Sunn 1200S Restoration project

[nevetslab]

I had set this project aside for a couple months back in Jan 2016, so I could earn funds to keep me going. When I got back to the project in mid-March, 2016, I reviewed where I left off.

After substituting a different power transformer as well as the entire power supply (from my HP 712C Tube Amp power supply), nothing was showing a solution to the hum and buzz I had. I swapped out the slower 1N5406 3A rectifier diodes with UF5407 3A Ultra-Fast diodes, removed the 20uF/600V 2nd stage filter, vastly shortened the supply wiring and installed a rear-panel mounted Diode/Tube rectifier mode switch. Marginal improvement, but no cigar.

I decided to restore the original grounding, which was still easy to do, where all the grounds of the amp got to chassis by way of solder terminals via 45 yr old rivets. When I had powered it back up, the bloody amp was VERY quiet…Well, I’ll be buggered!! The original ‘RF- grounding method used in so many amps over the years, sometimes successful, other times with lots of hum, was in this case the best solution.



I cleaned up the wiring a bit, now moving forward to deal with the Tremolo circuit, which had it’s own high hum problems. The LDR-opto-cell was glued upside down to the PCB in the middle of the chassis, surrounded by the hum field of the HT power supply, was not quite like that shown in the two schematics I had found on this amp. When I had replaced it with a jumper, the preamp residual noise dropped down considerably. So, I ended up moving the opto-cell to sit between the Volume pot and the Treble pot. It ended up with a mu-metal shield folded up and stuffed inside a brass shield that I soldered together, with openings for the connection to the LDR and lamp terminals. Re-installing it into the circuit did raise the noise level a little bit, but it was tolerable.



I increased the filtering of the power supply that ran the 3-transistor oscillator circuit, which got that quiet. Next was the Reverb return circuit. The original single-transistor circuit was horrible, so I replaced it with a an op amp stage, and mounted it atop a standoff on the Tremolo PCB. The original Gibbs reverb tank worked, did have the locking mechanism for transport, but….it was dark and murky sounding. After borrowing an Accutronics tank from a Fender Twin, that sounded much better, and ordered one that was suited for face-up mounting to the inside roof of the cabinet (4AB3C1A). I ended up tweaking the voicing caps on both the drive and return circuit, as well as lowering the gain of the return circuit.

That left the input circuit. The original input jacks were not grounding-type, so with nothing plugged in and the volume turned up, it was noisy. I finally came up with configuration that kept the original Bright Filters as well as the Normal inputs, together with an attenuated set of jacks, all that ground when nothing is plugged in so the amp was quiet. That required using a pair of Switchcraft type 13A Transfer jacks added switching contact) along with the grounding-type12 jacks. I will load the entire amp circuit into my schematic capture program, so there will finally be a complete revised schematic coming, and will post that when done. I still haven’t found a schematic that shows the 3-deck pot Contour circuit. For some reason, the pdf sketch of the input jack wiring isn't coming up properly, so I'll have to fix this later



I finally got this amp completed in late May 2016. It came out really nice, both in performance, sound and appearance. Apart from increasing the power supply capacitance, and minor mods to the Reverb circuit, the tone circuits, gain stages and output stage remained stock. Was it worth the 6 months of agony and labor I invested in it? Not financially, to be certain, but was worth the effort to see one of these nicely restored

[nevetslab]

Sunn 1200S Revised Guitar Amp Schematic 2016

I finally got the revised 1200S Schematic composed on National Instruments Multisim 11.0, the schematic capture program I use the most. There are a number schematic symbols that are clumsy..the T/S + Normal jacks don't show the Normal contact shorting to the input leave spring, and they didn't have any T/S Transfer jacks (Switchcraft # 13) which I used on the input jacks together with the # 12 T/S+N jacks to solve the automatic grounding of the inputs when nothing is plugged in. The 4 input jacks now have 0dB Normal & Bright, along with -6dB Normal & Bright inputs, with the -6dB Bright curve being a bit different than the 0dB Bright, just for added voicing. There was originally a 470pF Brightness cap across the Volume pot, which is now on a Pull Switch of the Volume Control.

I added a second Bias Adjust pot so the upper and lower output tubes have their own bias adjustment. I Also added a rear panel Rectifier Mode switch. With 5AR4 rectifier tubes, the difference isn't that different, but with 5U4GB's, there's a greater voltage drop in the rectifier tubes. I never got arund to trying it with the 5U4GB's, which is what you find on the Mesa amps having that feature.

Somehow, by the time I finished the project in late May 2016, I never recorded all of the supply voltages thru-out the amp. The Voltage taps thru the power supply decoupling stages were calculated from the plate current values I got from an earlier schematic. In looking at my before and after photos on the original 4-section power supply cap can, it looks like there was a 100k decoupling resistor to the preamp tube section, instead of a 10k, which was shown on that earlier published schematic. Though some other photos I took of that original section, it does look like a 10k 2W Carbon Comp resistor WAS installed.

The schematic program didn't have power and output transformer symbols that were complex as is used on this amp, so I had to cobble symbols together to best-approximate them. I also saw in my notes that I never measured the inductance of the two chokes in the power supply.

Sunn 1200S Guitar Amp-3.pdf