single 12ax7 amp

Thread Starter

pedro cabral

Joined Feb 7, 2008

My name is Pedro

I´m portuguese

I´ve just finished building one of "all about circuits" projects: The Vacuum tube audio amplifier.

It uses a single 12ax7 valve to produce some 2 to 3 W power output throught a 8ohm speaker.

I´d just like to say that it works great! I´ve build it mainly out of scrap components ( the only "expensive" thing was the impedance matching coil which cost me 20€).

The mods I needed to make to the circuit were:

1-I´ve used a much larger filter capacitor in order to reduce the 50hz "hum",

2-I decided to use a 12v to 220v transformer to the plate voltage, since it is hard to find 110v transformers here in portugal. The result was that about 270V are arriving at the tube and still it works as expected.

3-I´ve reduced the input resistor to 50k


1-Where do I fit a volume control to the amp?

2-Is it possible to associate more 12ax7 tubes in order to obtain more volume?

3-I was recently able to obtain a large stock of PCL-80 and PCL-82 tubes. Are these usefull for audio amp projects?

I´m now using my new valve amp for guitar recording and I´me very pleased with the final result.

Thank´s for the DIY




Joined Aug 8, 2005
2-Is it possible to associate more 12ax7 tubes in order to obtain more volume?
I'm not much of a tube jockey, but:

Generally speaking, you wouldn't add more 12AX7's to get power. The 12AX7's would be used as a pre-amp that drives something larger like 6L6's, which then drive a larger output transformer.

If you're interested in a more challenging project, there are a lot of schematics out there for old Fender and Marshall amps that you may find interesting. Maybe look at the smaller Fender amps like the Champ. For something more complicated and powerful, Google on Fender Twin.

Here's one example from Google for the Champ:


Joined Jul 17, 2007
You could use a 50k or 100k audio taper pot across the input connections for a volume control. Connect the end terminals of the pot across the input, and your signal in to the middle terminal (wiper arm) and ground. If you connect the pot backwards, it will seem more like an on-off control than a volume switch.

The PCL82 tubes are also known as 16A8; heater is 16vac. They are a high-mu triode with a power pentode, and were used in TV receivers. The triode can be used as an AF amplifier, and the pentode as an audio output tube. (info from RCA Receiving Tube Manual, 1970)
Another manufacturer's datasheet is online here:
Another page:
Here's an article a fellow wrote about building a stereo amp using PCL82 and ECL82 tubes, with schematics:

I don't have data on the PCL80 tubes.