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
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
An externally hosted image should be here but it was not working when we last tested it.
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).
Hi,
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!
Klimon said:
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!
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!
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!
The 4-250A gets described as a 'radial-beam tetrode', meaning it has neither suppressor grid nor beam forming plates. I guess it isn't a 'pure' tetrode either.
I'm not able to answer your question but here's what I found: "6CA7 was originally an American counterpart to EL34. 6CA7 is a beam tetrode like 6L6 and sounds quite different than EL34. Today, Electro-Harmonix produces a true beam tetrode 6CA7"
Thanks for all your replies, that about rules the tetrode formally out: much too kinky
Simon
Klimon,
Thanks for the comments. I have also seen those remarks re the 6CA7 elsewhere, but the RCA Tube Manual lists the EL34/6CA7 as a power pentode (thus identical), and they are quite specific with other types to distingush petween pentodes and beam power tubes. I have not been able to find a 6CA7 for visual inspection. Perhaps others will still comment.
Regards.
Thanks for the comments. I have also seen those remarks re the 6CA7 elsewhere, but the RCA Tube Manual lists the EL34/6CA7 as a power pentode (thus identical), and they are quite specific with other types to distingush petween pentodes and beam power tubes. I have not been able to find a 6CA7 for visual inspection. Perhaps others will still comment.
Regards.
Johan,
The beam EL34 I have encountered in the past is a true beam power tetrode but with similar specs to the EL34. They usually carried the number EL34S. That is to say that it was pretty much a marketing ploy, someone choose a beam power tetrode with similar specs to an EL34 and called it an EL34S. The ones I have seen were Chinese (and so probably had a Russian Heritage) and were fat bottle shape more like a slightly shrunk KT88 shape. They sounded OK in a Blues Guitar Amp..
These days if you want to do a comparison of the EL34 vs a similar beam power tetrode then try the new JJ KT77. It biases at almost exactly the same point as the JJ EL34 but I think sonically it has better tops and bottoms. Just to really confuse the issue JJ also have an "Industrial Strength" EL34 intended for guitar amp (ab)use which biases at a slightly different point and I think they call it an EL34S.
Cheers,
Ian
The beam EL34 I have encountered in the past is a true beam power tetrode but with similar specs to the EL34. They usually carried the number EL34S. That is to say that it was pretty much a marketing ploy, someone choose a beam power tetrode with similar specs to an EL34 and called it an EL34S. The ones I have seen were Chinese (and so probably had a Russian Heritage) and were fat bottle shape more like a slightly shrunk KT88 shape. They sounded OK in a Blues Guitar Amp..
These days if you want to do a comparison of the EL34 vs a similar beam power tetrode then try the new JJ KT77. It biases at almost exactly the same point as the JJ EL34 but I think sonically it has better tops and bottoms. Just to really confuse the issue JJ also have an "Industrial Strength" EL34 intended for guitar amp (ab)use which biases at a slightly different point and I think they call it an EL34S.
Cheers,
Ian
Hi Johan,
I use an Electroharmonix 6CA7 instead of EL34's in my HF-87. Improved sound quality all around.
I must admit I was terribly confused about the same issue. Now I have come to accept that they are simply "functonally equivalent". So far I have come to prefer beam tetrodes over pentodes. I have an Eico ST-70 rebuild coming up that may change my mind. Who knows?
-Chris
I use an Electroharmonix 6CA7 instead of EL34's in my HF-87. Improved sound quality all around.
I must admit I was terribly confused about the same issue. Now I have come to accept that they are simply "functonally equivalent". So far I have come to prefer beam tetrodes over pentodes. I have an Eico ST-70 rebuild coming up that may change my mind. Who knows?
-Chris
I ran across a beam tetrode version of a 6CA7 when going through a big box of assorted unboxed tubes. This one had 6CA7 etched on the glass, and 6CA7/EL34 "Made in Japan" painted on the base in silver paint. There was no manufacturers logo or date code on the tube. It was unfortunately broken, so I can't say what it sounded like, but I did take it appart. It looked like someone stuffed 6L6 guts into an EL34 plate structure. It most definitely had beam forming plates where G3 should have been, but I couldn't tell you if G1 and G2 were aligned or not. There were other Japanese 6CA7's in the same box which have 3 wire grids, I still have one or two of them. Since they are functioning tubes, I didn't take any of them appart. They look very similar from the outside. Most of the tubes in the box were from the early 1960's.
Hi,
grid-aligned tubes usually have an extreme Ia/Ig2 ratio, simply for the fact that high efficiency (low "lost" Ig2) was sought for - that´s why aligned grids (regardless if employed in a tetrode or pentode) were introduced.
Now, such an extreme Ia/Ig2 ratio would void the goal of copying a true pentode EL34 as exactly as possible by using a beam tetrode design (to circumvent Philips pentode patent) and using aligned grids at the same time. Aligned grids are not necessary, but even contradictive to copy a true pentode EL34. (BTW, a true pentode EL34 has no aligned grids.)
Thus I doubt the existance of beam tetrode, grid-aligned "EL34 alikes" (regardless of naming them EL34 or 6CA7) because it simply wouldn´t made have sense (and also doesn´t nowadays).
Tom
grid-aligned tubes usually have an extreme Ia/Ig2 ratio, simply for the fact that high efficiency (low "lost" Ig2) was sought for - that´s why aligned grids (regardless if employed in a tetrode or pentode) were introduced.
Now, such an extreme Ia/Ig2 ratio would void the goal of copying a true pentode EL34 as exactly as possible by using a beam tetrode design (to circumvent Philips pentode patent) and using aligned grids at the same time. Aligned grids are not necessary, but even contradictive to copy a true pentode EL34. (BTW, a true pentode EL34 has no aligned grids.)
Thus I doubt the existance of beam tetrode, grid-aligned "EL34 alikes" (regardless of naming them EL34 or 6CA7) because it simply wouldn´t made have sense (and also doesn´t nowadays).
Tom
Hi Tom,
Confusion reigns. Thank goodness there are still some around who can put matters straight. Your explanation is of course correct.
I did some measurements long ago (in my youth) when still working in a research lab and with the necessary instrumentation. There is as you will realise quite some difference between the equivalent "ra" and the equivalent "gm" referred to the screen of a beam tube and a true pentode (both with all electrode voltages on, i.e. not just the screen as in a triode). This was to investigate the load that g2 in either of these topologies would present to the UL tap of an output transformer. It also had bearing on the effect of a series g2 resistor.
Thanks all; we seem to have sorted this business of "freedom" with annotations out for now. At least I am the wiser.
Regards.
Confusion reigns. Thank goodness there are still some around who can put matters straight. Your explanation is of course correct.
I did some measurements long ago (in my youth) when still working in a research lab and with the necessary instrumentation. There is as you will realise quite some difference between the equivalent "ra" and the equivalent "gm" referred to the screen of a beam tube and a true pentode (both with all electrode voltages on, i.e. not just the screen as in a triode). This was to investigate the load that g2 in either of these topologies would present to the UL tap of an output transformer. It also had bearing on the effect of a series g2 resistor.
Thanks all; we seem to have sorted this business of "freedom" with annotations out for now. At least I am the wiser.
Regards.
To add a little more confusion, I have a bunch of 828 transmitting tubes. The RCA data sheet calls them "beam power amplifiers." They have a box structure on both ends of the plate which I presume to be the "beam plates" (though they don't look like the beam forming plates in a 6L6.) Looking in through the glass it looked liked the boxes were attached to the screen support rods. Huh?
One of them had an open filament, so I broke the bottle and took it apart. Turns out that what I was seeing is a bona fide suppressor grid. The actual screen grid is quite a bit closer to the control grid.
The data sheet is from 1948 and the tubes are mid 60's. I suppose they might have changed the construction in later years, but I suspect they were always made that way.
-- Dave
One of them had an open filament, so I broke the bottle and took it apart. Turns out that what I was seeing is a bona fide suppressor grid. The actual screen grid is quite a bit closer to the control grid.
The data sheet is from 1948 and the tubes are mid 60's. I suppose they might have changed the construction in later years, but I suspect they were always made that way.
-- Dave
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