RH84 PPE – "Parallel Pentode Edition"

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.).

RH-307A

(the DHP RH amp)

An RH amplifier with the 307A (VT225) output tube has been a long delayed project for me, a standing promise since the time of the original RH807. The resemblance between the two tubes actually ends with the 5-pin socket and cap, among other reasons because the 307A is a true pentode – unlike the 807 which is a beam tetrode tube. At the time, datasheets for the 307A were unavailable on the net, and the only reference while I write is a WE datasheet: strangely, because I have never seen a WE 307A, not even a picture of it – the most common 307A (VT225) are Ken-Rad and National Union.

 

While the promised amp was obviously to be an RH type, when I finally decided to design it I was already over the “old” type schematics, but the relatively low anode voltage at which it would operate as compared to the relatively high bias required (i.e. cathode voltage: the voltage swing that the driver tube has to produce will be significant) meant that the tried, tested, and faithful ECC81 family might not be entirely suitable for this task, unless used in the original RH type schematics. This is where the ECC88/6DJ8 family enters: the very high trans-conductance allows this tube to obtain the necessary voltage swing (approximately 60V p-to-p) while being operated at relatively low anode voltage (low, that is, to ECC81 standards).

The RH-307A is largely based on the RH Universal schematics – the main differences are the different driver circuitry and the fact that being a direct heated pentode, the 307A has a slightly complicated “cathode circuit”. The anode of the driver tube is connected in RH 2^nd generation style (Rfb being at the same time the anode resistor of the driver tube), and g2 (the screen grid) is connected to B+ through a 51V zener diode. While providing a referenced voltage for g2, the zener diode value is chosen to ensure that in normal operating conditions, whichever rectifier is chosen, the voltage across g2 will not exceed 300V.

As the 307A is a direct heated pentode, its heater wire is at the same time its cathode. Besides the known AC hum related issues, this poses as well the voltage referencing problem – there is a definitive need for a reference point substituting the cathode connection in indirect heated tubes. This reference point can be created with a wire-wound potentiometer which can be used as well for hum-nulling, but besides the fact that a pair of resistors costs less than a pot, I tend to have more faith in a soldered connection than a pot slider. Since the hum-nulling spot will inevitably be the center position, the best solution is to match two resistors (100 ohms) and use them to connect in series the two heater-cathode terminals: in  this manner the mid-point between the two resistors becomes a cathode reference point.

Being a true pentode, the 307A has a suppressor grid (g3), and being a direct heated tube, this grid has its own pin. Since the suppressor grid in pentode mode should be connected to the cathode, in this case the connection should be performed between the g3 pin and the “cathode reference point”.

The current setting circuitry comprises the usual LM317 and a current setting resistor, in this case 27 ohms. This means a constant current draw at the cathode of approximately 46mA, of which 43mA are anode current, while 3mA are g2 current (estimates based on the datasheet, and on mathematical operations subtracting driver current draw from the voltage drop across the transformer primary). To bypass the cathode circuitry, each heater connection is decoupled to ground via a 100uF 63V (or 100V) cap, providing a direct AC path from cathode to ground.

The heating of the output tubes could warrant a debate of its own – choices vary between AC and DC, where DC can be un-regulated, regulated, or fixed current. Whichever choice is made, it is very important to keep the cathode circuitry intact: I can only stress so much the importance of the “cathode reference point” in the connection towards ground.

My choice is always the simplest solution – AC heating. AC heating has several theoretical advantages and some shortcomings. The most important theoretical advantage is having the same potential (DC voltage) across the entire length of the cathode (heater). The most important (and audible) shortcoming of AC heating is hum (AC mains 50Hz or 60Hz low hum) which can be heard oh-so-much without the proper hum-nulling potentiometer, or the cathode reference point solution which I advocate. Even nulled, hum can still be heard, depending on the efficiency of the speakers. On my “common real world speakers” of approximately 90dB sensitivity – hum can be heard from the vicinity of the woofer, but is completely inaudible a couple of meters away (listener’s position), even during the quiet late night hours. AC heater hum depends as well on the heater voltage and is definitely lower than with 6B4G tubes, due to the 5.5V heater voltage as opposed to the 6.3V required for the 6B4G: I would guess it is comparable to the hum produced by an AC heated 300B amp.

A further shortcoming of AC heating is maybe more theoretical and seldom mentioned: due to the sinusoidal nature of AC current, the heaters (cathode) are not at the same exact temperature all the time… which temperature changes 50 (or 60) times per second. With DC heating, the temperature is constant since the voltage is constant as well… but I guess this is more metaphysics than real world experience.

The main advantage of DC heating is the lack of hum – but the rectifying and regulating circuitry is directly connected with the cathode and thus very much in the signal path (while the cathode current source made with the LM317 and current setting resistor is not, since it is bypassed with the two caps providing a separate AC path). Frankly, for high efficiency speakers (96dB and more) DC heating is the only reasonable option. In that case, whichever solution is chosen (regulated, current draw…) the output terminals should be connected to the heaters terminals on the schematics, and the rectifying/regulated DC circuitry MUST NOT be connected to ground.

Last but not least, the output transformers issue… The operating point is largely chosen on the recommendations given in the datasheet, at 43mA anode current and anode voltage slightly in excess of 300V (across the tube), taking care not to exceed the 15W anode dissipation rating. The chosen anode load is 5k – basically the same load used in all the 1^st generation RH amps, which is very similar to the datasheet recommendation. The 9W output power seems too high if compared with a 15W anode dissipation – but it can be expected that usable power will reach 7W. While I have not performed any measurements on my amp, it does go very loud – way above 6B4G levels, and definitely louder than the RH84. It is thus safe to say that the “usual” console amp output transformers that can be used for an RH84 can be used for the RH-307A as well, but a decent pair of output transformers is definitely in order if the full potential of the amp is to be unlocked.

The power supply should foresee at least three low voltage secondary windings, since each output tube must have its own, and the driver tube must be heated separately from the output tubes. Another low voltage secondary is needed for the rectifier tube – if one is used (which I always advocate). I would recommend the choke input solution as documented – after all, due to the DH nature of the output tubes, this amp immediately commends more respect than a humble EL84 tube. Still, the most important requirement is achieving between 360V and 380V at the B+ point. This is the variation between 5Y3GT and 5U4G in the proposed schematics, the latter touching 15W anode dissipation (in my amp – slight variations are possible with different DCR values of chokes and output transformer primary).

The choice of tubes is in this case limited to the driver and of course the rectifier tube – the 307A having no compatible replacements that I know of, including those with a different socket. Thus, while the driver circuit is designed having in mind the ECC88 family of tubes (any ECC88 type will do, since expected anode voltage is 85-90V, way lower than the 130V limit for common ECC88/6DJ8). E88CC, CCa, 6921… all those tubes will perform flawlessly in the driver task. Besides those, direct replacements (same pins) that will allow the amplifier to achieve full power are 6BQ7, 6BQ7A, and ECC85 (6AQ8). The latter two tubes have lower (but still high) trans-conductance than the ECC88, but make up for it with a higher mu (the ECC85 has 58). While I have not tried it, I know that with a different socket wiring the 5670 (2C51) would do just as well as a 6BQ7A. All the tubes mentioned require 6.3V at the heaters and have internal shields: the shield should be connected with one of the heater connectors and directly to the ground, thus grounding the driver heater winding.

 

The rectifiers proposed are the usual 5R4 and 5U4, but due to the low current draw of this amp (110mA) it is also possible to use the 5Y3GT. The proposed power supply is choke first, and with the hybrid Graetz configuration each anode will “see” half the secondary voltage… at 260V and 110mA current draw, the 5Y3GT can be used in choke first power supplies without any worries or problems. If a different socket is used, 5Z3 and 80 are viable alternatives to 5U4 and 5Y3.

For the 307A heater windings, since 5.5V is an odd number, there is a simple and cost effective solution – 6.3V windings (again those 6B4G amps!) are perfectly suitable if 0.44 ohm resistors are placed in series with each terminal (at 1A current draw, 2x 0.4 ohms means 0.8V difference, i.e. exactly 5.5V – most 6.3V secondary windings give slightly more than 6.3V, and 0.4 ohms is not a standard value). 5V windings for 300B heaters might be probably used with success, since the 300B needs slightly higher heater current, thus the output will most probably be at least 5.3V at 1A.

The 307A (VT225) is a relatively odd tube which is poorly documented on the net. Besides a very popular high quality headphone amplifier, there is almost no other amplifier with this output tube -proposed or documented. Most amplifiers mentioned or seen use the 307A in triode strapped configuration, as some sort of “poor man’s” DHT – which is a shame, given the huge difference in power in comparison with the 15W anode dissipation DHTs (2A3 family including 6A3 and 6B4G). Actually, while the 1^st generation of RH amps was mainly challenging comparative designs with EL84, 807, EL34 – the 2^nd generation challenges further the more expensive and coveted DHTs, like 300B, 211/845, and the 2A3 family of tubes. As always, I prefer letting others judge the sound of RH amps, but in this case I will allow myself a hint: this amp sounds disturbingly good…