[Alex Kitic]

Hi all Audio Circle members.
Other than the 300B, 2A3 and the same,
is there any other tube that is the size of the 6L6 or the same ?
How about a triode tube the size of a EL84 ?

Guy 13

I guess you are referring to the anode dissipation as a measure for comparison? In that case, 300B is 40W and "equivalents" would be KT88, 6550, 7027A, 8417 - with 35W anode dissipation (and the additional 5-8W of screen grid dissipation that some add to the total when triode strapping pentodes and beam tetrodes).

Due to its Direct Heated nature, the 300B would probably yield slightly more power than the mentioned tubes in triode mode... but in the mode they were intended for (let's call it pentode, although this third grid or beam forming element is not "used" in audio applications, other than being tied to the cathode or the ground) they are capable of a realistic 15W output in SE mode. In the RH Universal those tubes will allow about 11W at 1% distortion... more at 3% and 5% of course, if that is needed.

The case of the 2A3 is slightly different - EL84s are just 12W anode dissipation vs the 15W of the 2A3 family (6B4G, 6A3, etc.) and due to the DH nature, the 2A3 easily outperforms them when they are used in triode mode... but in the mode they were intended for the EL84 fights back very nicely - the RH84 will yield about 4W at 1% which is more than the 2.5W at 3 or 5% of the 2A3. More that is, in the scale where 1W means something.

For me, a nice DHT killer is the 307A (VT225). This tube is very lilttle known, aside the relatively famous headphone amp by P. Millet which uses those tubes in TRIODE mode (g2 and g3 strapped to anode). That is why I have promised a long time ago an RH amp with those tubes, and finally built and published it in my blog a couple of months ago -  the RH-307A.

The 307A has 15W anode dissipation plus a couple more for the screen grid. When used in triode mode it basically equals the performance of a 2A3 family tube (we are not discussing the sound, and although I have not used 307A's in triode mode - not interested at all when I can use them as pentodes). But when used in pentode mode, the datasheet promises up to 9W of power, which is a little too much coming from a 15W dissipation tube. My model is quite limited, thus I have no simulated results for the RH-307A, but it goes much louder than the RH84 and gives way slightly to the RH Universal - meaning that we are probably at 7-8W of power at 1% distortion - which literally "licks" away the 2A3 family (I happen to have a very good classic no-feedback 6SN7 into 6B4G design with choke input filters etc... and it just does not play in the same league, at least on "normal speakers" -- i.e. 88 dB/W/m). This amp actually competes quite easily with 300B amps when it comes to power, and of course has better bass and overal control than a no-feedback 300B amp - those seldom sound borderline warm to syrupy.

I guess this gives a better image of the pentode-DHT battle

[Alex Kitic]

Pentode amp has intrinsic high output impedance without overall feedback applied to the output transformer.

Your circuit does not use overall feedback but uses 100K shunt resistor(using 12AT7) on the output tube, this I believe reduces the output impedance somewhat, did you manage to make some measurements(impedance) using this configurations. If you have any results please let us know.

I suspect the reduction in output impedance is not really significant using the circuit configuration. Maybe you can prove me wrong.

The anode to anode feedback might be considered a better option than the overall feedback encompassing the OPT since there are less troublesome elements and the loop is shorter (smaller if you wish). Of course, the effects of feedback are those which one might expect - besides the lower distortion, there is also lower output impedance. The EL84 in the RH84 has approximately 290 ohms output impedance in the circuit, while the more powerful tubes (those used at 100mA current draw) are about 150 ohms (in circuit) - just to try to quote and approximate a few numbers. From those you can guess for yourself whether it is a significant reduction... or not.

Thus if the sound of pentodes and beam tetrodes without feedback is "bad" due to high output impedance, when used properly the impedance drops at levels below the DHTs and obviously produces better control ("dumping factor"). But I would like to avoid this type of "simplification" - there is more to a good design than just one element, and even that element has to be accurately defined and applied.

[Alex Kitic]

A lot of the tubes mentioned in this thread are actually beam tetrodes, right?  Are beam tetrodes inherently lower in distortion, even when treated as pentodes in a circuit, than a true pentode?

All these Tetrodes tubes had bad sound when used as Tetrodes due the kink effect, they are used as Triodes for audio;
As perennialized by the KT brand(Kinkless Tetrode).

Most tubes mentioned are beam tetrodes - all the 6L6 family, 6550s, KT66 and KT88... EL34 and EL84 are pentodes, and the 307A (VT225) is a direct heated "true" pentode/

1) Beam tetrodes do not have an intrinsically lower distortion than pentodes. This depends on the design and application.
2) Tetrodes without the beam forming element are just - tetrodes. Those have the "kink" - but I still do not imagine they cannot be used better than just in triode mode (although the latter would require no knowledge, experience, and thinking - a resistor would do to strap the g2 to the anode). I think I have no "just" tetrodes at hand, and that is most probably because they are "long extinct", meaning that they were obsolete even during the heyday years...
3) Kinkless tetrodes (KT) are beam tetrodes, i.e. those WITH the beam forming element which is analogous in function to the third grid of the true pentodes.

[Freo-1]

As I understand it, most of the beam tetrode tubes are technically considered "Kinkless Tetrodes".  The KT is more or a marketing moniker than anything else.  The idea of the beam tetrode was to work around the pentode patent.