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Monday, February 27, 2023

The Parallel Single Ended 6CB5A Amplifiers


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



  1. Hi Thomas, wonderfull work. Do You match the output tubes?

    1. Thanks. I always match the output tubes also in the single tube per channel versions. 6CB5A can vary quite a bit. I match with a curve tracer

  2. Hi Folks, this is really great stuff, believe me!!! Thank you very much Thomas!!!