Sunn 200s problems

[alebeaum]

When I turn the volume up to 8, the amp start to oscillate, I think this is the right term. Weird noise same thing with the bass unpluged.
Does anybody input to resolve the problem.
I have good experience with high voltage and automatisation but, in audio electronic my knowledge is limited.

THX
alex

 

[B-string]

Easiest thing to do is first try a new 6AN8. Grid leakage in the pentode section can setup oscillation.

 

[alebeaum]

yes, This is the thing that make most sense to me, Since volume is just in front of the 6AN8 stage. I ordered a tube on ebay. Just waiting for it... Any other ideas?

 

[B-string]

Lots. If the power supply caps have not been replaced in the last 20 years (including the bias supply cap) it could be motorboating. Does it have the tube rectifier or SS diodes?

 

[alebeaum]

the amp had a cap job in 2001 and the tube rectifier was replaced by SS rectifier. The tubes are old. The oscillation start when I put the volume on loud (8 or more) or when I run my tone stack at 10 with a little bit less volume (6,5),

this is a new amp So I don't have any info about that. All the caps seems great and the rectifier seems to work fine. But if the diodes rectifers were not working fine, the amp should not work at all???

(sorry if my english is weird a little btw)

 

[B-string]

The SS rectifier eliminates a weak tube rectifier. When you say "tone stack" which controls, Bass, Treble, low boost/hi boost switches? I'm still leaning on the 6AN8 as it also has the negative feedback on the pentode section.

 

[alebeaum]

yes for the tone stack... I'm gonna change the 6an8.. Maybe I have a friend with one, So I can check it asap. They are unusual tube, this is the first time I see this tube

 

[B-string]

6AN8 although work pretty well in audio were used more in TV applications. The pentode allows a high gain driver stage and the triode a typical phase inverter. Very few used in audio.

If a different 6AN8 does not solve it, next check the 1Mohm resistor from pin #8 on the tube socket. It should be between 900K ohms and 1.1M ohms.

 

[alebeaum]

The 1Mohm resistor is OK. Still waiting for the tube

 

[vegedge]

you run the treble and bass at 10 both?
may i ask what cab you are using. i know this is unrelated but i also have a sunn 200s and very interested

 

[chadds]

When its working to spec, volume on 8 should give you some serious Grand Funk Railroad/Mountain tone.

 

[alebeaum]

NO i dont use it on 10. This is only test I've made to give more information for the troubleshooting ...

 

[alebeaum]

I use 15'' EV speaker in a vintage Peavey Cab for the moment

 

[alebeaum]

I think i've found the problem..
on the shematic, top left corner. The 680ohm resistor and the 1Kohms resistor. They are connected to the yellow wire on the OT.
Their resistance were much lower

http://www.prowessamplifiers.com/schematics/post/sunn_200s.pdf

 

[B-string]

Those are (along with the cap) the negative feedback loop. If they were lower than 900 ohms and 612 ohms good find.

 

[alebeaum]

::::Though much more accurate, amplifiers with negative feedback can become unstable if not designed correctly, causing them to oscillate.::::


wiki

 

[vegedge]

so what are your amp settings use are using? i also am looking to run my 200s through a peavey 2x15

 

[alebeaum]

My plan (for bass 'cause the 200s seems to sound great on guitar too) , when my 200s will be fully fonctionnal ( it's a new amp I barely tried it, just want to make sure everything is ok before) is to use my other cab, a vintage EV (yes again a EV) 2x15 with upgraded speakers : One 15'' goes to my 200s and the other 15'' will goes to my mid 70's; peavey century 120.
One run dirty (200s probably) and the other: big clean headroom.

My components are ordered, I just have to wait and replace the bad resistors.

 

[alebeaum]

Ok, so I changed the resistors and what I notice is they were ok as soon as I take it out from the circuits, this is probably the transformer who false my reading at the time.
I also changed the 6an8 tube, for a short time it was looking find but, no.. it oscillates again. Any other ideas?

 

[OneWayPunk]

I know you said it had a cap job....but I still suspect the power supply filter caps. The cap job was done over 12 years ago, and depending on what brand of caps used they could already be at the end of their life span. Especially if they used the lower voltage cap can to recap. Voltages in a 200s can be higher than the 525V that most of the replacement cap cans are rated at. There is a 600V cap can available that is a better fit for the 200s.