June 24, 2006

Sunn Sceptre!

I was still a teenager when I first got to play with Sunn. I couldn't afford one but a CW band in Andalusia could. The Mavericks had 3 new Sunn rigs when they hired me to play bass with them. I had to replace a power transformer in their Sorado bass amp. That's when I fell in love with the simple but rugged construction of these amps. I even built a Sorado as my first guitar amp. I never had the good fortune to own one of these... until now. I bought this 1971 Sceptre on eBay for $250.00. I replaced the 6AN8 tube, did a cap job, and replaced the tremolo optoisolator. This thing has a very full, classic clean sound and at 60 watts RMS, it can hang with a Twin Reverb any day. It's truly a keeper!


 


(I included a photo of these caps just because I had a helluva time figuring out the color code.)
 


Update August 10, 2010

 I finally put a new set of SED Winged C 6550s in the amp. And I replaced the stock bias circuit with a dual bias adjust circuit. The schematic has been updated to reflect the changes.


Here's the original bias circuit. The test point simply monitors the negative bias voltage fed to the grids.


The dual bias mod provides separate pots and testpoints to measure cathode current for each output tube.


Locking pots and testpoints make bias adjustments and tube matching a breeze.

 


Download the schematic

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