Fisher SA-16

[kevco]

An update on this amp and a question. Its been in almost daily use for a month and a half now with no further hiccups. The peakiness of the sound has mellowed as the power tubes burned in. Initially the amp sounded bassey and trebley with lack of midrange. It has since opened up nicely and sounds great to my ear. I was initially looking for a bit of project on this, but it seems that messing with it at this point would be foolish. I'm sure I'll get bored sooner or later though...
My question is this: What is the purpose of running an EL84 (or any other pentode) in tetrode mode as opposed to pentode? I understand the action of a suppressor grid just as I understand the purpose of beam forming elements on beam tetrodes. Why not take advantage of the pentode's ability to self regulate secondary emission? Running pentodes in tetrode mode seems very common. Please pardon my ignorance if the answer is obvious.

[dcgillespie]

Kevco -- If I am understanding your question correctly, I think some clarification is in order. Any tube that has 5 active elements to control the current flow at one plate (besides a heater) is called a pentode. It makes no difference if the third grid is an actual grid, as in what some call a "true" pentode, or it is a beam forming element, as used in a Beam Power tube. Both of these tube types by definition are pentodes. An EL34 and EL84 illustrate the former, while a 6L6 and 6550 illustrate the latter. However, how this third elements operates is somewhat different between these tube types.

When a true suppressor grid is present, it functions much like a control grid in reverse. That is, since the suppressor grid is most always at cathode potential, it retards the flow of any electrons that might try to flow back to the screen grid when the plate voltage is low, similar to the control grid retarding electron flow due to it's relatively more negative charge than that of the cathode.

In pentodes with beam forming elements, these elements slow the electron flow in the vicinity of the beam element, causing a space charge to form in that region. This space charge then becomes a virtual third grid that functions similar to an actual grid in repelling any lose electrons back to the plate.

A beam power tube has the additional advantage of employing aligned control and screen grids. This has the effect of letting the screen hide in the shadow of the control grid, allowing it to run cooler, and drawing less current for a given total current flow through the tube. As a result, beam power tubes are typically more efficient that pentodes employing a third grid.

In any event, both of these tube types are always considered as being a pentode, as both regulate secondary emission from the plate. Unless the screen is strapped to the plate to produce triode operation, they are always considered as operating in some form of pentode operation. If the screens are decoupled AC wise to the cathode, then they are operating in true pentode mode. If the screen is operating from a tap on the OPT primary, then it is operating in partial pentode mode (or partial triode mode depending on your point of view!), commonly call Ultra-Linear operation.

In the days before pentode tubes were available, true tetrodes were available, consisting of a cathode, two grids, and the plate. These tubes were an advancement over that of the triode, but have very quirky characteristics, as the plate voltage must never be allowed to dip below that of the screen, or otherwise, the screen will quickly be destroyed. With the advent of the pentode tube, that problem was eliminated, so that power went way up, as did gain and efficiency as well.

I hope this answers your question!

Dave

[EWBrown]

My question is this: What is the purpose of running an EL84 (or any other pentode) in tetrode mode as opposed to pentode?

As far as running the EL84s in "tetrode" mode, the EL84's suppressor grids (G3) are internally connected to the cathode at pin 3, and the person who drew the schematic just left the G3s out, probably just for keeping it simple (or else he just forgot to draw them). The tubes are still running in pentode mode. I've been "guilty" of this when drawing my own schematics...

That's a really weird bias scheme, looks like it puts 10v positive on the grids.

What the bias scheme does, is to run the connected-together cathodes at approx +20V (130 ohms, with 154 mA total current), but the more appropriate grid bias would be 10V lower than the cathodes, so they add in the voltage divider to inject +10V on the EL84s' contol grids, thereby setting them at 10V lower than the cathode voltage.

I suppose they could have used a 68 ohm, 10W resistor, and kept the control grids at "ground" potentialbut they had some other reasoning.

This method also allows the grid voltage to "float" at half of the cathode voltage, as the tubes age.

This scheme, as with the ST-35, would call for using a matched quad of EL84s, with each tube consuming approximately 38.5 mA cathode current. That IS pushing them, not only to the PD limit, but a watt or two beyons

/ed B

[kevco]

[/quote]I hope this answers your question!

Dave[/quote]
Thanks Dave, it does. I think two things have confused me, one is lack of representation of suppressor grids on schematics, but the other (more commonly) is the generalization of nomenclature, IE: all modernly used power tubes are in fact pentodes even though on beam type tubes, the fifth element is in fact the beam former. And Ed, thanks for helping me understand "tetrode mode".