Sunn 200s negative feedback smoked

[Sowhat]

I am trying something new and that's to repair a Sunn 200s remotely using email and chat with the owner. I have got past the first stage of things and identified the damaged components which are all in the negative feedback loop or node "E" as outlined on the schematic. There's a 1K/1W 750pf cap bundle which is burnt and a 47 ohm resistor between the node and ground which is burnt to a crisp. Some nasty current flowed through that node to fry those parts. All the tubes seem to be O.K. as is the output transformer although dynamic tests have not been done yet. My big question is probable cause. The owner states that it was running as usual just fine when all of a sudden smoke trickled out and the fuse blew. What possible failure(s) could have happened to cause such a surge in the negative feedback loop? I had one suggestion that the 6AN8 shorted out but I have not had the owner do a short test on that tube yet. I'm just looking for likely suspects at this point. It's getting all new tubes anyway.

[loudthud]

This is a puzzler. There is a small cap (39pF IIRC) from one of the 6550 screen grids to the feedback node. Did you check that? It's mounted on one of the 6550 tube sockets. There is a 680 ohm 1 watt to ground from the 16 ohm tap. Did that smoke?

[Sowhat]

This is a puzzler. There is a small cap (39pF IIRC) from one of the 6550 screen grids to the feedback node. Did you check that? It's mounted on one of the 6550 tube sockets. There is a 680 ohm 1 watt to ground from the 16 ohm tap. Did that smoke?

Yeah, that 39pf cap did raise my interest, seems the only thing that potentially can put screen grid voltage on the feedback loop. funny they call it a 390uuf which should be a .39nf which would be a 390pf cap, right? or am I figuring that wrong. I need to get the owner to check that one out first thing. The 680 is good as gold although the 1K-1W paralleled by the 750pf is toast . I'll keep you posted.

[loudthud]

You are right, it is 390pF. I was just going from memory. Also, it's not unknow for the tube sockets to breakdown. But it's usually between pins 2 and 3.

[Rhodesplyr]

This is, as loudthud says, a puzzler. BTW, this amp is basically a Dynaco MkIII with an extra gain stage, so you can get lots of help from Hi-Fi guys, too. The 390pF capacitor failing would be the most reasonable way to get excessive voltage into that part of the circuit, but then you'd probably also see damage to the 47 Ohm 6AN8 pentode section cathode resistor. The 390pF cap was usually 1,000V mica in the Dynaco amps; it's there to improve the balance of the split-load phase inverter.

And I just re-read your post and saw that there WAS damage to the 47 Ohm resistor. If it was a 6AN8 short, the 680 Ohm 6AN8 cathode resistor would probably also be damaged, so I'd bet on the 390pF capacitor. These caps are under a lot of voltage stress with the static plate voltage of the output tubes + the plate swing.

[Sowhat]

This is, as loudthud says, a puzzler. BTW, this amp is basically a Dynaco MkIII with an extra gain stage, so you can get lots of help from Hi-Fi guys, too. The 390pF capacitor failing would be the most reasonable way to get excessive voltage into that part of the circuit, but then you'd probably also see damage to the 47 Ohm 6AN8 pentode section cathode resistor. The 390pF cap was usually 1,000V mica in the Dynaco amps; it's there to improve the balance of the split-load phase inverter.

And I just re-read your post and saw that there WAS damage to the 47 Ohm resistor. If it was a 6AN8 short, the 680 Ohm 6AN8 cathode resistor would probably also be damaged, so I'd bet on the 390pF capacitor. These caps are under a lot of voltage stress with the static plate voltage of the output tubes + the plate swing.

Yeah, the owner wrote me today and said his little parts order came in. In there I put a nice 390pf 1,000V mica capacitor because I'm pretty sure (even without testing it) that thing shorted. Of course, once I get the owner to start checking parts per my instructions I should have a good picture of what's up. That can be a little like herding cats sometimes.

BTW... would that be coupled to the screen grid because that's where it's connected... maybe just a little less than the plate voltage and swing then?

[Rhodesplyr]

BTW... would that be coupled to the screen grid because that's where it's connected... maybe just a little less than the plate voltage and swing then?

My mistake at the end of a long day. Yes, it is connected to the screen grid. However, this is an ultralinear output stage, so the screens do swing a bit.

[Sowhat]

Thanks all for your input on this problem. The owner finally got around to testing the 390pf/1000v cap and it was as short as Napoleon! That was the cause of the burning stuff. I had him replace the 390pf cap, the 47 ohm resistor and the 1K-1W/750pf bundle and all is well in Sunn land now. As a side note, he took it to a local repair shop in Chicago and they quoted him $556.00 for the repair plus new tubes and other nonsense with an outrageous story about what was wrong with the amp. Naturally he yanked the amp back unfixed but not before suffering a $40.00 bench charge. What a ripoff! Our little repair cost him $7.00 in parts and he now has the pride of doing it himself (with a little help from me that is). Wow! there's a lot of con artists out there. I just charged him a minimum bench charge for this even though it was his bench he was more than happy with that.

[52 Bill]

Wow! there's a lot of con artists out there. I just charged him a minimum bench charge for this even though it was his bench he was more than happy with that.

Lucky the owner found you. Maybe you should join in on the other thread regarding online tech help and give us your thoughts on how well it works without being there.