TETRODE amplifiers

[Klimon]

Hi,

It seems hard to locate any amps or schematics employing tetrodes (excluding beam tetrodes); are they deemed unfit for high-end audio reproduction? What about triode-strapping tetrodes?

Just a thought... Would make my to do list a little less infinite

Simon

[ray_moth]

Tetrodes are insufficiently linear unless they have beam plates, which have the same purpose as the suppressor grid in a pentode, i.e. to avoid the 'tetrode kink' in the plate curves at low plate voltages.

[sorenj07]

many amps that use tetrodes employ them in an ultra-linear topology, such as the amp that i'm in the process of building.

i should say though that the 20watt output transformer should actually be 40 watts to take advantage of the full power of the schematic. the original amplifier had "reduced frequency response" at 40W, but any transformer rated to 40 watts or above should have full frequency response - this is a 40-watt amplifier.

here's the schematic

[anatech]

Hi Simon,
If you are using 6L6 types, they are a beam type tube. Good choice, they can sound great.

-Chris

[darkfenriz]

As far as I uderstand, tetrodes suffer from secondary emission due to lack of suppression, which makes them highly inferior to pentodes.

[anatech]

Hi darkfenriz,
Beam power types have a vitual suppressor. Problem solved.

-Chris

[ray_moth]

tetrodes suffer from secondary emission

Yes, it is that problem that shows up as the 'tetrode kink'. In the KT series of tubes (KT66, KT77, KT88), the KT stood for 'Kinkless Tetrode' because they had beam plates to prevent secondary emission, like the 6L6, 807 and 6146 before them.

I am not aware of any 'pure' tetrodes in use in audio - only beam tetrodes (and, of course, pentodes).

[Geek]

Hi,

Originally posted by Klimon
What about triode-strapping tetrodes?

A triode strapped tetrode can sound really nice. Problem is they are horribly inefficient. Consider a beam tetrode, 6L6 or 807 as previously mentioned. In SEP, they crank about 8.5W for a good 25 or so watt input to the plate. In triode, they give a wee over 1 watt, for the same input!

[Johan Potgieter]

I think this is the right thread to clear something up that had me puzzled for a while - the beam EL34. Kevinkr has mentioned this on occasion, and I see other contributors here who would certainly be able to clear this up for the benefit of all.

But first, the classic beam power tube (6L6, KT66 et al): I always had these as having three distinguishing features: (a) the alignment of g2 and g1, thus giving rise to the mainly "field effect" in the action of g2 and thus improved efficiency, (b) the beam forming electrodes situated at the posts that would have supported a g3 in pentodes, thus keeping electrons away from the ends of the operational electrodes where the geometry would interfere with the intended operation, and (c) the elimination of g3 by virtue of a space-charge effect.

The omision of a g3 was possible because the geometry of g1, g2 and anode led to the forming of an electron space charge of electrons temporarily bounced off the anode exactly where g3 would have been situated, thus acting as a virtual g3.

All this as described in the classic RCA Tube Manual, my early introduction to tube technology. In that sense the desciption of "beam power tube" was to me using the less important feature in the title, still.

My question is now if someone could describe to me what exactly a beam EL34 is. I have seen repeated mention of it without being able to ascertain how exactly it differs from the classic EL34 pentode construction. I have consulted several dealers who have never encountered this type. Photos on the internet are just not clear enough to show detail. But as far as I can see, I do not notice aligned g1 and g2 sets, one important feature of a classic beam power tube. It was difficult to see whether there was a g3.

Thus, when talking of a beam EL34, does that include (a) aligned grids, (b) beam forming electrodes (presumably attached to the g3 posts), and (c) space charge effect near the anode? If only (b), how does this make a beam EL34 different in operation from a classic EL34?

Thanks in advance!

[jane]

Originally posted by ray_moth
I am not aware of any 'pure' tetrodes in use in audio - only beam tetrodes (and, of course, pentodes).

I think there was an article in Glass Audio some years ago of a 4-250A tetrode PP amp, a quite powerful thing.