My Sunn Sentura II that I've had since it was new has been in storage for many years. The power transformer failed. My brother replaced it, the electrolytics, and the 6.8K 2W resistors ten years ago, but the amp is still not working. A couple of weeks ago, I dug it out of storage and decided to dig through it. The circuit is simple enough, but it sure is hard finding where things are located with no parts layout diagram.
It may have been discussed elsewhere, but there are some errors on the Sentura II schematic. In particular, there is a connection missing from Pin 6 to Pin 2 of the 6AN8. Further, The plates of the 12AX7 and one side of the 12AU7 are shown as connected to "D" in the power supply. That's clearly an error as the plates should be connected to the point marked "260V". I've corrected the schematic by labeling the point as "E" and attached it. If anyone knows of other errors, please post them. If there is anything wrong with my corrections, I'm happy to fix them.
My journey to bring this amp back to life is in its early stages, but I'm sure I'll be back for help. One issue that bugs me so far is that voltage out of the 5AR4 is 604v. Since the amp is not working, that supply is probably not fully loaded, but I'm surprised it's 100 volts over the number on the schematic. Maybe that's normal or acceptable.
Since I don't have it here with me, I'll have to take a look and get back with you with the exact model number. If it turns out to be the wrong one, where can I get the right one?
Triode Electronics makes an exact replacement. http://triodeelectronics.com/mq782.html
The filter capacitor becomes an issue in these amps because of it's 525V rating. Antique Electronic Supply sells a replacement but reliability has been questioned, some people report problems. The most reliable option is a small PCB that has radial leaded caps with equalizing resistors. These are sold as retrofits for Dynaco amps. No good place to put it in an amp with tremolo though.
WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personel.
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