1200S Restoration

[Ryan Phelps]

Hi Sunn Guys!
You may recall my recent post regarding paint removal from tolex. This was the start of a restoration to my 1200S. Since then I've installed a 3-prong AC cord, disabled the polarity switch and removed the death cap, installed new discrete filter caps, cathode caps, bias caps and tremolo / reverb board power supply caps. I brought the amp up to full line voltage slowly with my Variac and checked DC voltages to be sure everything was working correctly. The amp sounds great, but I have a couple of questions:

1. The tremolo works great, but there is an obvious ticking or beating when not playing through the amp. I'm familiar with the fix for Fender amps with this problem. Have any of you dealt with this issue on a Sunn amp? Recommended solution?

2. The main amp output is wired for 16 ohms from the factory. This head was paired with a 612 speaker cab. My 1200S schematic (for the later version with SS rectifiers) shows pairs of speakers wired in series and then all three pairs wired together in parallel. If all the speakers were 16 ohm, the cabinet load would be 10.33 ohms. I wondered why Sunn didn't wire the main output for an 8 ohms? If you use the extension output jack only, your are connected to the 8 ohm tap.
I think I will wire the main output jack to the 8 ohm tap and the extension output jack to the 4 ohm tap, leaving the feedback circuit wired to the 16 ohm tap. It seems this would be more useful with modern cabinets. Thoughts?

Thanks!

[BassHog]

I once had a Solarus with a pretty pronounced tick/beat like you describe. What it ended up being was the ground on the RCA jack plate for the Trem/Reverb footswitch. The bolts holding the jack plates for both the reverb tank and the footswitch jacks had come loose and there was corrosion between the jacks and the chassis. I unbolted the jackplates and cleaned up the chassis and the jackplate contact points with steel wool and reinstalled them making sure the bolts were nice and tight. Problem was gone.

As far as the outputs, wire them how every works best for you and your cabs. I would make sure the Negative Feedback stays on the same tap though.

[Isaac]

Weren't the 12" drivers in the 1200S cabinet 12 ohm drivers? That would give a nominal cabinet impedance of 8 ohms. Still doesn't explain the 16 ohm wiring for the head.

[Ryan Phelps]

Basshog....Thanks for the tip! I'll try it.
Issac......Yeah, I can't figure out why they used the 16 ohm tap. I'll need to research the old catalogs.

Thanks Guys!
Ryan