SUNN Beta-Lead // Resources

The schematics I was able to find online were scanned at a very low resolution … I tried my best to improve their legibility … what I have is posted here below, …

SUNN Beta-Lead front

SUNN Beta-Lead back

the amp had a few issues that turned out to be minor … leaky 2.2uF NP caps at the front end that made the guitar controls sound scratchy // every 2.2uF cap I could find and all the pots are highlighted here …

SUNN Beta-Lead pre amp schematic


replacing these caps involves removing the preamp board from the chassis … while in there a 100k channel Master pot with broken shaft is replaced … I used a 24mm Alpha 100kLIN pot in lieu of the original // works fine …

SUNN Beta-Lead pre amp layout


the 470 ohm biasing resistors (pin 14 of the CD4069 chips) dropped voltage down to 8.4 volts on both IC104 and IC105, the CMOS chips that perform clipping duties …

SUNN Beta-Lead pre amp chip layout

Output stage was working fine and biased exactly as in the factory drawings …

SUNN Beta-Lead pwr amp schematic


SUNN Beta-Lead pwr amp layout


I was able to get some great tones out of this thing … at high gain the preamp gets fairly noisy, but with the master turned up and the gain turned up not too high I was able to get some pretty sweet blues and rock tones … definitely a fine sounding SS amplifier

2 comments on “SUNN Beta-Lead // Resources
  1. David Navarro says:

    My red channel is dead LED lights up but no sound. I can hear the hiss and the tone stack works when I did a little diy audio probing but something in the front end ain’t passing signal. Any segestion?

  2. jcm says:

    Hi David,

    The tone stacks come pretty late in the preamp, and run on op-amps which are not too likely to die … I would look at socketing/replacing the CMOS IC on your dead channel (these IC’s are listed as IC104 and IC105 and appear before the tone stacks in the circuit … while you’re at it, check out R122 and R153 (470r) that feed/bias these CMOS inverter IC’s … if these resistors happen to be too low in value, or the supply voltage higher than normal, these factors can certainly cause the CMOS IC’s to run very hot (since they are operating in analogue mode) … if one of these IC’s died from over-current (like I said, not impossible) the corresponding resistor could be damaged too // … I’d take a second to measure these, look ’em over for signs of heat damage; as well as associated traces and pads on the foil side

    best of luck !

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