Single Ended Amplifier Concept, Part 5

Hi!

The previous part of this series showed a scaled down version of this concept based on the 6CB5A which minimized cost mainly by reduction of the iron content. But most people who were interested in an amp with the 6CB5A asked what can be done to get the best possible sound out of it:

The most obvious way is of course to choose the best possible quality of interstage and output transformers. While the Lundahl transformers which were mentioned in previous posts will provide excellent sound quality, this can be improved by the use of suitable transformers from the Tango range. These come at a much higher cost though.
The very early prototype of this amp has been built with the Tango XE20S which is an excellent performer. One level up is the type FC30-3.5S. Tango makes one of the best interstage transformers available, the famous NC20, which is especially well suited for the highish plate resistance of the 6N7. A change to these transformers does not require any other change to the circuit.

The photo above shows the most elaborate implementation of the 6CB5A amplifier so far with Tango NC20 and FC30-3.5S transformers.

Another possibility to scale up is the power supply. Rather than splitting the amp up into mono blocks, a split between amp and power supply is the better option in my opinion. Not only does it provide good isolation of signal section and PSU but also more room to use a heavy duty power transformer. The external PSU also provides enough space for 4 tube sockets which enable the use of an all vacuum tube rectifier bridge instead of the more traditional full wave rectifier scheme with a center tapped secondary.

The schematic above shows the power supply as I use it in amps with external PSUs. It has a LCLC filter section as the previously shown supply. The transformer secondary only needs to be about half the voltage. No center tap required. The advantage of this rectifier scheme becomes obvious when you compare the voltage waveforms after the rectifier, or the secondary voltage on a scope. In the common full wave scheme the two diodes 'fight' with each other during the switch over between phases. This causes a distorted waveform. The full wave 'Graetz' bridge as shown above has a much smoother switching behaviour which means less potential interference. The rectifier bridge is composed of 4 6AX4 TV damper diodes. The 6AX4 deserves a separate post on it's own, so I'm not going much into detail about it here. All heaters of the 6AX4 can be wired in parallel and fed from a dedicated heater winding which should be referenced to ground, simply connecting one end of the winding to ground is sufficient. All other aspects of the PSU are the same as in the previously introduced version.

Here some more photos of this implementation. As you can see, there is one deviation from the rectifier scheme above. Half of the bridge has been done with 866A mercury vapour rectifiers. This was done mainly for cosmetic reasons.
Check out the blue glow of the 866As in operation and you understand why. Using mercury vapour rectifiers adds some complexity to the circuit since their filaments need to be preheated before the high voltage can be applied.

So a separate filament transformer and a delay circuit was necessary to implement this preheating mechanism. A manual override switch for this mechanism is added on the back to allow longer preheating of 866As when new tubes are used the first time. All connections are brought out on the back side of the chassis.

The main power transformer is mounted under one of the two black covers on the PSU. The second cover hides the oil caps. On the amplifier chassis, the caps are visibly mounted. Only six of the eight caps are placed on the top, two more are mounted inside of the chassis.

Two vintage style panel meters have been added for the plate voltage and current. The current indicator shows the total current which is consumed. They are mainly there for stylish reasons. No manual bias adjustment in this amp. However they give some indication if the tubes age and the drawn current drops.

As mentioned above, the 866As need some precaution. The main power switch connects power to the filament transformer and a small control transformer which powers a relais control circuitry. The secondary of that is rectified, smoothed and provides the voltage for a simple LM317 regulator. The resistors and capacitor at the adjust pin of that regulator are sized such that the output voltage of the LM317 rises slowly. After about a minute it reaches the threshold voltage of a relais which is connected to it. This relais then powers up the main power transformer and applies high vollage to the plates of the 866As. Since the Two 6AX4s which form the second half of the bridge are heated from the same mains power transformer, current doesn't flow immediately but only after the TV dampers cathodes are warmed up. This avoids voltage overshoots when the relais closes. As a nice side effect the blue glow in the 866As comes up slowly to full brightness as the 6AX4s warm up. The power supply schematic with the delay scheme is shown below:

Not shown in the circuit is the manual override switch which allows to condition the 866As when they are new. New mercury vapour rectifiers should be preheated for 30 minutes before high voltage is applied. This ensures that all the mercury gets vaporized and no drops remain in the system which could potentially created shorts and damage the tube. Such a preconditioning should be done whenever a tube is first used in the power supply. Transport or horizontal storage will distribute the mercury all over the inside of the bulb.

This photo shows a close up of the 866As. This tube will get it's own post in the Tube of the Month series in the future. And here another shot of the PSU in the dark so you can see the nice glow of the 866As:

The amp as shown here is about as far as it makes sense to go with the 6CB5A. Of course the whole amplifier concept can be improved even further. But that would require to go to directly heated triodes. This will be covered in upcoming articles which will show how the output stage can be converted to directly heated triodes. 45, 2A3, 10Y, 801A and 300B tubes have been used with the same concept, keeping the same driver stage and power supply configuration. For the 300B it is recommened to use the 6J5 instead of the 6N7. The 300Bs grid needs a driver with a bit more power. For all the others, the 6N7 is perfect.

I can provide all the parts needed for such an amplifier concept. Since I only use the Graetz rectifier scheme, I can only supply power transformers for these, none with center tap. Such power transformers can be easier specified for unicersal usage. My power transformers each have two different secondaries which can be combined in many different ways to obtain secondary voltages from as low as 100V to 600V in very small increments. So the power transformers can be used for many different projects and easily allow the change of the B+ voltage for different output tubes. The power transformers have separate heater windings for the rectifier bridge and signal tubes. The transformers are available in 3 sizes, with 100, 200 and 400mA current ratings of the secondary. Besides the use with Graetz bridges, they are also suitable for voltage doubler schemes to obtain volatges for procects with 211, 845 or 6HS5 tubes as well. 211 or 845s however will require a different concept from this one. This will be covered in another series of articles.

Best regards

Thomas