Sunn Musical Equipment > Q & A

New to the group, question about Beta Lead output problem

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engineco16:
I'm new to the group. I've had my Sunn Beta Lead since 1978 and I recently started playing again. I've been building a new pedalboard and the other day I had the amp on a clean setting and I was getting a little breaking up, this is where I screw up. So I decided to check the 45 year old speaker for any resistance and my finger goes thru the paper. Not the end of the world so I bought two news Celestion Seventy 80 8 ohm speakers, awesome! Not so much, I clean everything up, put the amp back into its enclosure, connect the speakers, no sound. I tried everything I could think of, nothing, but the speakers work when I plugged them into another amp. So I'm lost right now, can I connect alligator clips to an output jack and connect another speaker, or how can I tell if the amp is working. Thanks in advance.

loudthud:
On the back of your amp, find a jack marked "Power Amp Line In". Plug you guitar in there. You should get some low level sound through the speakers if they are properly connected to the amp. There are two more jacks marked "From Accessory". Those should work the same. These jacks have switch contacts that send the signal through when nothing is plugged in. The contacts become corroded when not used. Inserting a plug several times will usually "wake up" or refresh the switch contacts. A small amount of contact cleaner could be used, but you would have to open the amp to apply it properly.

To avoid this problem you would need short patch cables like the ones used on a pedal board and connect from the "To Accessory" to the "From Accessory" jacks or from the "Master Line Out" to the "Power Amp Line In" jacks.

engineco16:
Tried what you said, nothing. I literally took the old speakers out, with the power off and put the new ones right in. Six months ago when I pulled it out of the closet I totally went through the head and cleaned all connections and pots, much needed and it sounded much better. I plugged my the speakers into my VS265 and they worked fine and sounded so much better than the Marshall speakers in it.

I'm baffled. I took the amp out, looked inside and everything looks fine, clean, nothing burned, no smells, no loose wires. Everything looks fine from the outside. I got this amp and my Gibson "The Paul" for my 16th birthday/Xmas, I still have both and now that I'm into playing again it means a lot to have this amp working. Thanks for the help so far.

Isaac:
Here's what I think I'd do.

You know that the speakers work. You have another amp on hand.

I'd start by running a cord from the MASTER LINE OUT to your other amp. Use a line in if the other amp has one, but the regular input is okay, too. Just make sure it's turned down, because, if this works, you'll be putting in a pretty hot signal. If the preamp is good, then you'll get sound from the other amp. Might be distorted, but that's from the hot input signal overloading the input. Don't worry about that, at least not at this point.

If that doesn't work, then do the same thing with the individual channel LINE OUTs.

If those all work, then you know the preamp is not the problem. You already know that it's not the speakers, so that leaves the power amp and interconnecting wiring. Never forget that it might just be a cord!

Loudthud suggested plugging your guitar into the POWER AMP LINE IN. I've tried that before and got nothing, even on known good amps. Might be that your guitar doesn't have a strong enough signal to drive the power amp to an audible level. If you know that you're getting a good signal from the MASTER LINE OUT, then running a short cord from that to the POWER AMP LINE IN gives you a good signal into the power amp. Thud suggested that, but I'm not clear from your answer whether or not you tried it. If you have another, known good amp with a line out, go from that line out to the Beta's POWER AMP IN. That's to prove that the problem is in the power amp.

Doing all that should tell you where the problem is. Once you know that, you'll have to decide how to proceed. If you have the technical skills, you might be able to fix it yourself. Otherwise, you'd have to take it to a tech. Only you can decide whether or not that's worth the cost.

Isaac:
This might help: https://www.audioservicemanuals.com/s/sunn/sunn-beta/5017163-sunn-beta-partial-sm

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