Nikola Nikolov Single Ended 6BQ6 / 6CU6

Baltimore-based tube amp builder Nikola Nikolov is a bit of an explorer. Here is someone who is building one of a kind hifi amps designed around tubes that are unknown or forgotten; neglected gems that have been waiting decades for someone to notice their sweet plate curves and potential for a few watts of clean single ended power. Nikola has a bit of an adventurer’s spirit, and over time he has built up a hefty portfolio of tube amp builds, most of which are made with tubes you and I have never heard of. More of Nikola’s builds can be seen on his web gallery page here.

In this case he introduces us to the 6BQ6 (or 6CU6), a beam-power pentode designed primarily for use as horizontal-deflection amplifier in early TV receivers. The second Nikola amp to come through my room, overall this is a much cleaner build than the first amp I auditioned, which was based around the EL-11, a slick but forgotten (secret?!?) German pentode. Now he brings forward an obscure but generic 1950’s TV tube. Tube pedigree aside, I found this one be a supremely good sounding and musical amplifier.

In Nikola’s circuit, the 6BQ6 proves itself to be a very fine sounding audio tube, capable of triode-like holographic portrayal of program material, and with a drive and pace that keeps your toe tapping. Making 3-4 watts of single ended power, this cap-topped tube has a warmer sound than the EL-84 Vaughn amp that I first paired it against. Nikola’s amp is a softer and more relaxed sound than stereotypical Pentode, but is still quite energetic and lively feeling. It has a bit of 6V6ish sweetness to its sound, but the power delivery and grip on the speaker seems a little more like a 6L6 or EL34. This has a relaxed drive that is irresistible.

While the build quality of this amplifier is definitely head and shoulders over the previous sample from Nikola, its still a bit of a rougher feeling compared to many others out there in DIY tube audio. Parts wise this is ok, the connectors are good and the Edcor transformers are a solid budget choice (and do have a good sound). I would certainly love to hear an amp from Nikola made with top shelf transformers like Tamura and the like and placed on a heavy duty custom chassis. I think those are the final details needed here, but even as delivered, this amp’s sound transcends these concerns. Overall, at $4 each for NOS 1950’s tubes(!) this one is a real eye opener. Does anyone really need more audio than this one delivers for $4 a side? I hope we see more projects based on this little forgotten gem! I’m certainly looking forward to the next example from Nikola’s workshop.

The 6BQ6 is driven by a 12AD7, a superior quality 12AX7 that was used in microphone preamps. This particular tube came with the famed Sony TC500a reel to reel recorder and is one of several treasures hidden within that old diamond in the rough.

The blue Edcor output transformers are good performers and a solid choice if budget is a big concern. That said, the output transformers are not the place to skimp on a tube amp build. In a DIY project, using a low cost tube like the 6BQ6 could potentially expand a project’s budget to include top shelf transformers.

A 5 amp "TV Tube Single Ended Shootout". A fun rainy weekend project that proved these old TV tubes do audio really well. It was close, but the 6BQ6 was the runner up to the "Columbia" badged 6BX7 on the front right.