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 2nd
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.
