(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 2nd 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 1st
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 1st generation
of RH amps was mainly challenging comparative designs with EL84, 807, EL34 –
the 2nd 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…
