Mods

This is by far the most complex project I did so far – involves the most parts, the highest voltages and LDR channel switching – yummy. Of course, I wanted to add some mods to the basic idea (who wants 100% clones anyways):

• Modify power amp to use EL91 tubes for total power around 4.8W (that’s right).
• Use 6N2P(-EV) tubes in preamp instead of standard 12AX7. The only change made to accommodate these beauties is different heater wiring.
• Added Depth control.
• Reduce fx loop Send level to be more pedal friendly. I added 1K resistor in parallel with existing 2.2K resistor on Send triode cathode resistor. To compensate gain loss, Return triode cathode resistor is bypassed with 1uF capacitor.
• Switchable treble bleed capacitor on OD channel gain pot, a.k.a Warren Haynes mod.
• Only 8ohm output jack installed without impedance switch.
• Removed choke from the power supply. With less current draw from power tubes, RC filters should provide enough filtering for B+ (tested in Duncan PSU Designer).
• Changed bias power supply to AX84.com style. This is a slight change, I just wasn’t sure if SLO bias can go as low as 10mA per tube.
• Added switch to choose between 2 and 8 ohm OT taps for NFB loop. 2 ohm means less NFB and it makes amp sound a bit more aggressive.

Components

For output transformer I got Hammond 125C which is capable of taking up to 8W of power, so it’s more than enough for tiny EL91. Secondary is wired to lugs 2 and 4 which gives about 22.5K primary impedance. Data sheet calls for 20K which is close enough to what I have. Few more words on 6N2P tubes. I got a few of 6N2P and military version 6N2P-EV tubes that should have the same gain as 12AX7 but with slightly different heater wiring. There is no option for running heaters at 12.6V, but only 6.3V wired to pins 4 and 5. Pin 9 is used as internal shield between triodes that should help reduce noise and it should be grounded.

Since EL91 tubes work with only 250V on plates and SLO preamp would not sound the same with that low voltage I had to make separate power supplies for preamp and power amp. For power transformers I’m using custom wound toroids from local transformer specialized company Trafomatic which can make a single transformer with any given number of primaries/secondaries. All that at a very good price (much cheaper than EI style Hammond transformer with similar power). Here are transformer specs: 220VAC primary, 300-0-300V @ 40mA, 200-0-200mA @ 100mA and 3.15-0-3.15mA @3A secondaries.

With only ~5W of power, B+ line needs less capacitance to filter out AC ripple. I got JJ 100uF+100uF 500V cap can, so that each power supply uses one 100uF section as first filter cap. The rest of filtering is mounted on the board, so I do not have a dedicated power supply board.

Pictorial

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Sound clips

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Useful links

SLO Clone Forum