Tube Power Amplifier 2 x 50W Please note: The pictures show an older version of the amp rather than the schematic. This version used EF 86 in the phase splitter stages similar to the QUAD II design and 6550A output tubes.  Input stage The amplifier comprises four stages, input stage, phase splitter, driver and output stage (see schematic). Input tube is a UHF twin triode ECC 81 (12AT7), operating in single-ended differential mode. System I is a cathode follower that feeds the signal into the cathode of system II. Hence the input has very high impedance, determined only by the volume control (or grid resistor). Input capacity also is very low, for there is only static capacitance but no dynamic capacitance (Miller capacitance) as usual in common cathode triode circuitry. Besides cable capacitance, the preamplifier output only “sees” the static capacitance of grid/plate C GA  and grid/cathode C GC  respectively.  System II is a differential amplifier working with the input signal from the cathode follower at its cathode and local negative feedback at its grid which is defined by two resistors of 270k and 68k. By shuffling these resistor values the local NFB and thus the input gain can be adjusted. Main purposes of local NFB are enhancing bandwidth (250kHz at -3db point in my application) and reducing adverse effects of aging or tolerances in tube data on tonality and stereo image. The   cathode   idle   currents   of   the   ECC   81   automatically   set   to   a   ratio   of   4mA   to   1mA   due   to   plate   of   system   I   being   directly connected   to   the   +rail   ("through   grip"   effect).   This   is   sonically   advantageous.   A   cathode   follower's   distortion   is   directly   related   to its   current   swing   because   internal   plate   resistance   and   transconductance   of   the   tube   change   with   current.   If   the   current   of system   I   is   substantially   higher   compared   to   system   II,   current   changes   with   signal   and   hence   changes   in   internal   plate resistance   and   transconductance   are   low.   For   similar   reasons   the   cathodes   are   coupled   by   a   current   source   instead   of   a conventional resistor. Current   sources   look   like   resistors   of   almost   infinite   resistance   and   still   allow   to   keep   currents   high.   The   cascode   of   the   BF245 and   BC548   build   a   "rock   solid"   current   source   with   an   internal   resistance   of   several   GigaOhms.   The   BC548   can   be   considered   a current source for the BF245. Another   advantage   of   the   current   source   is   an   absolutely   constant   load   at   the   +rail.   The   total   current   (I AI    +   I AII )   across   the   two systems   stays   absolutely   the   same,   no   matter   whether   heavy   bass   or   diminutive   transients   are   run   through   the   input   tube. Replacing   the   ordinary   tail   resistor   by   a   current   source   gave   a   remarkable   improve   in   stability   of   tonality,   stereo   image   and   DDR (Downward   Dynamic   Resolution,   as   Allen   Wright   calls   it   in   his   very   remarkable   tube   preamp   cook   book).   The   serial   regulator   HIP 6300   reduces   hum   to   almost   zero.   Serial   regulators   normally   should   be   banned   from   audio   circuitry,   but   in   this   case   in   worked fine.  Phase splitter stage The ECC83 (12AX7) converts the unbalanced signal into balanced signals. It works as a long tail phase splitter and has the advantage of high gain, good frequency response and large voltage swings. Negatives are a poor balance and the necessity to tweak the plate resistors for balanced voltage, if you do not use a current source!  For optimal symmetry the cathodes must be coupled by a stable current source instead of a tail resistor. Then the current of the first system can only rise to the extent the current of the second system is lowered and vice versa. If the current in system I increases by 0.01µA, the current in system II must decrease by 0.01µA. So the phase split signals are forced to be perfect mirror images, depending only on optimum similarity of the anode resistors. And again, you have the advantage of an absolutely constant load at the +rail. Differential amplifiers were always considered less precise than single ended amps. From my experience differential amps with conventional tail resistors are loose and they indeed loose signal detail because the circuit parameters can shift with voltage swing. A good current source locks the total current down and differential amps become as precise as single ended amps. The anode resistors of the 12AT7 are relatively high in value for good linearity at large signals. This is because the KT88 beam power tetrodes are run in an anode/cathode coupled circuit similar to the QUAD II amps by Peter Walker (the McIntosh circuits also look similar in the first, but they work differently). The voltage swings of the phase splitter must cover the grid voltages necessary to drive KT88 plus the voltages that are fed back into the KT88 by the cathode coils of the output transformers. Driver stage 270 kOhm at the anodes of the phase splitter and low idle currents of 1.1 mA give a relatively high output impedance of the 12AT7. Thus with respect to the low grid resistors of 68 kOhms at the KT88, cathode followers were added in galvanic coupling for low impedance. The ECC82 (12AU7) was chosen, because it has acceptable transconductance for low impedance and allows 180 volts between cathodes and heater. Again current sources were chosen as load resistors at the cathodes for the reasons discussed previously. They allow the ECC82 to run at idle high currents, but current swing is only depending on the tetrodes grid resistors of 68kOhms. With ordinary tail resistors of 30kOhms and idle currents of 5mA the current swing would be between 3mA and 11mA at signals of 80 volts peak to peak. With a current source the current swing is limited between 6mA and 8mA. The MOSFETs IRF 830 can handle up to 500 volts and work very reliable. To dissipate their wattage, they are mounted on small heat sinks. The gate resistors at the MOSFETs are essential to avoid RF oscillation. For the same reason the ECC82 and KT88 have grid stopper resistors. The currents of the cathode followers are adusted to 5mA by the source resistor of 2.4kOhm.    Output Stage As already mentioned the KT88 tetrodes are run in an anode/cathode coupled circuitry (ACPP in the schematic stands for anode-cathode-push-pull). This circuitry goes back to Peter Walker and is also used in the bigger Jadis tube amps as far as I know. The trick is a local feedback from the plates into the cathode/grid loop. The voltage controlled feedback grant the tetrodes characteristics similar to triodes with low impedance and low distortion while maintaining their ability to high output power. With 430 volts at the plates the output power is 50 watts.  Contrary to ultra-linear stages the anode/cathode coupling allows free choice of optimum voltages at the screen grids. Stabilizing the screen grid voltage at 370 volts gave further improvements in sonic performance in my application. I tried the IRF 830 as regulator but I could not keep it from oscillating. So I used a BUT13 Darlington transistor. The plates of the KT88 dissipate around 30 watts. According to the data sheets the tetrodes should have a minimum distance of 10 cm (4 inches) from each other to avoid picking up radiated heat from the neighbor tube. The amplifier has a negative feedback loop from the secondary coil of the output transformer into the phase split stage. In my experience NFB can be somehow compared with pharmaceutics. They have effects and they have adverse effects. To achieve best benefit, you carefully have to adjust the dose and in some cases the best dose may be zero. However, the stability of the stereo image is very much dependent on an equal gain in both channels. Even the smallest gain fluctuation blurs the image especially when it comes to the reproduction of depth. That NFB is the best tool for stabilizing is often overlooked, despite the fact that everyone agrees on the importance of gain stability. A tube amplifier is more prone to gain fluctuations than a solid state amplifier. Small movements of the electrodes due to heating change the properties of a tube at least to some degree. In my application a good compromise between woofer control, stability of the stereo image and vitality was achieved with NFB between 10db and 12db. The capacitor was tweaked to 240pF by adjusting to best square wave response at the output. See schematic of the power source unit. Back to top of the Page Back to main Page
I built this tube amp back in 1994. It served me well until I replaced it in 2017 by my new single-ended hybrid tube / solid state power amp.