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6C33 single ended OTL

Output Power @ 8ohms             20W

Bandwidth @ Full Power            10Hz - 2 MHz

T.H.D.                                     <1 %

Input Sensitivity                       0,5V

Damping Factor                        2

Slew Rate                                400V/us

Weight                                    25kg per channel

Power Consumption                 600W per channel

 

OTL, top of the tops and final target….. many people says

This single ended OTL is my prod and realization of all my wishes about lifetime amplifier

Amplifier was faced with the extremely high requests during development but after lot of experimental and theoretical work all dreams became true.

Of course, lot of OTL designers made easier choice and choose some of Futterman’s or cyrclotron topology but it vas not my cup of tea. Class A single ended topology is simply the best. Of course, class A topology has some negative side effects like weight, heat dissipation but all of them are small victims for me.

 And so, the final topology is arrived to the 600W power consumption (per channel) but who care.

And can’t be different, total of 5 russian 6C33 tubes are employed in the every channel. All of them are parallel connected for the signal but each of them has independent bias regulation. This bias arrangement was necessary due to the well known static variation between sample to sample of the (almost all) Russian tubes. Not big deal in the diy construction, I am not industry and don’t need to save every cents of production costs.

Here 6C33 works in the cathode follower configuration. Low output impedance of this configuration means that amplifier circuit can be arranged in the “no global nfb” topology. I don’t like so much global nfb and using nfb only where is necessary. Here is not necessary, output impedance of the amplifier is around 4 ohms and it is more than enough for use with full range single drivers (my choice of loudspeakers)Supply to the output tubes is feed through the2 coils, one of them is huge and one is small. Both coils together with output capacitor and speaker forms the some kind oh Zobel network which obtain almost constant and frequency independent load for output tubes. Small coil is really small but has very important role for obtain desired bandwidth.

Of course, this approach give the best results with the full range single driver loudspeakers.

So, output stage which has this capability need proper driver. The best and probably the easiest way is to implement a separate driver for every 6C33 as is done here. Drivers are SRPP stages with ECC88 tubes. Only unusual thing is use of small compensation coils between the lower and the upper triode in the SRPP stages. Again, target was a MHz range bandwidth.

What’s about these 2 vacuum diodes? There are used as non linear attenuator to compensate small non linearity of 6C33 at maximum output level. Actually, diodes are formed from the ECC88 triodes.

In the input stage is again ECC88 in the SRPP circuit. This stage also employs small compensation coil between 2 triodes.

Power supply is really beefy, filtration for 6C33 anode supply consist 8 electrolytic capacitors (total of 75000uF), one inductor and separation resistors.

Little bit bizarre bandwidth test was performed. Video signal is feed to the fully loaded (8 ohms) OTL amp. Output video signal is attenuated to the level of 1Vp-p and sent to the TV. Picture quality was comparable with the picture from the used VHS video tape.

Really not bad for no nfb amplifier intended for the audio use

Many people asked me why spent 600W of power consumption to obtain only the 20W output power. After listening nobody……



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