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A 2-Stage Single_Ended Amplifier

    Thanks to 6c45-pe's performances, the first stage drives without problems the     211's grid. "Soft" A2 Class it's possible. The 6c45-pe is a low power triode (8 watts max. plate diss.) with high-m, very high gm and low rp. It's cheap and readily available. Further, the equivalent noise resistance is 50ohm!


The power supplies are very simple and effective: no vacuum rectifier in this context.

Alls caps in the H.V. power supplies are in polypropilene film.


Photo 1 - A Lab Prototype

During the last two years this design concept has evolved and here I'll present the full development including the simulation decks.

Fig. 1 The Block Diagram

Basically this amplifier is constitued by six sub-units, The driver circuit (that included the 6c45 circuitry), the output stage circuit (with 211 circuitry) and four power supply unities respectively. Local GNDs are joined in a common ground bus solidal with  an attentively evaluated  point on the copper plane. The copper plane represents also the mechanical support for irons and power supplies capacitors. The signal paths has been keep very short. The Driver and the Output Stage, very close beetween their, join a very small area close to the chassis front-end.



Photo 2


Photo 3


Photo 4

Fig. 2 The Amplifier

Many ways exhist to realize a two stage SE amplifier. Base on my personal taste (derived both sonic and technical consideration), I have matured the following staircase of execellence:

a) 2 stage SE with Interstage Transformer;

b) 2 stage SE with LC Coupling;

c) 2 stage SE Loftin-White based;

d) 2 Stage SE with RC Coupling.

The solution with the Interstage Iron it's very difficult since the high cost requested to posses this kind of iron often is not supported by an adeguate electric response. The design of a good interstage transfomer is true White Magic Art!! So I have preferred the option b) relying in the convincement that tube require magnetics as load for the best sonic response. The simplicity of this amplifier can be admired in Fig. 1. Take in mind that a two stage SE amplifier having a 211 tube in the output is not an easy task!  In fact as pointed in other section of this web you can obtain the cumulative requirement of an high amplifiyng factor in union with good current capabilites with the special category of tube having an high s

s = gm *        

There are so little triodes having the specification required. The adopted choice is based on the use of EC8020, WE437A, CV5112/3A167M. Unfortunately this triode are hard to find and uncheap. The introduction on the western market of the wonderful Russian triode 6C45-PE in the late '90 opened new way. I found this first set of 45's in the 1996 thanks to an Hungarian channell and subsequently by the Russian dealer Navigator. Unfortunately in this latter years due to an increasingly worlwide request the price of the 6C45 boosted without (fortunately) approaching that of previously mentioned triodes.

A key concept often negleted in similar design is that the major efforts are in the power supply side. I mean thechnical and economical efforts!

Fig. 3 The Power Supply (Click on it to enlarge)

In fact any SE amp cannot produce a good sound without an attentively evalued and designed power supply. As the amplifying section you can consider a staircase of execellence but there the choice is more simple because you has facet with:

a) Choke input filter power supply;

b) Capacitor input filter power supply.

Vacuum Diodes with no form of electronic regulation often produce better results.  Taking a look on Fig.3, you observe a DC regulation for the 211' cathode (with the only IC regulator of the overall amplifier). The monitor is a simple circuitry able to temporize power supply events (more details later).

I have  evaluated the performance of this block with a PSpice simulation, Fig.4.

Fig. 4 The schematic used in the power supply simulation

You can observe the absence of any kind of parasitics element in the schematic of Fig.4. This condition was purposely introduced in order to run worst-case simulation set.



Steady-state voltage are a bit higher of real values due to the adsence of parasitcs resistor inside the magnetics. Although the grid voltage is intentionally kept to the nominal value from the start you can observe "strange movement arount the first temporal events subsequent to the power on (in this simulation I have supposed no intervention of the monitor circuitry). A zoom reveals more attentively what happening, Fig. 6.

Fig. 6

In the time interval 150..250ms the output is subject to a strong stress due to the cumulative result of positive bumps on the plate voltage and (mainly) on the grid voltage; the resulting dynamic dissipation is depicted in the Fig. 7.

Fig. 7 The dynamic power dissipation on 6C45 and 211 plates

Real world results are however mitigated by parasitic resistor of magnetics .

Fig. 8 Diode pulse currents at start-up

Fig. 9 Repetitive diodes peak current

The contribution of the C-L-C filter is evident observing the residual hum on the output pin. Afte 5s this component is negligible, Fig. 10.

Fig. 10 Residual hum at the output pin after 4s from start-up

A great deal of attention was reserved also to the simulation of the amplifier block. For the performances in terms of square-wave response I have used the schematic in Fig. 11.

Fig. 11 The simulated amlifier block for the square-wave response evaluation

This schematic is based in a worst case evaluation of main parasitc elements involved in the amplification process. RLC block take in count parasitcs of the connections on supply rails, ground bus and devices used for the amplifier building.Clearly the values are depending strongly by the quality of wiring.

Fig.12 The 100Hz Sqaure Wave Output

  Fig.13 The 1kHz Square Wave Output


  Fig.14 The 10kHz Square Wave Output

Solid State designer can be a bit amazed about this poor electric response but Vacuum Tube base ones say that results can be considered good particularly if we think that this results refer to the full output power. However as know the main limiting factor for a SE triode amplifier is the output transformer.

For the evaluation of other parameter I have particularized again the schematic of  Fig.11 by the introduction of a simple passive model for the loudspeaker that replace the output resistor, Fig.15 and Fig.16.

Fig.15 Impedance response for the loudspeaker model


Fig. 16 The simulated amlifier block including the loadspeaker model

The first consequence deriving by loudspeaker model introduction is a worsening in terms of linear distortion. Respect to resistor load case, Fig. 17

Fig. 18 Frequency Response with resistor load

Fig. 19 Frequency Response with loadspeaker

Fig. 20 Frequency Response with loadspeaker detail

The Zoom of Fig. 20 shows in depth this harmful mechanism however kept within 1dB. 

Fig. 21 The Output Power Supply Rejection Ratio (Driver Stage injection)

Fig. 22 The Output Power Supply Rejection Ratio (Output Stage injection)

Depending by Loadspeaker characteristic we have different performances in terms of output power and THD. In genreal a rise in the load impedance produces a lower output power with a lower THD and mutatis mutandis to low loads correspond more power and higher THD. This behaviour is perfectly mapped in this amplifier as you can observe, Figg. 23-28.

Fig. 23 The 100Hz Output Sinusoidal wave in union with the Output Power

Fig. 24 The 100Hz THD


1 1.000E+02 1.697E+01 1.000E+00 8.210E+00 0.000E+00
2 2.000E+02 1.680E-01 9.903E-03 9.910E+01 9.089E+01
3 3.000E+02 1.763E-02 1.039E-03 1.302E+01 4.806E+00
4 4.000E+02 1.708E-03 1.007E-04 -6.306E+01 -7.127E+01
5 5.000E+02 5.403E-05 3.184E-06 1.532E+02 1.450E+02


Fig. 25 The 1kHz Output Sinusoidal wave in union with the Output Power

Fig. 26 The 1kHz THD


1 1.000E+03 1.567E+01 1.000E+00 3.555E+00 0.000E+00
2 2.000E+03 3.325E-01 2.122E-02 7.195E+01 6.839E+01
3 3.000E+03 5.669E-02 3.617E-03 -3.507E+01 -3.862E+01
4 4.000E+03 1.400E-02 8.930E-04 -1.513E+02 -1.548E+02
5 5.000E+03 4.896E-03 3.124E-04 9.561E+01 9.205E+01



Fig. 27 The 10kHz Output Sinusoidal wave in union with the Output Power

Fig. 28 The 10kHz THD


1 1.000E+04 1.783E+01 1.000E+00 -1.601E+00 0.000E+00
2 2.000E+04 1.579E-01 8.859E-03 7.546E+01 7.706E+01
3 3.000E+04 2.009E-02 1.127E-03 -1.628E+01 -1.468E+01
4 4.000E+04 2.295E-03 1.287E-04 -1.138E+02 -1.122E+02
5 5.000E+04 3.067E-04 1.720E-05 1.548E+02 1.564E+02


After this extensive set of simulation results take a look to building phases......... sterting from the detail of regulated power supply for the 211 heater. Remember this is the only solid state element inside this amplifier!

Photo 5 The 211 Cathode Power Supply Module (Top View)

Photo 6 The 211 Cathode Power Supply Module (Side View)

Photo 7 The 211 Cathode Power Supply Module inside the amp


Photo 7 The Copper Plane

Photo 8 Bottom view of the incomplete chassis

Photo 9

Photo 10

Photo 11