by Fred Nachbaur, Dogstar Music ©2002

Part 1:
Part 2:
How It Works
Part 3:
Part 4:
Part 5:
Parts List
Part 6:
Sound Sample


I went to pains to design this project around parts that are readily available. The parts list offers suggested sources for most of the parts. The only one that might be a challenge to find at low cost is the output transformer. I used a transformer from Triode Electronics, the model AA-1334 on their other transformers page. However, this has too high a turns ratio (about 70:1) so I took it apart and added another layer to the secondary to bring the ratio down to 35:1. However, I can't recommend this technique to any but the most enterprising experimenters. A reasonable substitute is the model TO-9105-1 on the same page. However, its turns ratio is off in the other direction; it would however work fine for 8-ohm speakers, and in fact give a bit more output power (perhaps as high as 1.5 watts). Better distortion response would, however, be obtained using 16 ohm speakers.

If you can afford to pay a bit more, an excellent choice would be the Hammond 125CSE. While the smaller 125ASE or 125BSE devices would also work, the 125CSE is actually cheaper! (This must be one of those supply-and-demand things.) The nice thing about these is that you can experiment with the primary taps to see what happens at different load impedances. The 35:1 ratio as per this design is on the black and green wires.

Construction of the MiniBlok SET Amplifier is relatively straightforward and non-critical, as electronics projects go. You could take any number of approaches to the mechanical layout; as you can tell from the pictures, I chose the "recycled" look by housing the prototype in a blue-anodized extruded aluminum case that once housed a small DC-AC inverter. Here are a few more ideas for you to toss around:
I'm going to assume at this point that you're reasonably skilled at soldering, since this is an ability that is best learned by doing (preferably with the assistance of a coach) I'm not about to try explaining this in a web-based article. If you're completely green in this respect, I'd suggest enlisting the aid of a local amateur radio operator, electronics store employee, or repair technician. Bribes are usually gladly accepted [wink]!

Be careful about wiring, especially in the comparatively high-voltage plate circuits. If you're unsure about how the tube connections are numbered, click here (use your browser's Back button to return to this page afterwards.) Be sure to use capacitors rated at the specified (or higher) voltage rating. I suggest staying away from "carbon composition" type resistors, especially for R2; otherwise you can get another classic tube sound - lots of hiss. That being said, if you want to use carbon-comp resistors for authenticity's sake, try to find the later Allen-Bradley ones; these will have a smooth finish and sharp, square edges (as opposed to the older and cheaper ones with rather rounded corners and a dull look; these can be awful as regards tolerance and noise).

A word about grounding: The ground symbols (three horizontal lines suggesting a downward-pointing arrow) indicate a connection to a common grounding point, which is in turn the only connection to the chassis (if using a metal chassis). This common point should normally be the ground end of the input jack. Since this is a relatively low-gain system, grounding isn't especially critical; so you could cheat and use, for instance, the ground lugs on your tube socket (if available) for tying components such as R3, C8, and tube socket pin 3 to chassis ground. The important thing is to avoid voltage drops caused by the high inrush current into the B+ filter caps (C1 - C3) from getting into your audio; therefore be sure that D2, C2 and C3 are connected together, with a wire connected to their common point at one end, and to the common ground point at the other.

Some more comments on the power supply: The MiniBlok uses two separate power transformers; the first is rated 12.6 volts at 2 amperes, and the second at 12.6 volts at 1 ampere. There is no reason you couldn't use two identical (2 ampere) transformers; you'll end up with a slightly higher B+ voltage if you do, which might marginally increase your available output power. Note when wiring up the power supply that transformer T2 (the high-voltage step-up transformer) is wired backwards; what used to be its primary is actually our secondary winding.

Modifications for 240 volt supply:

It is entirely possible to build a MiniBlok for use with 240 VAC, 50- or 60- Hz. power. Transformers T1 and T2 will be 240V to 12.6V transformers, rated at 2A and 1A (minimum) respectively. However, since the output of T2 will be on the order of 220 volts already, the voltage doubler for the B+ supply is un-necessary. Diodes D1 and D2 are therefore replaced by a 1A, 600 volt (or higher) "full-wave bridge" module (or four discrete diodes, if you prefer). Capacitors C1 and C2 are replaced by a single 220 µF, 350V unit. Other than that, everything else remains the same.

Power Supply mods for 240VAC

Wiring and Testing

I recommend testing (and debugging, if necessary) the power supply first. Be very careful about diode and capacitor polarities! The diodes have a band at the cathode end (this is the end that corresponds to the "bar" in the schematic symbol). The capacitors will usually be marked at the negative end with a bar or line consisting of a row of "-" symbols. At the low-voltage filament supply output on the tube socket's pins 7 and 8 (no load) you'll typically read about 13 volts AC. At the bias output you should read about -36 volts. At the high-voltage output you'll read about 300 volts under no-load conditions. Be careful about accidental shorts; these can make the Magic Smoke come out of your transformers or other components, and render them useless. If you chose to install the optional LEDs, you should see the green LED (D5) light up when power is on. The red one (D7) should only flash briefly while the capacitors charge, then extinguish.

The high-voltage filter capacitors will hold a charge for a long time after power-off, with no tube installed. To avoid an unpleasant surprise as you're working on this or any tube circuit, discharge the high voltage capacitors after turning off the unit and unplugging it from the wall socket. The best way to do this is to clip-lead a 10K resistor between the positive and negative terminals of the power supply (the terminals of C3 in this case), and leave it on for at least 30 seconds before proceeding with your work. (Don't forget to unclip it before applying power again!)

Wire up the rest of the circuit. Be careful that every connection is correct as you're making it. While in this relatively simple circuit it is quite easy to go back and fix errors later, it's good to err on on the fastidious side to save yourself possible trouble (and time wasted debugging) later on. Don't even think about plugging the tube into its socket just yet, though!

Do the following final tests:

  1. With the unit not plugged into the wall outlet, and after being sure the filter capacitors are discharged, connect the negative lead of your ohm-meter to common ground, and make the following measurements:
    • Tube socket pin 7: zero ohms
    • Tube socket pin 8: very near to zero ohms (may read zero on your meter)
    • Tube socket pin 3: zero ohms
    • Tube socket pin 6: about 820 ohms
    • Tube socket pin 4: zero to 100k, as volume control turned from zero to maximum.

  2. Switch your meter from ohms to the highest DC voltage range (leave the negative lead connected to the common ground point), plug in the amplifier, and turn it on. Note that the tube is not yet plugged into the socket. Make the following voltage readings:
    • Tube socket pin 1: at least -30 volts (may be higher, depending on your voltmeter's input resistance)
    • Tube socket pin 2: about 300 - 320 volts
    • Tube socket pin 3: zero volts
    • Tube socket pin 4: zero volts
    • Tube socket pin 5: about 300 - 320 volts
    • Tube socket pin 6: zero volts

  3. Switch your meter to a 20 volt or higher AC voltage range (leave the negative lead connected to the common ground point). Make the following voltage readings:
    • Tube socket pin 7: zero volts
    • Tube socket pin 8: about 13 volts

  4. Turn off the amp, and plug in the tube. Connect a 4 to 10 ohm dummy load resistor to the speaker terminals. Switch your meter back to the highest DC voltage range (leave the negative lead connected to the common ground point), and turn the amplifier back on. Make the following voltage readings:
    • Tube socket pin 1: at least -30 volts (may be higher, depending on your voltmeter's input resistance)
    • Tube socket pin 2: about 200 - 220 volts
    • Tube socket pin 3: zero volts
    • Tube socket pin 4: zero volts
    • Tube socket pin 5: about 130 - 150 volts
    • Tube socket pin 6: about 1.5 volts (you may have to switch to a lower range)

Everything ok? Great! Turn the amp off, button it up, disconnect the dummy load resistor and connect your speaker in its place. Connect your input source, turn on the amp, and enjoy!

MiniBlok underside

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