Strange Sunn 200s modifications

[shuksan]

I recently purchased a Sunn 200s on eBay, and it's in working order and sounding excellent, but I noticed a few questionable things about the amplifier. Namely, the power cord seemed a bit flimsy—it was a three-pronged replacement cord—so I opened up the amp to replace it. However, when I opened it, I noticed that numerous modifications had been made to the amplifier.

It's been recapped, but only in certain places. The big Dynaco cap had been taken out of the circuit entirely and replaced with an array of capacitors. Other caps have been replaced here and there, but the strangest replacement I've seen is that of the death cap. It wasn't removed from the circuit when the three-prong cable went in; rather, the cap was just replaced. Additionally, a capacitor has been added to the power switch between its bottom and top leads. I'm not so sure about this cap, but I'm worried that it's creating a short.

In addition to this, it appears that a second resistor has been added to the amplifier's first input in parallel with the 68k.

Earlier today, I took the polarity switch out of the circuit, removed the death cap, and put the power cable in place as it was before. The amp is working now, but the output of the amplifier seems to be humming—it didn't do this before. Could it be the capacitor on the switch, or something else?

EDIT: Having checked the amp again, it appears that the hum is coming from my guitar's pickups and not the amplifier itself. The hum goes away when I turn the volume down on my guitar or briefly unplug it, and it quiets down when I move my guitar away from the amp. I'm guessing that this is due to the fact that I haven't placed the amp back into its case just yet.

I've attached some photos that were taken before I changed anything.

Thank you!

[Soundmasterg]

The work looks somewhat sloppy, and some of the values used for caps could be a bad idea. It would help if you traced out the wiring changes and identified which caps were replaced and with what values, when compared to a stock schematic.

Greg

[shuksan]

The work looks somewhat sloppy, and some of the values used for caps could be a bad idea. It would help if you traced out the wiring changes and identified which caps were replaced and with what values, when compared to a stock schematic.

Greg

I haven't checked the coupling capacitors thoroughly yet, but the filter cap replacements seem to be the most dramatic and obvious change made to the amp, and I'm primarily concerned about this now.

It appears that two of the filter cap sections that lead to points A and B on the schematic were replaced with two 450V 47µF caps in series. The section nearest to the standby switch on the schematic was replaced with two 400V 100µF caps in series, and this section was grounded in two places on the chassis for some reason. The section leading to point C on the schematic was replaced with a single 450V 47µF cap—perhaps a second cap that would have been placed in series was omitted due to a lack of space. The last capacitor's working voltage seems to be underrated, especially considering that it is smaller than the original cap's. The working voltages of the other capacitors seems okay, but the capacitance values have been increased somewhat.

I'm pretty sure that cap should be replaced, but should I replace the others with capacitors that are closer to the original capacitance as well? If I have to replace all of the filter caps, I might go with the SDS board from Triode Electronics just to simplify the process and to clean things up inside the amp.

Also, any ideas about the capacitor on the power switch? Could this have been implemented to reduce popping when powering the amp off? The amp still pops periodically when I switch it off, leading me to believe that the cap was installed incorrectly or something.

Thank you for your help!

Steven

[Soundmasterg]

When you connect caps in series, you have to connect a resistor in parallel with each cap so that the caps will share the voltage. If this wasn't done, one cap will try to hog the voltage and eventually blow. A good resistor value to use would be 220k 1w.

Also when connecting caps in series, the voltage rating of the now single cap doubles, and the capacitance value drops in half. So for section A and B, you now have a 22.5uF 900V cap for each section. Stock was 30uF 525V for section A and 20uF 525V for section B.  The single 47uF 450V cap for section C is more capacitance than the stock 20uF and less voltage than the stock 525V. Having more capaictance in this section as opposed to sections A and B will change the tone of the stage they supply because it is RC coupled here whereas the plates and screens are not for the power tubes. When I use series connected caps in the Sunn bass amps, I use two 220uF 350V caps for section A and two 100uF 350V caps for section B. This results in a 110uF 700V cap for section A and a 50uF 700V cap for section B. I've done this for the Sunn guitar amps in the past too, but some people don't like to change the filter cap values that much. I try to stay close to stock with the other cap sections, but I make sure to have a cap rated for at least 500V in section C as it is often really close to that voltage at that stage. Keep in mind that if you go over 60uF for the first filter section, you can no longer use a tube GZ34 rectifier and must use a solid state replacement, or a Weber Copper Cap WZ34 or WZ68.

The coupling caps probably function just fine in the amp....the brown blob ones aren't particularly great sounding, but they usually work fine. The green Cornell Dublier's are probably fine and sound good. The Black Cats if you have them may leak, but if they don't they also sound great and I would leave them.

You could use the SDS board from Triode...a lot of people like it.

The cap on the power switch is not needed. Good thing you removed the death cap too.

Greg

[shuksan]

Thank you for all of your help! I decided to go with the SDS board just to ease the process of replacing the caps and to make things look a little neater inside the amp. As of yesterday, the SDS board has been installed, and the amp sounds excellent!

I think the next step will be replacing the rectifier tube, since it appears to be the wrong type for the 200s; the amp came with a Sylvania 5U4 rectifier tube

[Soundmasterg]

Thank you for all of your help! I decided to go with the SDS board just to ease the process of replacing the caps and to make things look a little neater inside the amp. As of yesterday, the SDS board has been installed, and the amp sounds excellent!

I think the next step will be replacing the rectifier tube, since it appears to be the wrong type for the 200s; the amp came with a Sylvania 5U4 rectifier tube

You you should get the proper GZ34 in there, as long as the SDS board isn't using first stage filter caps larger than 60uF. If it is, then you will need to use a solid state rectifier or a Weber Copper Cap in place of the tube rectifier. You will have to re-bias when you change the rectifier tube too.

Greg