1.5W single-ended power amplifier as used at the Århus Triode Audition 2000


This amp was a desperate attempt to get something playing for the Århus Triode Audition (2000) in Denmark this august. I am currently waiting on parts for my 71A power amps and thus used the chassis for these amps to get something up and running. This project is the result of that attempt.

Despite the very simple filament supply of the AV5, there was absolutely no hum at all in the system we were playing. The KC3 is also known for microphonics but not even a tad of microphonics in these amps. I guess something finally worked okay from the start for a change :)))

The KC3 is biased around 3V, this means it only takes about 2V RMS input before drawing grid current and thus clipping. It's gain of about 20 times gives me enough to drive the AV5's to reasonable power levels (2Wrms). The AV5 is biased at 65mA with about 49V on the cathode (Ua=285V). It dissipated about 18.5W and using the 5K load it produces up to 2W of power.

The 50nF/47K filter gives a low roll-off at around 90Hz. This means the amp is not meant to play the low-end of the spectrum. If you want to play full-range change the resistor to 250K and the cap to 100nF. The reason I use a high-pass filter is that I use my amps for the Oris system which only runs from 150Hz and up. The less power I use up in the low end (which is not audible throught he Oris anyway) the more control the speaker will have in the region it is reproducing.

Listening to some  music with lights dimmed at Bjørn Aaholm's place after getiing back from the ATF, I noticed an orange glow coming from the tubes. The plates are shiny nickel so I thought it was a reflection from the filament off of the glass. A few minutes later when my eyes got used to the dark I noticed the entire anode was glowing dark orange. Whoah! I quickly hooked up my meter to check out what was going on. No, everything tested as it should... 18W on the plate, it should handle 20W. A quick check confirmed the other channel was also glowing a bit, same thing. I contacted AVVT and they said this was okay, no problem, "they like to run hot". Okay then, let them play, they sound very nice!

With my Puppy power projects ended I rebuilt the chassis to it's final version using the AV5. As the setup is almost identical to the first Århus AV5 version I listed it on this page. I made some big changes in the power supply, just like with the Puppy Power 3. For the AV5 I reconfigures the use of the four 10uF capacitors to get as much voltage as I could (more power). What I got was 275Vdc, this is what I have to work with. The AV5 is biased at 30mA (-27V) and dissipates just a tad over 10W, very nice! The KC3 got a different bias at 150V/2.5mA and a cathode bypass to get a bit more gain in the low end. Not much else to say, take a look at the schematics. Ah yes, filament supplies are also current regulated, I replaced the LM317 for the 71A to a LM350 (3Amps) to handle the 1.3A of the AV5 so that it won't overheat.

I had to put the puppy to sleep. It was just too old and worn out. One of the AV5 tubes seemed to be playing a bit softer than the other. After some checking I noticed it's emission had dropped from 50mA to about 15mA in one channel, and only after a month or two of playing. Tubes went back to the factory, no problem getting replacements they said. As I am planning a new chassis and setup, the AV5 will be replaced by a different tube. Back to the good old stuff I guess... UX245.


I'll try to put up new pics as well. The ugly black paint has made way for the nice copper that was underneath. Looks totally different.

Technical specs:

Frequency range -1dB with 1Vrms output: 32Hz - 24kHz
Frequency range -3dB with 1Vrms output: 23Hz - 43kHz

note: The OPT's were designed and made with a bottom frequency of 60Hz in mind, the somewhat "quick" roll-off is a result of this (and thus intended to be like this!).

Frequency range -1dB with 1 Watt rms output: 61Hz - 15.8kHz
Frequency range -3dB with 1 Watt rms output: 37Hz - 30kHz

High range is a bit less then I would have expected.

Maximum output before clipping: 1.49 Watts rms.

One of the AV5 amps next to Hartmut Quaschik's turntable.


Here's the AV5 amp with a 31 as driver instead of the KC3.
Here shown with clip-leads for charging the battery.


In the beginning...

Signal section.


B+ supplies for both KC3 driver and AV5 output tube.


filament supplies for the KC3 and AV5.