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



18 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

  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 …

  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!

  10. Laurent says:

    Hi, I have a huge difference with the treble control, between the 10 and the 9.5 position, the loss of treble is massive. Then when I go below 9.5 , treble is decreasing smoothly.
    I have this on both channels, so I guess it’s normal, just want to check if you have the same ?

  11. jcm says:

    HI LAURENT, … THE AMPLIFIER USES LINEAR TREBLE POTS, WHICH MIGHT NOT BE THE BEST TAPER FOR THE CIRCUIT … YOU MAY WANT TO TRY REPLACING THEM BY REVERSE-LOG 100K POTS, WHICH WOULD “STRETCH OUT” THE 9-TO-10 ZONE … LEMME US KNOW IF YOU TRY AND HOW WELL IT WORKS, OR NOT …

  12. Laurent says:

    Thx JCM, I’ve ordered the pots and see when I have time. I already replaced all electrolytics and it took me a full day ! I’ll also try a log pot on the master as I find it too loud too soon 🙂

  13. Laurent says:

    Master change to log taper, Treble changed to rev log taper, both are now much easier to control. Wrong initial design by Sunn.

  14. jcm says:

    THANKS FOR YOUR FEEDBACK LAURENT, DIY IS ALL ABOUT MAKING OUR GEAR MORE PERFECT – I DON’T KNOW IF I WOULD CALL THEIR ORIGINAL DESIGN WRONG … REALITY IS WHAT IT IS, THIS DESIGN IS SONICALLY INTERESTING TO MANY PLAYERS AND PHYSICALLY RELIABLE FOR THE MOST PART – THAT’S THE IMPORTANT THING HERE … THE SUNN DESIGNER(S) DID A GREAT JOB IMO

  15. Laurent says:

    I’m only talking about the choice of tapers, the reste is excellent for sure. BTW I also changed the level pots to log and it feels better now.

  16. jcm says:

    FAIR ENOUGH LAURENT, THANKS FOR YOUR INPUT – MUCH APPRECIATED

  17. TJ Oehmen says:

    I have one on my bench right now that is giving me fits. I have a good 1kHz sine wave through the input section, sound coming out of the speaker is highly distorted, even at low volume with the gain on low. With a guitar plugged in, you have to hit the strings hard to get any sound, and it is highly distorted. The 1kHz signal develops a strange spike as it travels through the circuit. I rarely work on SS amps, more comfortable in the tube world. Any help would be appreciated.

  18. jcm says:

    THANKS FOR YOUR QUESTION TJ … FIRST TRY TO MAKE YOURSELF AN 8~10 OHM RESISTIVE LOAD TO REPLACE THE SPEAKER WITH, SOMETHING THAT IS RATED FOR 100 WATTS AT LEAST – AND BE PREPARED FOR IT GETTING HOT WHEN TESTING AT HIGHER SIGNAL LEVELS … I WOULD START BY SCOPING THE INPUT OF THE OUTPUT STAGE WHILE VARYING THE TEST SIGNAL AMPLITUDE APPLIED AT THE INPUT OF THE WHOLE CHAIN … THIS WILL TELL YOU TO EITHER FOCUS ON THE FRONT (PREAMP) OR BACK (OUTPUT STAGE) OF THE AMP … YOU COULD ALSO DISCONNECT THE PREAMP FEED TO THE POWER AMP AND APPLY YOUR SIGNAL GENERATOR DIRECTLY AT THE INPUT OF THE OUTPUT STAGE TO DOUBLE CHECK THAT PART ON ITS OWN … IF THE OUTPUT STAGE IS FAULTY YOU SHOULD BE ABLE TO FIND WRONG DC VOLTAGES AND GO FROM THERE … DOUGLAS SELF HAS SOME GOOD BOOKS ON SOLID-STATE AMPLIFIER DESIGN – THE MAIN THING IS TO LOOK AT THE OUTPUT STAGE AS A HAIRY CHESTED OP-AMP, IN OTHER WORDS SOME OF THE TECHNIQUES FOR ANALYZING OP-AMPS APPLY HERE AS WELL (IE., OBSERVE HOW R7 56K AND R31 1K5 SET THE OVERALL GAIN OF THE OUTPUT STAGE)… THE PREAMP SHOULD BE SIMPLER TO SUSS OUT OR FIX, START BY CHANGING THE CMOS IC’S AND AGAIN SCOPE AGAINST VARYING SIGNAL AMPLITUDE … HOPE THIS HELPS

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