The RH300B project was the first envisaged in the 2nd generation series, but the last to be developed. The development of this amplifier was long stalled by material reasons, mostly because I had no 300B tubes to build and breadboard it. Thanks to the generosity of a DIY friend who sent me two pairs of these DHT tubes, I was finally able to breadboard, troubleshoot, and finalize the project.
While an initial circuitry was published on some forums and sent to some DIY-ers back in 2005, when the time came to finalize the project some design choices were to be reconsidered. Thus the RH300B introduces a different driver stage in respect to all previous RH amplifiers, while simplification was introduced in the power supply section.
For the first time, I was hesitant and reluctant to publish the schematics of this amplifier. A lot of work, knowledge, and insight have gone into the design of the amplifier, and the various difficulties encountered on the way clearly show that these projects are not to be taken lightly. Good ideas and lots of knowledge are not enough; building an amp definitely requires some material means. RH amplifiers have met the criticism of all those who felt threatened by the publicity gained in particular by the RH84, and while many DIY-ers have built and enjoyed one of the several designs, the RH series has also spawned lots of copy-cats, and word is out that some are building RH amps for their local clients – mostly DIY-ers without the time at hand, or the experience required to build their own amps: while the clients should not be judged for feeling challenged by the prospect of building a tube amplifier, since it is among other issues dangerous as well (these voltages could kill you), the “hired builders” should at least give some credit to the original designer and avoid changing important parts just to make it look different at first site. There is a site (I will not paste shortcuts, but you can easily find it based on the description) showcasing a parallel SE amplifier with 807 tubes which is basically a copy of the original RH807 adopting 2nd generation solutions taken from the RH88, while the only difference is fixed biasing with negative voltage instead of a cathode resistor under the output tubes; the site owner has reluctantly agreed to mention my work by adding a blurred statement how his amp is in fact inspired by work of a compatriot of his, whose triode connected 807 amp (sic!) was in turn inspired by one of my designs… is there any shame left in people? Thus this amplifier represents the last in the 2nd generation series, published for all DIY-ers.
As it is evident from the schematics, the driver section now has a cathode follower directly coupled to the driver itself. While the driver is a rather standard choice for RH amplifiers – an ECC81 family tube, with anode-to-anode feedback connection to the output tube – the cathode follower portion of the driver is not. The cathode resistor of the driver tube is split in two, of which one resistor is bypassed with a capacitor – which is also a first in RH amps. This particular split is necessary in order to maintain a correct biasing of the driver tube while increasing the gain, in order to keep the input sensitivity below 2V RMS for full output (as declared at 1% distortion).
The cathode follower is not strictly necessary, as the driver tube is well capable of driving the output tube on its own. This particular detail appeared as a necessity while contemplating biasing methods for the output tube, since fixed bias requires the adoption of a grid resistor lower than 50k. With such a load, the driver was not capable to perform its task adequately, and the cathode follower represents the best way to solve the problem, since it does not invert the signal nor add any gain, while it is possible to couple it directly to the driver tube – avoiding additional capacitors in the signal path.
In the end, fixed bias was discarded, since the only apparent benefit would have been a lower B+ voltage. On the other hand, the negative bias power supply represents and additional cost, complication, and a source of potential trouble for the DIY-er. Furthermore, it would require checking the bias voltage of the tubes from time to time, as well as adjustments every time the output tubes are changed. All the amplifiers I have designed so far were easy to build and straight-forward in use, which to me represents an important feature – thus fixed bias (i.e. bias by negative voltage applied to the grid) was discarded as it brings more potential problems than eventual benefits.
Lowering the output impedance and increasing the current draw has enabled lowering the B+ voltage which mostly compensates for the only possible gain from fixed bias – as a result, DC RMS voltages are never higher than 470V (with 5U4 rectifiers) which helps keep costs down while the second cap in the power supply remains safely in the 500V WKG category (of course, with both channels operating, i.e. drawing current).
Finally, the cathode follower has remained in the schematics, since it obviously helps the driver tube perform its task, and keeps distortions at even lower levels. While the driver tube should be an ECC81 family member (12AT7, ECC801, 6201, CV4024, etc.), the cathode follower can be almost any small signal triode tube – almost, because once you choose the socket and the basing connections, there is little variation possible – 5687 is one example - and of course you have to keep in mind the operating conditions for this tube, which preclude some choices (most ECC88 tubes cannot be used, and even E88CC types are not advisable). But if we stick to the more common noval tubes, it could be an ECC82/12AU7, ECC81/12AT7, even an ECC83/12AX7 – while in the octal domain, a 6SN7 would do the job perfectly well (12SN7 would suit just as well, but you will have to provide 12.6V for the heaters, and connect the heaters of the driver tube in series mode instead of parallel). Whichever tube you choose, as the cathode follower section is directly coupled to the driver tube it will be biased to approximately the same conditions. What will vary is the bias voltage of the CF tube (grid to cathode differential) as well as the input impedance (approximately ranging from 2meg to 5meg) and the gain (in CF slightly lower than 1, depending on the mu of the tube used). It is thus not unimportant which tube is chosen or used – as it will have an impact on the sound of the amplifier.
The cathode follower section requires referencing the heaters supply for the driver tubes at approximately 20% B+ voltage. Most datasheets recommend not exceeding the heater-cathode differential of tubes by more of 100V DC, thus it would not be a good idea to have the cathode of the CF tube at approximately 160V DC without doing something about the potential of the heaters. The best option is to reference the heaters to a potential between the cathodes of the two driver tubes, and a 220k/47k voltage divider will perform this task perfectly well.
Of course, having two triodes per channel helps employ one double triode per channel in a mono-block amplifier configuration… there is no reason why you could not use the ECC81 as the CF tube as well, and as a matter of fact it provides the highest input impedance among all the proposed tubes.
The output tube, as you already know, is the 300B. NOS 300B tubes are probably extinct by now, or too expensive, and I cannot expect that DIY-ers – people who are not ready (willing, or able, choose your game) to spend 5 figures in USD or EUR on Hi-End amplifiers (Hi-Cost no doubt, the Hi-End sound is in the ear of the beholder) – would spend 4 figures on the output tubes alone. Thus the amplifier was designed, bread-boarded, and finalized keeping in mind the current production 300B tubes.
The biasing of the output tube is automatic as customary in RH amplifiers, set by a current source (or sink, if you like) made by a simple LM317 and current setting resistor. Regardless of the tube inserted in the socket, the current draw will always remain the same, and since this is a triode, there is no second grid to take into account – all current drawn is “anode current”. The current is set in such manner as to be near the upper limit for 300B tubes, while the voltage drop across the tube, deriving from the combination of current draw and B+ voltage, puts the anode dissipation between 30.5W and 33W, depending on the chosen rectifier tube (5R4 or 5U4 types). The anode dissipation is on the upper conservative level, which basically means that you will not unnecessarily shorten tube life while enjoying the best sound those tubes can provide. This relatively low anode dissipation is certainly adequate for the mesh anode types, either “mesh anode” or “plate with holes”. Those seeking additional thrills can always use a GZ34 (5AR4) rectifier for higher B+ voltages resulting in higher anode dissipation (about 36W) – this is feasible but not at all necessary.
As for heat and tube life, I have measured the temperature on the hottest point on the glass of the globe “mesh” 300B tubes after several hours of operation at approximately 33W dissipation, and it does not exceed 100⁰C. Real datasheets for current production 300B varieties do not present the same richness of data as the datasheets from the golden age of tubes, but I guess that an output tube in an SE amplifier, operating at temperatures lower than that of the rectifier tube in the same amplifier, where the rectifier tube is definitely running inside the safe envelope of operation, means that the output tube is not stressed and will not encounter a premature end – at least due to anode dissipation – regardless of what some tube dealers would like you to believe. On the other hand, filament defects (breakage) are definitely not caused by the ability of the anode to disperse heat, and should be addressed in a different manner.
Last but not least, the output tube cathode circuitry is virtually the same as the configuration already explained for the RH307A amplifier. Since the LM317 can only handle up to 35V input, the inserted resistor keeps the environment safe for the current regulator. Regardless of 300B type, or rectifier used, the voltage across the regulator cannot exceed 30V. Another solution would be the TL783, which is good for up to 125V input – but besides being relatively difficult to source, it would have to dissipate up to 7W, requiring a very serious heat-sink (temperatures inside the amplifier should be taken into account as well). The voltage dropping resistor should be powerful enough to withstand up to 5W dissipation for a long period of time (11W types and higher recommended).
The heating circuitry is AC, but 50Hz hum is absolutely low and completely inaudible on my speakers, due to the cathode circuitry solution and the general schematics of the amplifier. Even on more efficient speakers the AC heaters hum should not be prominent enough to annoy the listener. While the question which heaters power solution provides the best sound – AC, DC, unregulated, voltage regulated, or current regulated – it is my opinion the AC probably sounds best due to the fact that the whole filament is kept at the same potential level. But leaving this discussion aside, since each and every amplifier should be regarded as a whole, another great advantage of AC powered heaters is the simplicity of the solution. If the inevitable mains frequency hum is not an issue (in this amplifier it should definitely not be a problem), AC is the best, cheapest, and most practicable solution by far.
The Power Supply
As already mentioned, the power supply is simplified in respect to previous RH amplifiers. This is a normal cap input power supply, where the rectifier is combined with solid state diodes in a hybrid bridge. The hybrid bridge is more efficient than the standard full wave with central tap rectifier, and puts less stress on the rectifier tube. It is also by far less complicated to set and use than an all tube bridge rectifier, which would require more tubes and additional secondary windings. The sound (contribution to the sound of the amplifier) of the hybrid bridge is virtually identical to the sound of the same rectifier tube used in a classic central tap arrangement, providing a win-win combination. Rectifiers can be switched easily, either to tune the sound of the amplifier, or to increase (or decrease) anode dissipation. Since output tube biasing is automatic and always the same (set by CCS), there is no need to adjust anything when changing either output tubes or the rectifier tube.
The power supply is cap input, and this first cap should be a 600V poly or oil type – a good choice would be motor run caps rated at least 400V AC. The second cap should be a 500V type, and it can be a good quality electrolytic. There is no need to bypass any of the two caps with smaller caps. The choke is not critical as the only important element is DC current handling – a 200mA type is the minimum requirement. As for inductance, 5H or more is recommended for a good level of filtering.
The RH300B can be built with just one power transformer, or with several, depending on the size of the enclosure and the willingness of the DIY-er to order custom wound transformers. I have built mine employing a 150VA custom wound toroid for the high tension and rectifier heaters, and another smaller custom wound 30VA toroid for the heaters of the output tubes and the drivers. The two cannot be squeezed in a 150VA core, but could be wound on a 200VA core which would probably be a more expensive solution due to the large number of secondary windings. This even larger toroid would also require more space, both as sheer physical size, and probably due to the more complicated placing of the secondary windings output wiring. There is absolutely no need for a more powerful high tension secondary than provided by the schematics: while the DC current draw of the B+ is slightly above 210mA, the AC current draw as measured on the HT secondary is 270mA – less than foreseen by PSUD2 or calculated using the usual formulas. The temperature of the transformers in operation is up to 60⁰C and 40⁰C, respectively, in the box, after several hours of operation – thus it can be regarded as absolutely recommended.
While the schematic requires 5V secondary windings for the output tubes, it is advisable to adopt 6.3V secondary windings and drop the voltage to 5V across 0.56 ohm resistors on each leg. This will limit the current drawn by the cold filaments at power-up and definitely increase the life of the tubes, since filament failure through breakage at power-up is a common problem with many current production direct heated tubes.
A word about the output transformers – as shown in the schematics, those should be 2.5k primary impedance types. The secondary, of course, should be adequate to the nominal impedance of the speakers used. As for size, while size does matter, manufacturers tend to rate their amplifiers as output power, which is a relative point of view. You could build a 2A3 amp and a 300B amp using the same output transformer, as both tubes would work into 2.5k loads – but while a classic SE 2A3 amp will provide approximately 3W, the 300B will provide approximately 8.5W, thanks to its higher anode dissipation, and in particular due to the higher current draw.
Thus the most important part when choosing output transformers for the RH300B should be whether they are built to operate with 100mA DC current across the primary winding. If that request is met, all the remaining details (physical size, DC resistance of the primary, etc.) will work towards achieving a more or less extended and defined output, introducing additional distortion and loss in a more or less pronounced manner. While the RH300B thanks to the circuitry will have an excellent damping factor (DF), and will sound perfectly well on most output transformers that are fit for the current, primary impedance, and output power – since this is really a top-notch project, you should try not to spare on the output transformer. What is true of the RH84, re-built by many with high quality parts and output transformers to enjoy its qualities even more, relates even more to this amplifier, as the parts quality will be rewarded with increased sound quality.
End Results and Remarks
This long awaited amplifier, while being the first to be designed in the 2nd generation series, was the last to be finalized and built. The RH amps started as an alternative to classic DHT SE amps, succeeding in offering better sound and at least similar output power, step by step, DHT by DHT. The RH Universal easily over-powers classic 300B amps while providing better bass and definition, while the RH307A offers approximately the same power of classic SE 300B amps from a less expensive tube (while providing better bass and definition), and adds the thrill of direct heating, the particular sound of direct heated tubes… What is to be expected of the RH300B, since this is the tube mostly addressed by previous RH amplifiers with pentode (and beam tetrode) output tubes?
All RH amplifiers sound quite similar – since they have been designed by the same person adopting the same scheme type, which is quite rigorous on the tubes, imparting a particular type of sound. The main difference, beside output power, is the output tube itself, as the intrinsic sound quality of the tube will define the overall sonic quality of the amplifier. Direct heated tubes definitely have some interesting edge when it comes to sound, and the 300B is probably the most coveted of all DHTs – due to its intrinsic sound qualities, and the relatively high power it can provide, which is necessary in our world of relatively inefficient speakers spawned by several decades of powerful transistor amplifiers rule on the commercial Hi-Fi market.
Well, this amplifier is virtually a nail in the coffin of classic SE DHT amps. Whatever the 300B tube used (since there is currently a relative multitude of current production 300B types, and of course the almost extinct NOS WE) would provide in a classic SE design, it will do better and provide more in the RH300B amplifier. This is not any more the case of comparing pentodes with the 300B – but using the same “weaponry”. As a matter of fact, this was an easy win.
The simulation clearly shows 12W output at 1% distortion, with almost 2V RMS input sensitivity. I do not have any equipment that I could use to provide measurements, so DIY-ers will have to believe my word – or the simulation presented. Nevertheless, what matters is the sound, and the relative loudness that the amplifier can achieve in a system. In practice, the RH300B goes just as loud as the RH Universal does – on my normal real-world 88dB/W/m speakers – quite loud by anyone’s standards. There is no hint of the usual syrupy sound of SE 300B amps, which is so good for taming wild high efficiency horn speakers… instead, the sound is extremely detailed and controlled, with great bass definition and depth. Such statements are not to be taken lightly, but those who have successfully built any of the previous RH amplifiers probably know that their expectations will be met and exceeded.
If you ask me, my only regret is the fact that the 300B, being a triode, does not have a second grid that I could use to my advantage, like a lever to increase performance even further… It is interesting how many attempts have been made to copy the 300B, including indirect heated prototypes, curve clones, that were made out of pentodes in which the second and third grid were missing, not wound on their supports during manufacturing. All this trouble to construct a good triode, when there is so much to be had from pentodes – only because the knowledge required for that task has been lost or misplaced. Humans sometimes find it easier to replace an object, or resource, than to learn how to use it properly.
This amplifier represents in a way the end of the 2nd generation series. The next, 3rd generation of RH amps will strive to go further, including different and maybe unexpected output tubes, and the circuitry solutions necessary to make these tubes work at their best.
Designing these amps takes a lot of time and thought, as I try to see the eventual problems in advance and design including simpler and more effective solutions, never straying from the path of excellence: the sound comes first, since those amplifiers are designed to enable enjoyment when listening to music, not winning awards. Bread-boarding an amplifier requires resources which are neither cheap nor easy to source (particularly in my country). The RH300B exemplifies this more than any of my previous amplifiers, as it was not possible to bread-board and build the finalized amp without the output tubes, and the lack of other resources, like output transformers and chokes, hit me more than ever before: for instance, the output transformers had to be extracted from the RH307A (those are 2.5k into 4/8 ohm transformers, when connected to 8 ohm speakers they present a 5k load to the output tube – the same ones pictured in the RH84 PPE prototype), and I had to find a way to design a good sounding power supply using only one choke in a cap input configuration…
There are no publicity banners, nor a Pay-Pal button on this blog. My efforts have always been totally non-commercial. But without help from DIY-ers who respect my work, I am not going to be able to continue designing and sharing amplifiers. All those who would like to help with output transformers, chokes, tubes, sockets, caps… are welcome to contact me.