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Friday, September 29, 2023

The Compactron Phono Preamplifier - part 1


Work is in progress for a Compactron Phono Preamplifier.

This time in Orange!

This is a stand alone phono stage based on the phono section of the Compactron Preamplifier. It is for MM cartridges and has variable EQ adjustments to adapt for the recording curves of different vintage records. Bass turn over and treble roll off are independently adjustable. Load impedance and capacitance can be selected as well. In addition there is a phase switch which allows to change the polarity of the output signal.

The outputs are transformer coupled and offer both RCA and XLR connections. The gain is selectable via a switch on the back in 4 steps.

The top plate is kept in aluminum color and it is glass pearl blasted for a matte surface.

Some of the initial assembly steps.

The internal modules:

The module with the main signal section.

Power supply section:

Another plate holds the gain selector switch.

Signal sub module mounted to the main plate and partially wired.

Front plate:

A choice of different knobs is available.

Stay tuned for an update when this phono stage is finished. Contact me if you want your own Compactron preamplifier. Of course it can also be built for MC cartridges or switchable between MM and MC.

Best regards


Friday, September 22, 2023

Tube of the Month : The 6BD11


This month we have yet another Compactron cutie: The 6BD11.

The 6BD11 is very similar to the 6AF11 which I presented last month. It has two triodes and a pentode integrated into one tube.

There is a medium mu and a high mu triode section and the pentode is of the sharp cut-off type. The two triode sections are almost identical to those of the 6AF11 except for slightly different maximum plate dissipation ratings. The pentodes also differ in plate dissipation and also have somewhat different plate resistance and transconductance. The 6BD11 and 6AF11 share the same pinout. In most applications these tubes would be pretty much interchangeable. I wonder what the reason was to develop two such similar tubes and register them under their own designations. Unsurprisingly the similarity between these two types also reflects in their plate curves. Here we have the data sheet curves and actual curves of each section, starting with triode nr 1:

Triode section 2:

The Pentode:

The pentode triode wired:

Moving on to some tube pics.

The 6BD11 from different angles:

Some more views of the tube:

The glass removed:

Removing one of the triode sections from the rest.

As expected the internal construction is very similar to the 6AF11.

The heater:

Cutting off the screen between the two triodes:

The second triode:

The pentode:

And lastly the tube in operation:

Stay tuned for yet another Compactron tube next month!

Best regards