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

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

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-schematic

SUNN Beta-Lead pwr amp layout

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



9 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 !

  3. Rob says:

    I just scored a 212 beta lead from someone, but the amp has a hum even with all the dials turned to zero. So, the cleans are basically non-existent if I want to play some or if I want to mute everything at high volume it’s virtually impossible to do so. What issue is this most likely, the caps?

  4. jcm says:

    HI ROB, NEARLY IMPOSSIBLE TO TELL W/O LOOKING AT IT … IT COULD BE FILTER CAPS, ONE SIDE OF THE POWER SECTION COULD BE BLOWN, OR SOMETHING ELSE … FIRST CHECK WITH A DMM THAT YOU HAVE PROPER +/- 40VDC ON THE DOWN SIDE OF THE FUSES (YES, CHECK THE FUSES !!) … IF YOU HAVE A SCOPE CHECK THE RAILS FOR HIGH RIPPLE // THIS WOULD INDICATE BAD CAPS … WHILE YOU’RE AT IT, CHANGE THE MAIN FILTER CAPS ANYWAY … MOVE ON TO THE OUTPUT STAGE AND MEASURE DC VOLTAGES AT IDLE AND COMPARE AGAINST SCHEMATIC VALUES, GO FROM THERE … GOOD LUCK

  5. Carson says:

    I was having the same scratchy guitar pots issue with the beta lead with accompanying hiss at problem levels. I suspected the early stage coupling caps and stumbling across your page confirmed it for me. What did you end up using as replacements? More np electrolytics? Or did you change to Mylar style coupling caps?

  6. jcm says:

    THX FOR ASKING CARSON … I NEVER USE MYLAR

  7. Carson says:

    Ok, I replaced most of the np 2.2 caps on the board and that solved the scratchy guitar pots issue. I still have quite a lot of hiss which more than doubles if I turn the guitar volume or tone knobs down. I checked the voltage at the 470 ohm bias resistors and I get 10v on chan. B and 9.5v on Chan. A. The resistors read roughly 480ohm each. The supply side is at 14.9v so that seems fine. I don’t know much about troubleshooting cmos ics, is this evidence that the ics are bad? Or do I just need more bias resistance?
    I really appreciate that you made your experience with this amp public. It’s been really helpful so far.

  8. Carson says:

    Forgot to mention that the hiss increase when rolling off guitar pots happens only when using the “both” jack. There is hiss when using the other jacks as well just not increasing with guitar controls. This may point to ic106 or 103 but I’ve had a little trouble finding good replacements for those. I’d like to learn a good way to troubleshoot this so I don’t just replace a bunch of stuff that wasn’t a problem.

    Can’t wait to get back to my tube projects.

  9. jcm says:

    HEY CARSON, LOOK FOR CD4069UBE’S – I THINK YOU CAN GET THEM FROM SMALLBEAR, YES HERE : CD4069UBE SMALLBEAR … THESE IC’S CAN HANDLE UP TO 20VDD … GOOD LUCK!

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