Glasshouse 300BSE Amp - Circuit Description
Circuit Description by Andy Grove
This circuitry of this amplifier follows the basic tenet of avoiding the use of identical cascaded stages, this way colourations are not compounded. Today the most commonly cascaded stage is the capacitor coupled common cathode amplifier using a triode. Bearing in mind that a large number of stages will end up tagged together if used all the way from phono cartridge through to power amp, it is too easy to end up with an amp setup with a very dominant colouration.
Dogmatic belief has created two major factions in the commercial marketplace:
1) The People's Triode Front regard valves as inorganic gain blocks, preferring op-amp like valves which give excellent paper performance. For these guys transconductance is all. Their circuits use cascades of ECC88s and Russian RF valves. A black and white picture of a rainy day in Milton Keynes comes to mind. Know them by their complex, maximalist circuitry.
2) The Front for Triodes for the People, prefer "classic triodes" over the frame grid types of the PTF. The problem is that this preference leads to habitual over use and a xenophobic attitude towards any other type of valve.
An Excellent Mix
A good dish however comes from a careful balance of ingredients, too much of any one taste and it will be spoiled. In an amplifier each stage will add it's own flavour to the overall taste of the amp. For some reason there is a great focus upon the passive components and the output valve but little attention is paid to the input and driver stages.
EF86, The Input Valve
Here I have used a pentode as the input valve, V1/V2, the venerable EF86 which was developed as a low microphony audio type. A pentode can deliver high gain from a single stage, allowing an amplifier with only one gain stage before the output valve. I have used two 100k resistors (R21/R22 and R23/R24 in series to give a 200k anode load for the EF86, this gives fairly high gain at the expense of a high stage output impedance, which will be similar to the anode load resistance. I deliberately used two resistors in series to reduce the voltage across them. Too high a voltage across a resistor will cause it to become non linear with respect to the signal voltage. This phenomena is particularly pronounced with carbon resistors but it is present in all types. The screen dropper resistor has a constant DC voltage across it and the screen voltage is bypassed by C23/C24 so there isn't so much of a problem there. R25/R26 and C21/C22 are conventional biasing components. The EF86 could just about drive the familiar 300B (V5/V6) directly, but the high impedance drive causes very hard clipping when the output valve grid reaches 0V and draws grid current.
5687, Cathode Follower
In this amp I have used a 5687 parallel cathode follower V3/V4 as a knuckle duster for the EF86, it provides a low impedance drive to the 300B grid and buffers it from the delicate input stage. The 5687 is direct coupled to the 300B grid, and a fixed percentage of the required voltage is provided at this via R29/R30 from a potential divider at the power supply. The rest of the 300B bias voltage is provided by it's cathode resistor R39/R40 and stabilizes the operating point. The cathode resistors for the driver stage R35/R36 and R37/R38 are high power metal oxide types to reduce thermal and voltage effects. Grid stoppers (R31/R32 and R33/R34) are essential in this configuration to prevent oscillation of the 5687 sections. The direct coupling of the cathode follower driver to the 300B grid allows slightly more power output, as the follower can supply grid current and push the grid positive. The 5687 is a seriously beefy valve and is ideal for the purpose.
The Power Supply
The power supply is fairly traditional. The HT for the 300B is provided by a Russian 5U4 rectifier (V7), a type which has proved itself over the years. Stacked capacitors are used (C1,C2 and C3/C4) to give plenty of voltage rating. R6 is the dropper resistor for the EF86 stage, this voltage is bypassed by C5. R7 is a bleeder to keep the voltage on C5 down during the switch on surge. The driver stage power comes from a centre tapped bridge using soft recovery diodes to provide plus-minus 195V rails. It uses simple two stage RC smoothing, the bias potential is provided by R14/R15 and C12 filters out any residual hum. The heater supplies are conventional using LM323 5V regulator ICs which are very rugged. The two 6.3V heater supplies use a couple of diodes in the LM323 return leg to trick it into supplying an extra 1.3V at the output.
Valve Alternatives
There are various types of EF86 available The 5687 is very similar to the 7044 and E182CC valves and they are literally plug in replacements in this amp, so a bit of experimentation can be carried out here. E182CCs are getting a bit expensive, but the 7044 is quite reasonably priced. Both the 7044 and E182CC have a tighter and slightly more dynamic sound, at the expense of the 5687s darker, more juicy sound. At the input the type of EF86 used will have a large impact on the overall sound. My favourite is the Marconi/ Osram Z729 whichs is clean and clear without being hard The Mullard is also very good but there are types from other manufacturers to try as well such as Mazda and Telefunken, whom some regards as the pinnacle of valve manufacture. There are a few types of 300B available now, the TJ mesh plate is getting a lot of attention I don't think it really sounds much like a 300B, more like a 2A3. The best is of course the Western Electric but there is some doubt as to whether they are going to be available in the future. A new player on the field is the EAT (Europe Audio Team) from the Czech Republic, I haven't listened to the pair I have yet but they look very serious, and measure brilliantly on the bench.
Future Upgrades
With regard to component mods the sky is the limit, a black gate for C21/C22 of the N type will yield superb results. Also replacing the coupling capacitors with better paper types, possibly even silver foils if you want to go that far. At some time I would like to offer a better output transformer with an improved winding and speciality C-Cores of HiB silicon steel and even Radiometal as used in the best Audio Note output transformers.