Sunn 200S Power Supply Help

[misterc57]

So I changed the first stage cap to (2) 120 uF 450V caps along with the series 220K resistors. Choke wired per the Sunn schenatic. Choke does not get warm. Amp is dead quiet now.

[loudthud]

As I remember, those chokes measure about 50 Ohms.

[soundmasterg]

The re-wiring had the B+ fed at the first filter cap, before the choke (like a Fender Bassman 100 or Bassman 50 layout). The choke is still in circuit. I moved the B+ connection to come after the choke (as per the Sunn schematic) and the 120 hz hum was gone. This is with the single 30 uF 600V cap at the first stage.

The amp has been set aside for now, but when I get back to it my next step is to see if the choke gets hot (would this happen quickly or take some time?). And eventually, go back to the series caps and voltage divider resistors, at the first stage after I order some caps.

It sounds like someone did the mod and didn't rewire it correctly, or wasn't familiar with how Sunn did it as opposed to Fender and screwed it up. It hummed with the B+ tap before the choke because the one RC stage isn't sufficient to get rid of the hum, but when the choke is there also then it is. When you add back the series connected caps with the larger values and have the choke wired up correctly then it should be quieter than stock. I bet the choke is fine since it is working now with the smaller cap but with the correct wiring. With the larger caps it will sound quicker in the bass, but the amount of bass won't really change. It is just a little tighter and quicker, and it hums a little less too.

Greg

[misterc57]

The re-wiring had the B+ fed at the first filter cap, before the choke (like a Fender Bassman 100 or Bassman 50 layout). The choke is still in circuit. I moved the B+ connection to come after the choke (as per the Sunn schematic) and the 120 hz hum was gone. This is with the single 30 uF 600V cap at the first stage.

The amp has been set aside for now, but when I get back to it my next step is to see if the choke gets hot (would this happen quickly or take some time?). And eventually, go back to the series caps and voltage divider resistors, at the first stage after I order some caps.

[soundmasterg]

soundmasterg, did the 110 uF capacitance at the first stage have any 120 hz hum issues?

Nope. Imagine the circuit the same exact way it is now but with a 110uF 700V cap in place of that one that is there. That is essentially what is being done. The caps are added in series and also need to have resistors in parallel with each cap to share the voltage. It becomes a node with essentially the equivalent of a 110uF 700V cap (assuming you use 220uF 350V caps). 220k 1W resistors are good to use in parallel with the caps. Assuming all soldering was done correctly and there is nothing wrong with the choke, then it should work well with some quicker bass response and less hum, since larger caps are more efficient at removing 120 cycle hum. There could be a problem with the choke, especially if someone bypassed the choke with the previous mods. Triode Electronics probably still sells an exact clone choke, though you can get them elsewhere too as long as it has the same or better specs.

Greg

[g1]

I think it should be fine, but yeah, make sure it's not getting too hot. I'm probably being overly cautious and mistaken in my idea of size vs. current capability.

[misterc57]

Should I look for that small choke getting hot or some other indication?

[misterc57]

No pic showing, is it same as the one on the left in this pic?




Looks kind of small for plate current to me, but soundmasterg mentioned it running the plates on another forum, so I defer to his expertise that the schematic is correct.
( https://www.talkbass.com/threads/196...#post-11684766 )

Yes looks like the same choke. Not sure why you do not see the pic I posted, I see it in the thread. Here it is again. Thank you

[g1]

Hmmm. Not sure, here is a pic. Looks like 27-3100 606942.

No pic showing, is it same as the one on the left in this pic?




Looks kind of small for plate current to me, but soundmasterg mentioned it running the plates on another forum, so I defer to his expertise that the schematic is correct.
( https://www.talkbass.com/threads/196...#post-11684766 )

[misterc57]

As long as it's the stock choke (or equivalent). Sometimes people will use a cheaper replacement that can not handle the current, so they move it to the other side of the OT supply node. A choke that can run the power tube plate current is big and expensive.

Hmmm. Not sure, here is a pic. Looks like 27-3100 606942.

Now that you mention it, looks like someone intentionally moved the B+ tap before the choke (because the wire was extended to reach it). soundmasterg, did the 110 uF capacitance at the first stage have any 120 hz hum issues?

[g1]

Voltage to the output tube plates were being fed before going through the filter choke. I rewired to go after the filter choke and no more hum. I guess someone made a mistake when they upgraded to the series caps.

As long as it's the stock choke (or equivalent). Sometimes people will use a cheaper replacement that can not handle the current, so they move it to the other side of the OT supply node. A choke that can run the power tube plate current is big and expensive.

[misterc57]

Using a solid state rectifier allows you to use higher than 60uF capacitance in the first stage. These amps use pretty high voltage that often goes over 500V for sure and approaches 600V on occasion. The earlier 200S amps with the Dynaco transformers will have lower voltages than the later amps with the Schumacher transformers. That high voltage in these amps is why series connected caps in the first stage are a good idea because they get you a higher voltage capability. The 220uF in series for a 100uF stage is probably from a post I did on the Sunn forum over a decade ago. The higher capacitance in that first stage makes for a tighter bass response and for a bass amp that is a good thing for most bass players. The amp holds up just fine with the higher capacitance too. Sounds like you have it working again with a lower value, which is fine. If you like it roll with it.

Greg

Interesting. I am thinking of upping the first stage capacitance. Especially since there is about 585V there.

Anyway, good news. I finally got rid of the 120 hz hum. Voltage to the output tube plates were being fed before going through the filter choke. I rewired to go after the filter choke and no more hum. I guess someone made a mistake when they upgraded to the series caps.

MC

[soundmasterg]

Using a solid state rectifier allows you to use higher than 60uF capacitance in the first stage. These amps use pretty high voltage that often goes over 500V for sure and approaches 600V on occasion. The earlier 200S amps with the Dynaco transformers will have lower voltages than the later amps with the Schumacher transformers. That high voltage in these amps is why series connected caps in the first stage are a good idea because they get you a higher voltage capability. The 220uF in series for a 100uF stage is probably from a post I did on the Sunn forum over a decade ago. The higher capacitance in that first stage makes for a tighter bass response and for a bass amp that is a good thing for most bass players. The amp holds up just fine with the higher capacitance too. Sounds like you have it working again with a lower value, which is fine. If you like it roll with it.

Greg

[g1]

I just read about a guy who's garage door opener had developed reduced range from the transmitter. He tracked it down to new LED lights in his garage.
Lots of new tech causing odd issues.

[misterc57]

So there's still some ultrasonic signal (around 42kHz).

Could be either an oscillation of V1 or a signal picked up from nearby equipment (switching power supply, phone etc.).

Interesting. Beyond human hearing. Will see what I can figure out.

Thank you!