DISCLAIMER: Vacuum tube circuits work with dangerously high voltages. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.

All rights of photos and text reserved. Usage of photos or text from my blog on other websites or for any other purpose only with prior permission. If you want to use any material from my blog please contact me by email.

Sunday, November 30, 2014

Tube of the Month : The 6AG9


Last month I showed the 6GE5 compactron beam pentode and mentioned it's usability as an output tube in a single ended amp. I went through the compactron series tubes to find a suitable driver. And this is what I found: The 6AG9

Since I have a preference for all triode amps, I concentrated on triodes only. There are quite a few of them in the compactron series. Some combined with other triodes, diodes or pentodes in one bottle.

The 6AG9 contains a triode and a pentode. The triode section has an amplification factor of 39 and a plate resistance of 8500 Ohm. Quite similar to the values of the 6N7 which I like to use as driver for small output tubes. This would make it usable in a 2 stage amp with transformer-, LC or RC coupling. The heater current is a bit on the high side for such a triode, but that is because it also needs to supply the heater of the pentode section. A quick glance into the data sheet shows some nice plate curves of the triode section. A check of prices showed a nicely low cost of $3-4 for the tube. It seems to be an ideal companion for the 6GE5! These are the plate curves as shown in the data sheet:

And this is how it looks like in real life:

Quite alright if the region to the left is avoided!

Most 6AG9s in my stock came unmarked in bulk pack like those above.

Here a 6AG9 mage by General Electric:

The 12-Pin base:

Let's have a look how it is constructed inside!

Removing the glass reveals a better view of the details:

The pentode section takes up most of the space inside. The triode section (top) is separated by some space and a screen from the pentode (silver sheet of metal).

A view of the side of the pentode section:

The top:

The heater wire can be nicely seen coming out of the top and connecting across the tube between the sections.

The heater wire removed:

The part with more folds heats the pentode.

The triode section:

The triode electrodes separated:

The grid:


The triode grid is circular shaped:

The shield which separates triode and pentode:

The pentode section, cathode, control grid and screen grid on the left, plate and suppressor grid on the right:

The grids separated from the cathode:

The control grid has an extremely fine pitch between the wires:

A very well made tube, completely overlooked by audio designers!

In the meantime the tube serves as driver in a 6GE5 amp. Stay tuned for a report.

Best regards



  1. Happy Holidays Thomas! Have you had a chance to directly compare the 6AG9 to the 6N7, driving a DHT? The round grid structure and precision is interesting. Lower plate resistance can't hurt either.
    Best, Jason

    1. No, never did a direct comparison. Too many projects ongoing, no time of such things ;-)
      Happy holidays to you too

  2. It occurs to me that this might make a good sounding single bottle mm phono stage using a topology similiar to your octal phono stage. What do you think?

    1. The Octal phono uses two triode sections. The tube has a triode and a pentode. I haven't looked into usage for a phono with this. If you have a specific circuit in mind to utilise the pentode section as well why don't you try it on a bread board?

    2. What I meant was using the triode section as the input tube, then a passive RIAA network, then a triode connected pentode output. I would breadboard it but Im not knowledgeable enough to design it or select the operating points for this application. Really I was just curious if you the thought the tube had the potential for being used this way? If there was anything about that would disqualify it as a phono tube for some reason. Thanks for your response.

    3. I never characterised the pentode section in triode mode. So can't really say.