Turning Pentodes & Beam Tetrodes into Triodes
Separating the DC conditions from the AC


  Editors Note: This is a post from the JoeList posted here with Bill's permission. I have been encoraging Bill to make this thing a kit or a pre-assembled part (he has recently said he has found a better part to use for the zener which gives even better noise performance -- he has to buy them a 1,000 at a time and won't tell me what they are). Please email Bill and encorage him to release this if you are interested.

Eddie "Obivaughn Kentubi" Vaughn of
Vaughn Audio has recently posted a snippet that explains how this works.

Hi Kurt, Joes:

I don't care to keep this any great secret . . I just worked hard to get the theory into my head - much digging thru old texts on tetrode theory - and I didn't feel like giving it away . . . just wanted to have some fun is all, and maybe stimulate a little thinking and re-evaluation of some tired old "standard procedures."

The way I do this little trick - as a couple of folks already knew - is to bypass a zener with a cap. But, not just any zener and not just any cap. My favorite zener is a 1N821, a 15 cent, 1/2 W, 6.2 volt part that is both very quiet and of nearly zero tempco. I forget why 6.2V is the "magic" voltage for zeners but it is, the noise falls thru the floor and the low tempco yields a very stable voltage source . Maybe someone could post on this 6.2V business ...

To build this:

  1. string five of the 1N821s in series and set them aside (1N821 now unobtainium, use an appropriate substitute string of zeners)

  2. take two, 16V, 470uF Sanyo OSCon caps and put a 10k resistor across each one, then hook the two RC combos in series

  3. connect that assembly across the zener string being sure to observe the polarity

  4. then put a 1N914 diode & a 10R, 1/2 W resistor on the cathode end of the string and a resistive ferrite on the anode end

  5. connect the other end of the 10R to the plate of the tube to be "triode connected" and the ferrite end of the assembly to the screen, and there you have it.

The 1N914 prevents the screen's going into a runaway "negative impedance" mode where is it emitting, rather than collecting, electrons.

The 10R resistor is both a parasitic suppressor and a "fuse link;" ALWAYS use a 1/2W MF resistor there, not something stout like a wirewound.


Note that if you are using true, three grid EL34s that the suppressor is best connected to about -5V or so. Just voltage divide off your bias supply. Biasing that grid slightly negative reduces its electron interception to essentially zero.

Have fun discovering what a triode connected tetrode/pentode really sounds like.

Bill - PEARL, Inc.

And a bit of explanation in another post:

> Will someone PLEASE explain what the zener thing for strapping pentodes as triodes is all about?

Basically, this is just a method of "level shifting" the screen to a lower DC potential than the plate and doing so thru a network with *very* low Z at audio frequencies.

Why is this a good thing?

  1. Well if you look at the plate curves for pentode/beam tetrode operation you see that plate current is pretty much a function of screen voltage and that's why most amplification stages that employ these devices also regulate the screen supply. Otherwise plate current can be affected by the changes in screen voltage that will inevitably occur with the changes in screen current that accompany signal. The object being of course to provide a low-Z or "stiff" screen supply.

  2. While it's the desired case for the turns of the screen grid lie within the "electron shadow" created by the negative charge on the control grid, this seldom turns out to be achieved in practice. Metal ceramic transmitting tetrodes being a notable and *very* interesting exception ie. 4CX-125, -250, -350 and their ilk.

    When the screen is less than perfectly shadowed, its ability to attract electrons passing thru its accelerating, positive field on their way to the plate increases dramatically. Current at voltage equals power and consequently the screen must be able to effectively dissipate this, otherwise it will melt down eventually.
  3. The more nearly equal the screen and plate voltages, the greater the screen current, and the greater the changes in its value with signal.

  4. In "triode connected" pet/tets, this issue is typically "addressed" by the insertion of 100 to 1K ohms in series with the screen. While this *will* reduce the screen's voltage wrt to the plate and with that, the screen's current and consequent requirement for dissipation, it is far from an ideal fix.

  5. Referring back to (1), it will be seen that plate current is a function of screen voltage. If the screen is intercepting electrons destined for the plate and if, triode-like, its current flow varies with it voltage it can be seen that that the potential on the screen will vary with the voltage drop across the resistor connecting it to the plate.

  6. By this action, the effective impedance of the entire B+ supply is driven up by the insertion of a screen resistor. This action is the cause of the "softish," "mellow" sound universally reported when pet/tet/UL amps are "triode-connected"

  7. The solution for this is to "level-shift" the screen to a lower DC level - thus limiting its dissipation - and to bypass the level-shifting circuitry with a *low* impedance ie. a *very* good, high value, low voltage cap of the BlackGate, Cerafine, OsCon variety.

  8. 1N821s work well for this as they are sonically reasonably benign and completely bypassed down to very low audio frequencies.

And another:

The deal is that what you want to do is run the screen DC-level shifted some 30-odd volts -below- the plate's voltage.

Thinking then in terms of --electron flow-- (and not the usual, ass-about-face "current flow" thrown at you in text books) work thru in your mind the correct polarity for the zener string such that its operating in its Zener-breakdown mode when electrons are flowing from the screen back to the plate connection on the OT.

That done, and remembering that its perfectly possible for a screen's normal impedance to become what's known as "negative." By that is meant that instead of -intercepting- a certain percentage of the anode-bound electrons, it goes into a mode where it is -contributing- to that flow; we say at that point that the screen's impedance has gone negative and it being that the B+ supply's impedance is held as low as possible a little more thought will bring you to the realization that a screen running as a negative impedance is doing so in a mode where it sees no practical limit on the current it can contribute.

Now, current through any (non-superconducting) conductor tends to cause it to heat up and high current causes it to heat up a lot. Hot conductors more readily go thermionic than cold ones and the moreso they go the hotter they get ... a self-stoking loop, a self-destructive loop ... capiche ? ?

So, what sensible people do is put a small-signal diode in series with the screen such that electrons can move -only- from the screen to the anode.

All that done, screens, contrary critters that they are, will sometimes just up and cack ... period ... get over it.

Well you don't want your dead screen supplying AMPS of current to, say, your control grid so a sensible thing to do is to put a 1/2W 10R ** METAL FILM ** resistor in series with the screen right at the point the screen-dropping circuit ties to the anode. Use a low-wattage, metal film part because they make almost perfect fuses. Hit hard with a burst of current they fritz pretty much instantly...

Even after all that, things can sometimes just get sh*tty on you at very high frequencies . . . get over that too. Dick around with about a zillion different kinds of ferrite beads placed (oh yeah, this -is- tedious) right where the screen lead exits the tube socket. Some beads sound rather better than others, the "resistive" ones seem best.

Finally, you don't want all that -stuff- in the AC signal path so just bypass from the screen side of the 10R resistor to the other other end of the zener string. In other words, bypass around, and only around, the zener string.

And there you have it.

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