The RH84 seems to be my most famous amplifier – and probably the favorite EL84 DIY amplifier. Recent builds of the RH84 revision 2 confirm the good sound that many DIY audiophiles have enjoyed so far. Thus, the RH84 remains a good vehicle to further new ideas and improvements.
Critics have mostly criticized the choice of the driver tube – ECC81 – claiming that the application of a pentode driver tube would yield more power and/or less distortion. Still, to most using a pentode as driver actually means using it in triode mode (g2 and g3 strapped to the anode) – and while this is a valid if meaningless approach, it should not be mistaken for a pentode used in pentode mode as driver, which is what the critics claimed as superior, but have never shown in practice, either as one of the numerous copycats, or as an RH inspired improvement.
The Pentode Edition in the name of the amplifier stands for “all pentode”. The driver chosen for this amplifier is the 6AU6 (actually, 12AU6 – since I have got no 6AU6s, and the difference is confined to the heater voltage). At 6.2mA current draw it’s operating point is just where most would put it (if they knew how)…
For my taste, or more precisely, speakers – the RH84 could be improved by having more power. 5W is barely enough for 88dB/W/m speakers, if you like listening to music at moderate and high levels: this is obviously not an issue for fortunate owners of the various large Klipsch, Lowther, Tannoy, Altec, or JBL speakers, with efficiencies higher than 94dB/W/m. While many of those who built the RH84 over the years own such speakers, those that have lower efficiency speakers have stuck to this amp due to its good sound… and eventually gone further in the search of more power. Unfortunately, the power output is a limitation of the output tube – with 12W dissipation, an indirectly heated pentode cannot yield more than 5-6W. Since the EL84 cannot be driven into class A2, no additional power can be had above it physical limits – and there is no driver than can change that – as it should be clear to everyone with enough common sense and some electronics knowledge.
The only way to have more power from the cheap and affordable EL84 is the parallel approach: two EL84 in parallel have a combined anode dissipation of 24W, and output power is comparable to, if not slightly exceeding, the EL34. The parallel approach has been criticized by purists due to the fact that identical tubes do not exist and even matching cannot solve all the issues…
OK, so you cannot have everything from life, thus improvisation and compromise are necessary – this is not a perfect world. With the right design choices and approach, the most important issues related to parallel output tube implementation can be overcome and minimized.
Let’s start with the output tubes: as shown in the schematics, the two devices share the anode connection to the primary of the output transformer, and the zener diode connection to the B+ (22V of drop, slightly more than the drop across the output transformer, keeping the g2 at a constantly lower potential than the anode). The lower half of the tubes is actually where the separation occurs – each tube is controlled by a separate current sink, matched to equal current. The two grid stopper resistors connect to the common grid resistor for both output tubes. If the tubes are perfectly matched, the bias voltage across the current sink will be identical – but eventual mismatches will be automatically addressed as an equalizing voltage across the grid resistor: in practice, with reasonably similar tubes (not matched) this voltage is in the 0.02V range, meaning a grid current of 6.06x10-8 A (sorry if it’s more than the eclectic taste and knowledge of some allows). Even quite mismatched tubes (worn and almost new tube) will not lead to more than 0.4V, i.e. grid current of 1.2121x10-6 A… thus it can be concluded that the two tubes tend to balance without problematic repercussions.
The current sink device chosen for this version of the RH84 is the lowly and cheap 7805. Besides the need to provide some ideas and guidance to the DIYer, this was also done to keep costs lower: the 7805 is 33% cheaper than the LM317 (it that is of any importance) and if 10 pcs from the same batch are acquired, it should be easy and feasible to match accurately for output voltage two pairs of devices. Because, the output voltage is the reference voltage that will define and limit the current draw of each current sink, and thus each EL84. Furthermore, since the output voltage is 5V, approximately half the bias of the EL84 in this design, the resistor will perform half of the dissipation (approximately 250mW) leaving the remaining 1/4W to the 7805 which can handle up to 600mW without heatsink. If that was not enough, the screw hole is connected to pin 2 or ground, and thus is at ground level – meaning that it can be simply bolted onto the metallic chassis of the amplifier (if it is cold enough and not heated by the transformers and tubes…). The current setting resistor can be a 1/2W unit, and I have used 0.6W metal film precision resistors for this position.
Each EL84 cathode is separately bypassed to ground by means of approximately 100uF valued capacitors – I have chosen to place in parallel two 47uF/22V ROE tantalum caps, but any cap type above the 22V rating would do.
The driver in the RH84 PPE obviously is a pentode, used as a pentode (in pentode connection). While small signal pentode tubes can always be used as triodes by strapping g2 and g3 (if not already connected to the cathode inside the tube) to the anode – I see very little reason to pursue that direction, since there are many types of triodes readily available with a wide range of gain and current draw characteristics, thus an adequate driver can always be found without resorting to triode strapping pentode tubes. On the other hand, using a small signal pentode in pentode mode can bring some advantages over triodes, mostly in the available gain “department”… of course, using a pentode as driver in the RH amps (or any other similarly conceived amplifier which does not operate in class A2, or with such output tubes that are not suitable for class A2 operation, provided the pentode used as driver could drive the grid of the output tube with the current necessary… to be precise) will improve neither power nor distortion: it’s used to show that it can be done, and how it should be done properly, with excellent sonic results.
Small signal pentodes are used in a slightly different manner than output pentodes – the g2 resistor in small signal pentodes has a very important function in setting both current draw and gain, and the screen grid should be adequately bypassed to ground, since the combination of g2 resistor and cap forms an RC filter: in this case, 10uF to 20uF caps should suffice. Another detail to keep in mind is the voltage value of this bypass cap – while in operation the g2 will be below anode level and roughly at ½B+, at power-up the grid will not draw any relevant current and the cap should have the same rating as all the other power supply caps!
The 6AU6 was chosen as driver tube due to the fact that it is not some exotic hard to find tube, it should be relatively cheap – and because I had some 12AU6, mostly CEI (gray anode) and RCA (black anode), at hand. The 7 pin sockets are similar in looks to the common noval sockets and are easily available. I am not aware whether there are any xAU6 tubes from current production, but the NOS stock of those tubes does not seem to be dwindling yet.
Another small signal pentode I would use for the task is the EF86 – if I am correct, those can be sourced from current production as well. They require standard noval sockets and are therefore even more easily applicable to the driver task. I guess prices are slightly higher… anyway, I did not use it because I have got none, but here’s a simulated schematics with resistor values for those who would like to try the EF86 in pentode mode as RH84 drivers.
What about the old faithful, ECC81? Of course, it can be used as driver for the parallel RH84 as well, and the results of the simulation are, of course, quite in line with the results of the 6AU6 and EF86 versions – attached is a resistor configuration similar to the RH Universal which is actually optimized for use with ECC81. As a design issue, the ECC81 is a double triode, thus requires only one socket for two units (and half the space in the amplifier, not to mention less heater wiring). Furthermore, there are no additional g2 resistors and bypass caps – less parts equates not only to less cost, but more importantly less complexity and potential problems.
When it comes to sound, I guess we are on very subjective ground. Ultimately, higher quality tubes, drivers in particular, will lead to better sound – that is beyond subjectivity. The PPE is designed as to sound just as good with a garden variety xAU6 as the RH84 sounds with a garden variety ECC81 – just like the EL84 in the RH84 amps sounds in pentode mode just as good as in triode mode... But the power part of this well-known sentence does not hold water here – while in pentode mode the RH84 has (much) more power than in triode mode, the pentode driver does not add to the power, nor improve on the distortion values. On the other hand, there are several great ECC81 family members out there (6201, ECC801, ECC801S, etc.) that are quite tough to beat (in my book, and in the books of many others). Frankly, the RCA black anode 12AU6 are hardly a match for the Philips SQ 6201 – for my taste. As I already stated, this is subjective opinion terrain, and everyone should attempt to find his own best combination of tubes. To someone owning a box full of black anode RCA 6AU6 the acquisition of some Philips or Valvo 6201 (particularly at current prices, which inevitably increase in time), probably represents a nonsense – unless they are absolutely necessary.
It is quite obvious that the RH84-PPE lacks “universality”. You can use the xAU6 chosen, the output tubes are EL84, and the power supply as drawn requires a GZ34/5AR4 rectifier. But if your output transformers have 8 and 4 ohm taps, you can always play the game of “power vs finesse”! Provided your loudspeakers are a nominal 8 ohm load, if connected to the 8 ohm tap the anode load will be 3k, just as foreseen in the schematics. But if you connect them to the 4 ohm tap, the anode load will be 6k, just what is needed for a classic RH84. The schematics foresees the possibility to operate the amplifier with just one EL84 per side into 6k – with half the power: just unplug one EL84 per side before powering up, and you can check whether two tubes in parallel necessarily lead to a loss of focus, as some like to define it. This is basically the same game as “triode vs pentode RH mode” some used to play at the time the original RH84 was introduced. The output transformers will probably be quite large (3k at 100mA DC current across the primary) and thus represent an overkill for a single EL84 – if nothing else, power bandwidth should be at its best, and expected power output is just above 5W per channel. You could do it as well to consume less energy – up to 50W per hour less, as your contribution to a greener planet.
Operating the amplifier with just one EL84 per side will decrease the current draw for approximately 100mA, thus the B+ will be too high at approximately 360V. Therefore a change of rectifier will be necessary, and a 5R4 will provide the correct B+ value. Since the current draw will not exceed 125mA with one EL84 per side, a 5Y3GT can be used as well.
Last but not least, a few words about the power supply. It is kept deliberately simple to keep costs as low as possible, thus a simple C-L-C filter is applied. The first cap should be oil for the best results, while the second cap can be electrolytic. Any choke capable of approximately 200mA DC would do, preferably a 10H unit. Since the power supply is cap first, the choke will not be particularly stressed, and does not need to be particularly large or stiff (potted etc.).