direct coupled. We can set the output bias by adjusting the current through R2.

Here’s a more fleshed out version:

It looks like a lot in the schematic, but I’ve already got shunt cascode and grid driver circuits on small PCBs. The power supplies don’t need to be anything exotic in this case as the input has decent PSRR already. The higher current output could use simple CLC filters as well.

Will I build it? I hope eventually. By summer I hope to have the workshop basically finished. I’m already enjoying having all the tools and parts in one (heated) space!

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E-Linear: an interesting kind of feedback

Oops, I bought another pair of output transformers. This iron is 60W rated push-pull with a 6k6 primary, so the natural pairing would be 6L6GC.

I’ve used the 6L6GC in the past (see the Luciernaga). Because it is common in guitar amplifiers, it’s an easy to find tube both new and vintage. While it’s no power-house in triode mode, the 6L6GC is quite capable in pentode or ultralinear.

Operating the output tubes as pentodes means higher output impedance and distortion. The Quality Amplifier from a couple weeks ago avoids the issue entirely by operating the outputs as triodes. The other approach, which is actually more common today, is to sacrifice some of the circuit’s gain to bring output impedance and distortion back down. That’s called (negative) feedback.

The question is how do we want to apply it?

This post won’t get into the nitty gritty of feedback (that deserves its own page on the website), but there are generally two prevailing approaches. Global feedback is what we see most often; this takes a signal from the output of the amp and wraps it all the way back to the input. The Williamson amplifier is the quintessential example of this.

The second approach to feedback is to use local loops. These affect a circuit just like global negative feedback, but are isolated to just a stage or two. Local feedback, because it involves fewer stages and phase shift, is more stable than global negative feedback. That means they’re generally simpler to employ.

The circuit above is a variation of what seems to be unofficially referred to as an E-Linear stage. Feedback from the output transformer primary is applied via the ultralinear taps directly to the load resistor for the input stage. This local feedback drastically reduces the output impedance of the 6L6GC.

The input stage is commonly a pentode because the high plate resistance is a benefit here to applying feedback. In this case, the input stage is a hybrid cascode, which still has a high “plate resistance” owning to the MOSFET upper device. That also gives us more options for the lower triode tube.

I like the simplicity of this circuit quite a lot. In Class AB, it looks like a good 25W should also be available with a pretty modest B+ (or a little less in Class A). Seeing as how I’ve got the iron on hand, I hope to give this one a test at some point!

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Los Monos are coming

Here’s a sneak peak of the long-term push-pull project. The second monoblock is one cathode bypass capacitor away from being ready for playback. A bad tester tube took out the cap on one side during testing with a bang, but I’ll have replacements soon. Until then here’s the intro of the project write up.

The monkey on your back

Everything should be made as simple as possible, but not simpler.

-Einstein

There comes a time in every DIY builder’s life where he or she gets the urge to stretch beyond single-digit output power and single-ended amplification. There is no shortage of worthwhile projects to choose from: variations on Williamson, Mullard, or Dynaco push-pull topologies are easy to find discussed in forums and tweaked to compensate for modern parts. You can even find kits for something like the Dynaco ST-70.

When the double-digit power bug bit me I could not bring myself to abandon my usual no-feedback, triode output, class A comfort zone. This is the simplest (but not the only) path to good sound and my speakers are efficient enough. I’m also too lazy to do feedback math but that doesn’t mean open-loop, class A triode designs aren’t an engaging challenge. This build faced the following complications (which are common to many push-pull amplifiers):

  • Class A requires healthy current in the output stage: this needs to be balanced in the output transformer to preserve inductance
  • Two cascaded grounded cathode input stages is too much gain, but one stage is generally not enough
  • The input stage must have low enough output impedance to drive the triode output tubes

For the most part, my solutions to the challenges strive for simplicity. As is often the case in tubes and life, simplicity in some areas is traded for complexity elsewhere. This push-pull amp has only two stages, the outputs are cathode biased, and it requires only three tubes per channel. To make this seeming simplicity possible, I used solid state helper circuits on PCBs. While these helper circuits are not technically complex, they drive up the parts count and require some measurement and adjustment.

Here is the conceptual topology for Los Monos:

Pictured is a two-stage triode output push-pull amplifier. The output stage is garter biased and the voltage gain and phase splitter stages are combined in a folded cascode long tail pair. This is all described below with a full schematic (showing lots more parts).

More of this write-up is on the way as soon as I’ve got both channels playing and glamour shots are taken!

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“All-tube” MC phono preamp (continued)

Not long ago I wrote a short post about MC carts and the noise contribution of tubes when amplifying such tiny signals. I focused on step-up transformers as the solution to noiseless amplification, but there is another approach.  If you don’t like solid state, stop reading. Ok, now that you stopped reading and checked out the going prices for step-up transformers, you’re back. Good. Don’t worry, this approach uses the tubeyist solid-state device: the JFET.

A cascode is a compound amplifier in a totem pole arrangement. Here’s a great explanation by Valve Wizard Merlin. This allows you to achieve huge amounts of voltage amplification with fairly economic current usage and without coupling capacitors or multiple phase inversions. The driving force in this arrangement is the transconductance of the lower tube. The lower tube and upper tube do not need to be the same, nor do they even need to be the same type of device.

JFETs (junction gate field effect transistors) are voltage controlled devices, just like tubes. In fact, they bias in a very similar way: Rsource in the above raises the n-type JFET’s source voltage above the gate, similar to the way a cathode resistor in a grounded cathode amplifier raises the cathode above the grid. On the other hand, even the lowliest JFETs have a higher transconductance (gm) than the mightiest small-signal tubes. Icing on the cake is that JFETs, properly chosen and cared for, are lower noise devices. As such, they make a great lower device in a hybrid cascode.

The overall gain of a cascode simplifies to approximately:

gm(lower) * Rload

This equation is a simplified expression of the total gain of both devices:

[gm * (Rp + Rload) / (Mu + 1)] * [(Mu +1) * Rload / (Rp + Rload)]

AKA [JFET gm * load divided down at tube’s cathode] * [grounded grid gain of tube]

Rp and Mu are characteristics of the tube upper device. The choice of upper device affects how much of the voltage gain is performed by the JFET by affecting the load it sees. A high Mu and low Rp upper tube (i.e. high transconductance) presents a lower load as divided down at its cathode, thus less voltage amplification by the JFET (and more voltage amplification made up by the tube due to the higher Mu). A low transconductance upper tube does the opposite. But regardless of the tube (assuming an appropriately sized Rload), the overall gain remains the same: ultimately the transconductance of the JFET multiplied by the load on the upper tube.

So where’s this headed? Obviously there’s a full design coming to try out this idea, but the takeaway is that a hybrid cascode is potentially a great way to step up the tiny signals from a moving coil cartridge with very low noise and hand the now-larger signal off to a tube amplification stage without multiple supply voltages, coupling caps, or an expensive step up transformer.

The catch? Cascodes have poor power supply noise rejection and a fairly high output impedance. But there are ways to minimize these factors, too.

Further recommended reading: 1, 2, 3, 4

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