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Sunday, December 22, 2019

Tube of the Month : The 6A5


In this last tube of the month post of the decade I would like to present a rather unusual and seldom heard of tube. The 6A5.

The 6A5 is an indirectly heated power triode with the same electric parameters as a 2A3, except for the 6.3V indirectly heated cathode.

The 6A5 has an 8 pin Octal base. The pinout is shown on the left. It was derived from the 6B4 which is an Octal base version of the 6A3. The 6A3 is a 6.3V filament version of the 2A3. With a 6.3V directly heated filament the 6A3 and 6B4 are prone to hum when heated from AC, while the 2A3 with only 2.5V can be easily operated with minimal hum. This led to the development of an indirectly heated version. The 6A5 is basically a 6B4 with cathode sleeves on the filament. The cathodes are internally connected to the center tap of the filament and to pin 8. This pinout is identical to that of the 6B4 except for the additional cathode connection. Since the cathode is internally connected to the center tap and since the heater current is the same as the 6B4 filament current, the 6A5 can be used in circuits designed for the 6B4 without any changes. Obviously a good idea, but the 6A5 does not seem to have been widely adopted as the tube is rarely seen. I only have 4 of them, so don't expect as many photos as usual. But I find the tube too interesting to not show it as tube of the month.

Above the plate curves which are as linear as one can expect. All the 6A5s I have were made by National Union.

The tube is also dubbed 6A5G.

It has the same bi-plate structure as most 2A3s and at first glance could easily be mistaken as a 2A3.

Let's have a closer look.

Also from the top it looks very similar to 2A3. Only a closer look shows that it has a heater instead of a filament.

Here we see it lit up:

These photos show the cathodes. 8 individual sleeves pulled over the heater strings.

This was very likely extremely difficult to assemble, and is probably the reason the tube was never produced in volumes as large as the 2A3.

Since I only have 4 of them and they are all in perfect condition, I did not want to open one, although I was very tempted to inspect the internals. So some more photos of intact tubes have to do.

In this close up the cathode sleeves are nicely visible:

That's all for the 6A5. I hope you enjoyed this tube presentation.

Best regards



  1. A very nice tube, I like this classic ST shape a lot. It's always interesting to see what the engineers of that era came up with to answer the necessities of their time - cathode sleeves on filaments, I never expected that.
    Thanks for putting in the work to publish this series, it's always a joy to find a new article on your blog.

  2. I actually have on my bench a Paul Klipsch home brew 6A5G PP tube amp. We are working on cloning the amplifier. It was part of a project he used with a diy electronic crossover. Late 40's early 50's project. Instead of rebuild we are aiming to clone this project.

  3. Love your "Tube of the Month" posting. Merry Christmas from Colorado.

  4. Thomas, Hartmann is spot on! Agree totally. Engineers of that era are amazing.....what they were able to accomplish with limited technology. Merry Christmas from GA (USA)!! Randy

  5. Hi Thomas,

    I built an amplifier using this tube, a two-stage 3W SET design. It features a U18/20 rectifier, passive supply with Lundahl chokes / Clarity Caps, REN904 / MH4 IDHT driver, copper foil PIO capacitor coupled to the 6A5G, with Lundahl LL1620 OPT. The sound is very nice, I can highly recommend this tube if it can be found! Thanks for the lovely article.

    From my research, despite the branding, the 6A5G was only made by Sylvania and Visseaux of France under Sylvania license. Interestinly, I acquired a pair of the Visseaux 6A5G for my amplifier. To my surprise (and chagrin), while Visseaux manufactured the tube under the 6A5G label, they did not include cathode sleeves!!! It is directly-heated with center-tapped filaments, something of a hybrid between a 6B4G and a 6A5G. The hum is not too bothersome, and the sound is still quite nice. Whether it was a mistake or intentional, who knows, but an odd discovery.