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Oliver Acoustic Research AR-94 specifications
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Francesca 1999 is the previous version of my DHT amplifier. This was another intermediate result to get a "no compromise version" of Francesca. Majors differences (vs. Francy) are:

  • Coupling transformers are used in each stage.
  • In the input stage a paralleled couple of EC97 tubes
The use of coupling transformers has definitely improved the clearness of the sound and the resolution of little details in music. (now in choirs you can easily distinguish voices, or ear the decay of the sound in the recording room). The use of Ec97 has improved a lot the gain (EC97 's mu is 63 compared ti 44 of 5842) while the lowering of the Ra has improved the drive of 2a3. The EC97 is a low cost choice but with very nice sound, expecially in the high part of the spectrum. Here is the main schematic.

Audio project

click to enlarge


The power supply

The power supply described below can be used to supply Francy or Francesca. It can be used also to powerother tubes like SV 811 or 845, provided you fix the adequate heater voltage.

Rules to follow: As you're dealing with high voltages you must pay very much attention not to touch any part under tension. As usual I follow this general rules:

  • Verify several time the circuit. Avoid being in hurry when you do this, nor do it at night.
  • The first time you connect to power line, verify the circuit under low-voltage conditions. I do that inserting a 40-100W lamp in series to the circuit. This is an old trick that can avoid you great problems.

  • Start with one channel
  • Connect o nly onetrasformer each time.
  • Heather supply circuit , Verify the voltage output under low-voltage (lamp inserted) and no tubes.

If everithing is ok you should measure a tension a little above normal condition. Then insert the tube: the lamp should flash and if everithing is ok the light should decrease while the filamet glow red. Now you can exclude the lamp and verify the correct filament tension.

HIGH Tension Section (900v)

Disconnect filament circuit and delay circuit, take away tubes and insert the lamp. When powering the lamp should flash only one time indicating that the capacitors are charged. To measure tension use a highly insulated tester with wires in good conditions. While measuring high voltages you MUST:

  1. wear rubber shoes
  2. have dry hand, (if you fear a lot, you can wear thickrubber gloves, but i never use it).
  3. use only one hand, while the other is in your jeans' pocket, to avoid that your hart is passed by a discharge (that's highly probable if you touch the wires with both hands)
  4. you should measure a tension that's slighly higher than normal.
  5. Let about 10min. pass (check the voltage)to discharge the capacitors.

For the mid-tension (300V) I suggest you to use the same approach if you are a beginner.

  1. Fix the polarizing voltage in safe area (about -60/-70V)
  2. Then you may: insert tubes, connect the low/mid tension circuit directly to the power line BUT still put the lamp in series to the high voltage sectio n. Connect a "poor" speaker to the output, and switch on. If everithing is ok the lamp should flash, you may hear a little "hum" from the speaker. Check the bias to let some current flow to the 572.If the speaker sends a whistle the amplifier is self-oscillating (never happend even with my "creative" cabling), you must check the layout. Touching the input wire with a 1meg. resistorthe hum should increase a lot, indicating that the amplifier is working.
  3. Put the bias in a safe position (-60-70V)
  4. Take away the lamp and fix the bias checking the current in the tube by measuring the drop in the 10ohm resistor.
  5. My compliments ! you did a good job!

The bias should be checked very often during the first working time (30 hours) do it every ten minutes, then every half hour). Now I measure it from time to time (let's say every 15 days) to check the tube conditions. My anode voltages are very similar and costant indicating that the tubes are very good quality.. Each channel'sanode voltage is thesame within 1 volt for the 572 and 3 volts forthe 2a3. I have more problems to match the 5842 (10 v difference).

Best listening impression is obtainable after 30 minutes warming up.

High voltage section

Audio project

click to enlarge

Low voltage section

Audio project

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Delay Section

This's is necessary to avoid fuse blow-up, and not to stress tube too much during heather warming up. Transistor are allowed here... ;)

Audio project

click to enlarge


Here's a photo of the amplifier, still under construction.