Sunn 200S

I did some work i've been talking up for a while on my 200S. Basically two things, implementation of solid state power rectification and replacement of all electrolytic capacitors (from 1967).

The original power cap is one of those jumbo 4 in one caps which i am not interested in sourcing so i replaced it with several separate caps. The 30uf was replaced with 2 x 68uf in series (34 uf @ 800v), the 20uf's feeding A and B were replaced with pairs of 47uf's in series (23.5 uf @ 700v) and the last 20uf with a 22uf @ 450v. Overall this increases the total capacitance of the supply a bit resulting in a decreased power factor. Also to insure that the voltage is shared nicely between the capacitors in series i added dividers using 100k resistors. This will cause a bit more current to be drawn through the power transformer but since it is extremely large i don't think it will cause problems. (2.9mA on the B+, 2.85mA on 570 tap and 2.17 mA on the 433 tap = about 4 watts of power dissipated over the new voltage dividers). The 15k resistor was bumped up to 17k. Ideally the dividers should have been made with 220k and the 15k replaced with a 20k but its what i had on hand and it will work alright. With a 20k the voltages on the preamp taps would remain very close to the original design. with a 17k they are just a bit higher.

The caps on the negative bias voltage supply were also swapped out and replaced with the same type of 47uf mentioned above.

Here is the original supply schematic for reference:



Below is a schematic of the new power supply capacitor section. The voltages are the end result measured after changing to solid state rectification. Unlabeled resistors are 100k 3 watt.



Here are some pictures of the physical layout, first the pairs of 68uf and 47uf.



Here you can see all the electrolytics replaced.



The diode rectification is very simple to convert to. I have seen it suggested 10pf caps be put in parallel with each diode but i do not have any rated at the appropriate voltage so for now i have omitted them. Mostly you just have to make sure that the PIV rating of the diode is about 3 times greater than the rectified DC. In this case the resulting voltage is about 595 volts. And the PIV of a 1n4007 is 1000 volts. 3 X 595 = 1785. So if we use 2 diodes on each half of the rectifier we should be fine.

Here is a picture of the final result with the diode rectifier.



The amp was re biased for the new B+ and so far so good. It seems clearer sounding and punchier. It also seems to have a bit more gain. Possibly from the higher voltages in the preamp or possibly my imagination.

DIY Bass Repair

At some point I bought this bass online:



"you've got mail!"













Ive never seen something get quite so mangled in the mail. Anyways my money was refunded by the seller and they never filed the claim with usps so i still had the pieces of the bass. I asked some local luthiers about a cost of repair but the cheapest i could find was 80 dollars just for the neck. I contacted Dean and they were willing to sell me a new replacement neck for about the same price. so I decided i didn't have much to lose by attempting the repair myself.

I used wood glue and a bunch of clamps to glue both the neck and the body back together. I just used normal wood glue from the RONA near me and it worked perfectly fine. The hardest part was probably clamping the headstock area but in the end it turned out nicely.





this is the part of the fretboard that was originally cracked



the body "repaired"



various neck shots









Also the pickup was smashed so I replaced it with a seymour duncan "basslines" musicman pickup. It required some sort of preamp so I designed a very simple one with the option of some slight asymmetrical clipping using a RC4580 op-amp from texas instruments.

The preamp has 3 controls. A gain knob, a master volume knob and a switch which places 2 leds (used as clipping diodes) into the circuit. Below is the schematic i came up with (note that NE5532 and RC4580 are pin for pin compatible and both high performance low noise op-amps). Also the "volume" pot is Audio tapered not linear taper as stated in the schematic.







Without the preamp the output of the pickup is unbearably low.










The bass is now quite usable. I may repaint the body so that you cannot see the crack
but im not sure if I really care that much.