Sunn Concert Bass refurbish -- help?

[CrashAlpha]

I am refurbishing an old Sunn Concert Bass amp I have had lying around. This isn't my amp, but the same model - the old silver face:



It was always weak and distorted and I ignored it for ages.

Now a friend is refurbishing it for me. The sound is now beautiful, clean noiseless and has full-range response. I find it pretty loud for 150W @ 8ohms (currently going through a Yorkville Sound XC210 300W 2 x 10 + horn @ 8 ohms). This is going to make an excellent small-gig/practice hard rock/metal amp once I get it going because the sound with my Rick's Seymour Duncun bridge pickup is just searing.

But I have a couple of challenges:

Challenge 1: So far we have replaced the leaking caps and are working on a problem with the sound cutting out intermittently under load.

The volume pots were really dirty and we cleaned them and now there are noiseless but we suspect the problem may still be with these old pots as it is an early 70's unit that has done nothing for decades. Can an old volume pot sound clean and still cut out intermittently?

I am going through a Boss ME 20B bass pedal through the normal channel. Could I be overdriving the pre-amp and not knowing it, being deceived by the clean sound and could it make the amp cut out?

Challenge 2: One of the control knobs is missing and I would like to replace it. It seems to me I have seen these knobs once upon a time in Radio Shack - but that was ages ago. Does anyone recognize them and if so, can they give me key words to search for? (Someone was selling a set on ebay for $25, recovered from an old Sunn amp, they are gone)

Challenge 3: I believe that a footpedal is required in order for the distortion to work. I have seen discussion threads on how to build one, http://www.diystompboxes.com/smfforu...?topic=61126.0 but I have never seen one. If I understand correctly, it should be a single DPDT switch going to two RCA jacks. Apparently it's a real oddball. Can anyone confirm?

I appreciate any advice anyone may have.

[CrashAlpha]

Chalenge 3 may already be solved, these look like Fender Black Skirted amp knobs!

[nashvillebill]

Look for the schematic, it's out there. I had this amp, with the footswitch, but sold it a couple years ago so I'm going from memory.

One RCA jack controls distortion. The other controls the treble boost, IIRC. Do NOT short the two jacks together. For eachRCA jack, you just want the switch to connect (switch) the inner conductor to the outer shield.

[nashvillebill]

Well I found the schematic, if you need a copy PM me with your email addy.

Looks like my memory wasn't so good. Treble boost isn't controlled by the footswitch. But it appears the distortion is two-stage,each jack is tied into the circuit separately, so you might try two separate switches on the distortion. Simulate this now by merely shorting the RCA jacks to ground one-at-a-time...

[CrashAlpha]

Originally Posted by nashvillebill

Well I found the schematic, if you need a copy PM me with your email addy.

Looks like my memory wasn't so good. Treble boost isn't controlled by the footswitch. But it appears the distortion is two-stage,each jack is tied into the circuit separately, so you might try two separate switches on the distortion. Simulate this now by merely shorting the RCA jacks to ground one-at-a-time...

I read the same schematics, it shows two switches, but this could easily be a DPDT. Do you remember there being two separate switches? No one seems to remember, and the consensus appears to be that the switches closed to ground individually don't do anything impressive, but combined produce a desirable effect.

[nashvillebill]

My footswitch definately had two separate switches. I keep wanting to think that one of them was labelled "boost" and the other "distortion". And this was the real Sunn footswitch that came with the amp.

[GreaserMatt]

I had a Sunn Concert Bass head for years; great amps. Those knobs are hard to get anymore; check ebay. The older sunn stuff was always better than the 'red knob' era, imho...

[CrashAlpha]

Originally Posted by GreaserMatt

I had a Sunn Concert Bass head for years; great amps. Those knobs are hard to get anymore; check ebay.

It's a really unique sound, it's amazing to me that this thing was wallowing in my basement forever. Like I said, I am hoping the Fender black skirted knobs will make for a nice replacement. Did you have the footwitch, and do you remember what the functionality of the buttons were? My amp is in a rehearsal space and I haven't tried grounding the two individual RCA footswitch jacks to see what they do, but at least one person on other forums have said that grounding the individual jacks doesn't do anything pleasant, but grounding both at the same time produces a real effect.

[thetaxmiser]

Originally Posted by nashvillebill

My footswitch definately had two separate switches. I keep wanting to think that one of them was labelled "boost" and the other "distortion". And this was the real Sunn footswitch that came with the amp.

Bought one of these new around 1974. To my best recollection you are correct on the Sunn footswitch.

[CrashAlpha]

Originally Posted by thetaxmiser

Bought one of these new around 1974. To my best recollection you are correct on the Sunn footswitch.

Fender makes a two-button footswitch with RCA plugs. In order for it to work with the Sunn I believe that the switch has to not just close the circuit, but rather ground each circuit. I wonder if this will do the job?

[nashvillebill]

Closing the circuit, in this case, means that it's being grounded. The shield on the RCA jack is ground, so the connection from the internal wire to ground will be either made or broken by the footswitch. Don't overthink this and make it harder than it is! All you are doing is connecting (or disconnecting) the internal wire to the outer wire.

[skruch]

Originally Posted by nashvillebill

My footswitch definately had two separate switches. I keep wanting to think that one of them was labelled "boost" and the other "distortion". And this was the real Sunn footswitch that came with the amp.

You are correct. I bought one new and giged it forever. Always had the boost on. FYI, I had some issues with intermittent signal. I ended up rubberbanding down one of the stereo output jacks. It was not stereo, but the extra tab contact seemed to create problems. After banding it it was a solid tank workhorse. Good luck on your quest.

[CrashAlpha]

Originally Posted by nashvillebill

Don't overthink this and make it harder than it is!

I'm a software analyst. I overthink everything. I mean, the word "anal" is right in the title.