I recently acquired my first 40W chassis, a '69 Sonaro. I've got a lot of experience with the 60W and 120W amps but this is the first EL34 Sunn I've owned.
The only schematics for these I can find are the ones here, the Sonic I-40 and the Solarus.
I am...confused.
Mine seems to follow the Sonic schematic, but there are some exceptions:
1. It has the feedback tap on the screen on one of the power tubes, like the bigger amps. The schematics don't show this. Error on Sunn's part in drawing? The Dynaco EL34 schematics show it.
2. Instead of the suppressor grid being tied to ground, as in nearly every other true pentode amp (or tied negative, a la Traynor), it's lifted ABOVE ground by a 330 ohm resistor. Not only is this not on either Sunn or Dynaco schematics, I cannot imagine WHY one would do this
3. The Solarus power supply is a typical DC rail, with nodes of decreasing voltage as you get further from the power tubes. The 'B' node feeds the phase inverter, the 'C' node feeds the preamp tube. As on the bigger amps, and common practice. However, the Sonic (and my amp) flip the PI and preamp, with the preamp being fed higher voltage from node 'B' and the PI getting lower voltage from node 'C'.
4. My amp has the typical multisection can filter cap, but also an additional 40uF/450V cap on the preamp tube, mounted with a ring to the preamp terminal strip. I once owned a Studio PA that had this cap, but at least on that amp the schematic described it. Not so on either the Sonic or Solarus schem. It's riveted with the terminal strip and dates to the same period as the rest of the components, so I am confident it is original
5. It's got no cathode bypass cap on V1. To be fair, about half the Sunns I've opened up lack this, though all the schematics show it. Why did they leave it out so often?
