The Parallel Single Ended 6CB5A Amplifiers

Hi!

The owner of a single ended 6CB5A amplifier approached me with the wish for more power with the same sound. Since he was not interested in the big transmitting triodes like 211 and 845, I proposed a pair of monoblocks with 2 6CB5A in parallel on each.

In  order to have room for the needed parts, a hefty power transformer and big output transformer as well as all the necessary chokes, capacitors and interstage transformer, I built them in the tower chassis like the last pair of SE 6CB5A monos.

The driver tube can be a 6N7 as in the SE 6CB5A amps but the 6J5 is useable as well and recommended due to it's lower plate resistance for driving two of the output tubes.

The power supply is using a full wave bridge with 4 6AX4 tubes into a choke loaded filter like most of my designs.

Expected output power is a solid 15W per channel.

A few impressions from the assembly phase:

The top plate with the sockets mounted and wired up.

The interstage transformer also sits directly under the top plate for short signal wiring.

Adding two platforms which will hold the most critical capacitors close to their connection points.

Capacitors in place:

On the next level below the top plate is a subassembly with a big Tango output transformer ans B+ smoothing capacitors. And at the bottom the power supply.

Output transformer module mounted.

Fully assembled amps only missing the wooden enclosure.

Initial testing is done with the amps 'naked'.

Measurements show that the target output power was easily reached.

The Tango transformers have taps for 4, 6, 8 and 16 Ohm.

A great alternative to 211 and 845 avoiding costly tubes and high voltage power supplies but still providing two digit power numbers.

Need 20+ Watts? I can build these also with 3 6CB5As per channel.

Want your own 6CB5A amps? Drop me an email and we can discuss the ideal amplifier for your system.

Best regards

Thomas

The New D3a Phono Stage - Part 2

Hi!

Progress on the new D3a LCR phono.

I received more chassis material and continued assembly. Not finished yet. But far enough to give a glimpse of the final look.

The power supply will be in a separate chassis in the same look. The circuit is pretty much the same as the classic portrait style D3a phono with some minor updates.

The square with the holes is easily removable to get access to the tubes if a tube change is needed.

The side panels are wood in the moment but I will also offer a metal option.

The back side:

In the next part I will show more of the assembly. Stay tuned

Best regards

Thomas

The New D3a Phono Stage - Part 1

Hi!

In the last post I showed the Compactron Preamplifier Prototype and explained the motivation for a new chassis style which gives a more discreet look. I also plan to offer existing preamplifiers like the D3a phono stage in this style.

The fist chassis parts just arrived.

This will be offered as an alternative the the existing portrait chassis which I continue to make.

Some stuff for internal mounting:

Initial assembly:

Stay tuned for updates as the assembly progresses.

Best regards

Thomas

Compactron Preamplifier Prototype

 Hi!

In my previous post I teased the new preamplifier I am working on. Here some photos of the prototype build.

This is a full function preamplifier with phono section on board. In the moment I am only offering separate line and phono stages and these each have separate power supplies. So a 4 box set up. The Compactron preamplifier is all in one box including power supply.

Above the face plate design which still needs to be manufactured. While many people including myself like to see a landscape of tubes, transformer covers and capacitors on top of their amplifiers, others prefer a more discreet design which is easier to integrate into a living room. So the new preamp will have an enclosed chassis.

So far my D3a phono stages are MC only. The reason for that is that a MC step up transformer is already integrated in the unit and part of the biasing scheme of the input tube. Also the D3a tube is not too well suited for an input tube of an MM phono stage since it has a rather high miller capacitance. I also received requests for adjustability of the load impedance and the possibility to have multiply phono inputs. This new preamplifier has 2 each MC and MM inputs so can be connected to 4 tonearms at the same time. In addition the MC load resistance can be selected between 1000, 375, 150 and 50 ohm. And if that does not offer the desired impedance value, there is a 5th position on the selector which connects to a set of RCA jacks on the backside which can take plugs with a specific resistor to get another desired load impedance. MM load capacitance is selectable between 150, 200, 250, 300 and 470pF. In addition the prototype has 4 line inputs, two balanced (XLR) and 2 SE (RCA). Both XLR and RCA outputs will be available.

The prototype is completed and proofs the concept. The prototype only shows an example of possible configurations. This will be the first of a series of new preamplifiers which can be customised with various features. For example the possibility to change EQ for historic records. Even more phono inputs, or less. Different set of line inputs, tape monitor loop, mode switch, phase switch, etc. Also phono and line only units are planned.

I will show some possible configurations in an upcoming post and also share some photos of the build process. Once the prototype testing is complete a final version will be built which will have a much shorter chassis since a lot can be compacted compared to the first build. Stay tuned for updates on the Compactron Preamplifier series.

Best regards

Thomas

Coming soon ...

Hi!

A teaser for new things to come.

Stay tuned for updates on the Compactron Preamplifier and other new gear as development progresses

Best regards

Thomas

Tube of the Month : The 717A

Hi!

Here is another weird tube which Niclas of Sweden sent so I can feature it as tube of the month: The 717A.

The 717A was developed as a high frequency pentode. But this has not prevented intrepid amplifier designers to use it for audio frequency amplification.

The 717A is a peculiar tube due to its unusual shape which looks like a small mushroom and also resembles the look of a doorknob. This gave it the nickname doorknob tube. It is an indirectly heated pentode and can also be triode wired. The heater runs off 6.3V and only consumes 175mA heater current. The base is 8-pin octal. The base is made of brown micanol and has a metal shell wrapped around it. The tube has seen some use as driver for the 45. But due to its scarcity it has not been used in the audio world very much. The Tung-Sol data sheet lists an equivalent type called 713A which only differs in the base material (bakelite). I did not find any plate curves so I took them myself from a sample. First the pentode curves:

And triode wired, 50V per step on the horizontal axis and 5mA per step vertically. Grid step is 1V:

A nice set of curves! Now lets see some photos of this little gem:

The tube comes in a rather plain box which was typical for military tubes.

Although Western Electric branded and packed the tube is actually manufactured by Raytheon which is also clearly stated on the box:

Some close ups showing some of the details inside:

These must do since I do not own any of these tubes. So no dissection.

And lastly the tube in operation:

In the last photo the grids are nicely visible:

Thanks again to Niclas for lending these tubes to take photos. Stay tuned for more gems from his collection in upcoming Tube of the Month posts.

Best regards

Thomas