RH-307A v2 "Super"

The RH-307A has undergone further development, with the aim to improve output power and explore the possibilities of the circuit, as well as to improve reliability - achieving the same project quality other RH amps are known for.

The Driver Circuitry

I have initially considered the 15W dissipation rating of the 307A tube a limiting factor, and the 9.1W output foreseen in the datasheet as overly optimistic. Thus the circuit was developed around the 2^nd generation RH driver circuitry idea - using the feedback resistor as the only anode resistor. This circuitry inevitably leads to a limitation in power output: graphically represented, the sine wave of the driver tube is in opposite phase with the sine wave of the output tube – and the peaks of the two will inevitably collide at a given power output, unless the difference in potential between driver and output tubes is larger than the combined voltage swings. The latter is virtually impossible to achieve at common operating voltages, and represents a limitation to the output power – which does not necessarily represent an important factor if the output power cannot be much higher anyway due to other limitations, like the anode dissipation of the output tube.

The 307A has actually shown itself as an unexpectedly powerful output tube, which probably due to its direct heated nature largely surpasses the output power of other pentodes with similar maximum anode dissipation. Thus the next step was the introduction of a “classic” RH driver circuitry, albeit of the more “modern” style that can be found in the RH Universal v2. The resistor ratio (anode to Rfb) is slightly skewed, although the purpose of such values is not limited to confusing the critics or those who taught they knew everything that was to be known about the subject of anode to anode feedback… Anyhow, this type of circuitry does not limit the output power by colliding the two sine waves, thus power is only limited by the characteristics of the output tube.

Reliability

Some 307A (VT225) tubes will exhibit a tendency towards screen grid arcing at voltages near the maximum operating condition as shown in the datasheet. In practice g2 might arc (particularly at power-on) if operated near 300V, and this arcing actually happens towards the nearby g3, since the suppressor grid is connected to a low potential point (the cathode or virtual cathode point).

This does not occur with all 307A tubes, nor does it manifest itself regularly in affected tubes. Most documented cases where this tube was used in amplifiers are related to triode strapped operation, thus arcing has not happened as most tie both g2 and g3 to the anode. In pentode operation, tying the suppressor grid to the screen grid, or the anode – would cause the appearance of the “tetrode kink”, with all the negative consequences, and is therefore out of the question.

A very easy solution to this problem is operating the 307A in pentode mode with lower screen grid voltages – conditions that this “filamentary pentode” was actually intended for as a transmitting tube. I have thus made the choice to set approximately 200V as the g2 operating voltage – a value totally safe from arcing in all conditions on all of the 307A tubes I have tried.

With solid state devices in the circuit, like zener diodes and voltage/current regulators, the arcing which has otherwise not damaged the tubes or other parts of the amplifier, has succeeded in killing both the zener diode and LM317 on the interested channel… as a further reliability measure, and also after reconsidering carefully the datasheet, I have decided to connect the suppressor grid (g3) directly to ground, instead of connecting it to the “virtual cathode point” as it would have been customary and usual. When g3 is grounded, the arc from g2, if it ever happens, will miss the cathode and relevant circuitry (LM317). Also, it should be taken into account that the operation of the 307A can be controlled as well by the suppressor grid, and the potential at which it is set – another good reason to connect it directly to ground (i.e. 0V).

Additional Effects

Operating the screen grid at lower voltage means that the anode will draw less current with the same cathode to control grid voltage differential. If this amplifier had a cathode resistor, the value of the resistor would have to be adjusted. But since the current draw is controlled by a current setting device (LM317 with current setting resistor), the result will be a lower cathode to control grid voltage differential, about 15V instead of the almost 30V of the original version where g2 was operated at 300V.

Needless to say, loosing 15V less in the cathode circuitry means drastically cutting on the dissipation for the LM317, with all the theoretical and practical reliability improvements. This also means having 15V more across the tube (anode to cathode) which leads to an increase of anode dissipation since the current is fixed. While some have reported operating the 307A tubes at 22 or 25W dissipation, and 80mA current, I have chosen to stick to the values given in the datasheet – 15W maximum dissipation and 60mA maximum current (set to approximately 43mA anode current). Since the 307A/VT225 is not a tube in current production, and the stock is going to dwindle in years to come, although I think that life is too short to be squandered with sub-optimal solutions – there is no need to burn your (rare) tubes too quickly.

Since the tube is forced to conduct a set current, this means that the characteristics change – a lower input signal will lead to the same output power – the sensitivity of the amplifier increases almost twofold. The effect of this change is obviously audible – the amplifier is more dynamic sounding than the original version: the percussion attacks are more pronounced, it seems as if the amplifier has gained speed. If “syrupy” is how many would define classic 2A3 and particularly 300B SE amps, this is quite the opposite.

The change in screen grid operating point is achieved by increasing the value of the zener diodes. While it could be easily achieved using one 150V zener diode, two 75V 5W zeners are a far better choice. It goes without saying that the power of the zeners needs to be quite high, since their power rating is greatly derated with temperature (and tube amps tend to be warm or even hot). Still, a 150V 5W zener should do, but its dynamic resistance is much higher than the dynamic resistance of two 75V 5W zeners in series – this dynamic resistance is obviosly very important to achieve the particular sound in amplifiers where zeners are used to drop voltage and set the g2 operating point.

The zeners used to drop voltage to screen grids (instead of resistors) seem to be susceptible to performance degradation, and the result is a relatively fast decay in bass (low frequency) performance. This is a topic which, unfortunately, is totally un-documented elsewhere on the net! The results of zener performance degradation can be easily experienced experimentally by connecting a grid stopper resistor between g2 and zener diode – the bass will be filtered and bass levels lowered, as if some RC filter was introduced. Removing the resistor restores low frequency extension, and the same effect can be experienced when a degraded (but otherwise normal in operation) zener diode is changed for a new zener diode of the same type.

Of course, one way to avoid this issue would be supplying the screen grids from a regulated source, or even an additional power supply… but besides being large and complicated, this solution would also miss one important issue – the specific sound achieved when screen grids are connected to the B+ via zener diodes. Thus the “free lunch” solution would be using zener diodes of much higher power, possibly achieving the desired voltage drop by putting them in series, and, last but not least, keeping them as cool as possible by physically separating the zener diodes from sources of heat – for instance, not soldering the diodes directly to the g2 pin…

The series of two 75V 5W zener diodes, placed separately and not soldered directly to the sockets or to the anode resistors, definitely solves this issue with the least of cost and complication – while keeping the particular sound character.

The classical driver circuitry removing swing limitations allows to obtain full output power, but also more freedom in the choice of driver tubes. In this case, it meant getting back to the ECC81 family of tubes, and in particular to the 6201. It is more than obvious that an amplifier will work as foreseen by the simulation or mathematical calculations based on the schematics – but there is more to sound than simulation or mathematics. While circuit simulation with good models allows setting the best values for resistors and estimating frequency response, power output, and distortions – the quality and intrinsic characteristics of the tubes used will have an important influence on the sound, which cannot be simulated. Just like the 6AU6 pentodes are no match sonically for the 6201, or the E180CC, the E88CC is also not playing in the same league. While my choice of 12AU6 was relatively limited (although RCA black anode always means high quality in my dictionary), I had a lot of various ECC88 family member to play with… even the famed CCa does not come close to the sonic performance of the 6201 as a driver in RH amplifiers. Getting back to the ECC81 family, and in particular to the 6201 – is like the “return of the King”.

“Super”

Well, after solving the reliability issues, improving sensitivity and speed, and changing to a preferred driver, I became aware that the small output transformers are probably a limiting factor, since the amplifier is already capable of sound volumes way higher than the RH84 (the SE version). Thus the small output transformers capable of maybe 5W output have been swapped for larger units (E108 size lamination).

Larger than necessary output transformers are regarded as a trade-off, most are afraid of bandwidth losses (high frequency loss due to higher parasitic capacities). Not in this case… the output transformers used are 5k into 8 ohms, with primaries foreseen for 100mA DC current: if output transformers could be estimated by power, these would probably be rated around 25W. While this looks as a total overkill, the results are awesome with this amplifier: not limited by the small output transformers, the bandwidth extension is nothing short of astonishing, and output power is almost at RH-Universal v2 levels. This should come as no surprise, since the WE datasheet states 9.1W output power at 300V across the tube into a 4.5k load. With 350V across the tube, anode to anode feedback loop, approximately 43mA anode current draw… if the datasheet is of any relevance, no wonder there are about 9W of undistorted output into a 5k load. Thus the “Super” in the name of the amplifier – if built with adequately sized transformers, it will undoubtedly outperform classical 300B SE amplifiers both in sound and power output.

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…