On the bench: Zenith K731 (7M07)

This poor radio has seen better days and doesn’t quite live up to modern safety standards with regards to mains electricity.  But the look is great and there’s generous space inside for a small tube amp. Because the enclosure must be allowed to vent for the tube amp to dissipate heat, the speaker (which I also plan to modernize) will be a small challenge. This is a great candidate for a DIY tube radio restoration.

Probable features:

  • 3-5W single channel output
  • Bluetooth connectivity
  • Aux input (analog)
  • Volume control
  • EQ (either treble/bass knobs or loudness contour)
  • Safety!


DIY tube RIAA calculator sheet

All-In-One RIAA Calculator

Here’s a spreadsheet I built for calculating RIAA values in two stage tube phono preamps. When comparing results to other published designs using the same filter network, everything looks correct (within a few percent due to estimation of Rp). I used this sheet for El Matématico.

If you want to estimate values for something like a CCS loaded stage, you can set Rload on the appropriate stage to 1M or thereabouts. If you’re looking at using a cascode, mu follower, SRPP, etc 1st stage, you’ll need to make sure the Zout figure the sheet uses (cell I6)  reflects the Zout of the topology because it is used to calculate R1. Same thing goes for cell I3 (Miller capacitance of 2nd stage) if you use a gain stage after the filter that affects this (cascode, grounded grid, etc).

Go build a phono preamp!

“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

The tube shunt sickness

So many options so little time. On the other hand, if I stopped thinking and started building, I might be able to actually try one or two of these.

Upper left:

DC EL84 shunt regulator, output fed to a grounded grid amplifier (non-inverting), then to a cathode follower (non-inverting), then back into the shunt to amplify and cancel any power ripple on the output. A CCS feeds the shunting device to provide a high impedance.

Upper right:

Same shunting arrangement, now fed by a differential amplifier (non-inverting). You can think about the dif amp kind of like a cathode follower driving the cathode of a grounded grid amplifier. Weakness here is the high output impedance of the dif amp and low input impedance of the DC shunt (shunt could be switched to non fixed bias for a higher input impedance though, see next).

Lower left:

Auto-bias shunt (varies with wall voltage) fed by a cascode. The cascode is on the ‘other side’ of the CCS to take advantage of the higher B+ headroom.  This will also introduce an un-wanted ripple signal, so the upper triode is configured for 1x gain (Ra = Rplower + [Mu + 1] * Rklower). Injecting the ripple through the voltage divider to bias the upper grid would, in theory, cancel the ripple on the ouput. The ripple we want comes from the output and is fed to the grounded grid lower triode in the cascode to be amplified as an error signal and fed to the EL84. Multiple feedback loops here, may not be stable.

Lower right:

For the iconoclasts, a TL431  in the cathode of the grounded-grid EL84 controls the current. The tube is really here just to protect the low voltage SS part from the high voltage output. A cap feeds the AC ripple from the output to the TL431.


Coming soon to a workbench near you…

In a world where near-field and headphone listening has become an unstoppable force. Where every DIY builder is bored to death of rational and safe two-stage, single-ended triode designs. Where power supplies have become an afterthought and parts values are just plugged and chugged. Prepare your butt for a new madness. Prepare it for La Luciérnaga…


Note db scale for the predicted frequency response graph: +/- 1 dB from 40-20khz. If that’s not good enough, I’ll show you how to make it even better. There’s mucho tube math coming your way, amigo.

Here’s the worksheet I used to create the above: