Sunn 2000s- past updates to understand

[pbman1953]

I have a 2000s that was worked on before I got it. There are few items  I have questions on they did and why-


Off pin 8 they used a cap  of 10uf/160v instead of the 250uf along with the 1.5 k resistor across.


The yellow 16 Ohm OT wire wasn't used but was attached to a terminal with a 670 ohm resistor to ground. If a speaker output wire isn't used ,must it be connected? In some of my other amps with a 3rd connection ,when the 4 & 8 ohm were  used, the un-used wire was shrunk tubed

[SEL49]

1. Sunn used a 250µF cathode bypass on the input triode to reduce hum due to heater/cathode leakage. I suggest you replace that 10µF.

2. Sunn used the 16Ω tap for NFB. You should also see a 1K and 750pF connected to the 16Ω tap. I suggest you let it be.

Both of the above ideas were used in all of the Sunn amps of that era. They are part of the sound of a Sunn.

[pbman1953]

The closest I have is a 220 @ 35v, is the voltage important? Because I've seen other preamp schematics with the same position but with a lower voltage item.

[SEL49]

The closest I have is a 220 @ 35v

That's fine.

[pbman1953]

Thanks!

[pbman1953]

Another issue-


one of the 47 ohm, pin 3, resistors is bad,


is 100 ohm overkill


I checked others ,


Traynor & Fender don't have resistors on pin3


Ampeg uses a 5.1


Thoughts?

[SEL49]

Use 47Ω only.

[pbman1953]

Ok, thanks, I'll look for some

[SoundmasterG]

Ok, thanks, I'll look for some

You'll want to use the same type of resistor that Sunn used in that spot. I can't remember if it was wirewound or not, and am not in a place where I could look right now, but use the same type and wattage rating so that it will perform the same as the other ones that are not bad.


Greg

[pbman1953]

HI Greg!


I'm not sure if it was you who told me , but  it could be, to change all the 47 ohm grid resistors to 5w white stone resistors. One 47ohm/ 5 watt is the one that became bad. The schematic calls for 2 w


I ordered another one to match


Plus, from the direction of a master tech he told me to add a separate filament transformer so I could use KT-120's. There's plenty of room for it. I can say that there isn't a 2000s that sounds like mine. It's amazing and both transformers don't get very hot


Thanks


Rich

[SEL49]

The 47Ω connected to pin 3 is a plate resistor, not a grid resistor. There should be a total of eight 47Ω/2W resistors, four for plates and four for screens. IIRC, they are all carbon comp.

[pbman1953]

The 47Ω connected to pin 3 is a plate resistor, not a grid resistor. There should be a total of eight 47Ω/2W resistors, four for plates and four for screens. IIRC, they are all carbon comp.

Years ago all the carbons, 2W & 5 W were replaced with 42 ohm/5w stone resistors.


Are you saying for the 2w's, pin 4's,  to go back to carbon?

[SEL49]

Don't put words in my mouth. This is what I said...

The 47Ω connected to pin 3 is a plate resistor, not a grid resistor. There should be a total of eight 47Ω/2W resistors, four for plates and four for screens. IIRC, they are all carbon comp.

At the time that amp was made, 2 watt CC resistors were very common. But if Sunn was building that amp today, I'm pretty sure they would choose a different resistor and higher wattage and flame proof.

[SoundmasterG]

The 47Ω connected to pin 3 is a plate resistor, not a grid resistor. There should be a total of eight 47Ω/2W resistors, four for plates and four for screens. IIRC, they are all carbon comp.

Years ago all the carbons, 2W & 5 W were replaced with 42 ohm/5w stone resistors.


Are you saying for the 2w's, pin 4's,  to go back to carbon?

Keep the wirewound ones that you put in there. 5W is better than 2W and wirewound are way better than carbon comp too. Carbon comp can burst into flames when overloaded, and they fail more than WW do. The question is, why did that resistor fail? Do you have a tube that is going south?


Greg

[pbman1953]

The 47Ω connected to pin 3 is a plate resistor, not a grid resistor. There should be a total of eight 47Ω/2W resistors, four for plates and four for screens. IIRC, they are all carbon comp.

Years ago all the carbons, 2W & 5 W were replaced with 42 ohm/5w stone resistors.


Are you saying for the 2w's, pin 4's,  to go back to carbon?

Keep the wirewound ones that you put in there. 5W is better than 2W and wirewound are way better than carbon comp too. Carbon comp can burst into flames when overloaded, and they fail more than WW do. The question is, why did that resistor fail? Do you have a tube that is going south?


Greg

No worries, they're staying. Once I get the new resistor installed , I can check the tube in that location


Thanks

[pbman1953]

Another question, for the bass boost , does the resistor in the circuit control the amount of boost? I'd like it to be less. It's on the boomy side

[sluckey]

Another question, for the bass boost , does the resistor in the circuit control the amount of boost? I'd like it to be less. It's on the boomy side

No. Replace the .0075 cap with a smaller cap for less boost.

[pbman1953]

Thanks for writing in, good to see you again!




What value would you start with?

[sluckey]

What value would you start with?

.001. Let your ears be your guide.

[pbman1953]

.002 does the job!


Thanks!

[pbman1953]

Sorry to bother you all with more but after using the amp a weird issue has come up


Over all the amp is fine and sounds like it should.  While playing I noticed a buzz. I went what to the tone controls to readjust. While turning the Treble control, and playing, there was a spot on the pot that created the buzz. Either side of the spot was clean. Then I figured replace the pot and I installed a new 250k Linear and it was still there.


The Volume, Bass & Contour controls do not exhibit any issues.


What should I look for?


Thanks