Hello sunn forum!
I am a first-time builder who became enamored with the Sunn 200S but had difficulty finding one, so I decided to build it! I've been using this forum as a touchstone and it's been very helpful but I'm struggling with a problem now (or maybe a few problems) and perhaps someone here can help. I used this schematic
, almost to the letter but with a few modifications:
- I got rid of the polarity switch and death cap and added an equipment ground for a 3-prong cord
- I used a 6.3v lamp wired up from the PT tap along with the filaments instead of a 120v lamp
- My rectifier is a WZ34 Copper Cap solid-state replacement
- I did (somewhat foolishly) elect to stick with the old filter cap, I know that combo with the WZ34 means I'm pushing the 525v limit but I am using a Variac to dial down the wall voltage to compensate for now
- I did away with that weird output impedance-switching silliness and just hard-wired in a 4 ohm and 8 ohm jack (I will only ever use 1 cab at a time for it anyway)
- Wherever possible I got the exact value capacitors, but some like the 50uF bias caps are hard to find so I just used 47uF caps
- The only other notable departure from the schematic is the 390PF coupling capacitor coming off point E near the OT there. I got lazy and didn't feel like waiting for another mail order, so I popped in an extra 470PF cap I had lying around. I'm not sure if that was ok or not...
I used the Dynaco Mk III-type transformers from Triode, tubes from AES (JJ KT-88s), the filter can is the CE repro, and I used a turret board with a bizarre mix of eyelets and turrets instead of terminal strips, so as a result the interior looks like it was wired by a blind 6 year old.
I would be banished from this forum if I posted a picture. Seriously. It looks really bad. But as far as I can tell after checking and rechecking continuity and probing for shorts and whatnot, it is true to the schematic (well, it might be more accurate to say it's a faithful, if not somewhat creative interpretation
of the schematic). Which brings me to the problem:
For this early phase of the amps life I have it hooked up to a beater 8 ohm 12" speaker that I don't care about. When hit the power switch, lamp comes on, filaments glowing, all seems good. AC voltage checks out, breaker holds, so far so good. When you hit the standby switch there is a little pop through the speaker, followed by a little burp, then the faint hum of a functioning amplifier The amp seems to work fine at low volume, up to about 2. After that though, it doesn't really get any louder, it just distorts, fuzzes out around 5, and I haven't dared to crank it up past that, but it just sort of....stays the same volume, but fuzzier. It's a nice effect to be honest, but that is not what that control is supposed to do. The bass pot works fine, but the treble pot is completely insane. If I keep it in the middle, say, between 3 and 7, it sounds fine (the volume issues notwithstanding). If I turn it past 7 it starts going BANGBANGBANG like a loud repetitive popping sound, accompanied by alarming blue flashes inside the power tubes...this speeds up as you go higher. if the treble pot is around 6 this happens only when there is a loud signal but happens on its own if you turn it past 7. If you turn the treble pot DOWN below 3 you get basically a quieter version of what happens above, it starts as a sputtering hiss (and activity inside tube) and fades into a static hiss as you go lower. If you turn the pot all the way down you get a sound like a vinyl record being very slowly scratched, sort of a ripping, farting sound. Very unpleasant. The two knobs seem to play off each other, the treble pot is more erratic the higher the volume, although I have elected not to fully explore this interaction since I don't want to keep the device on for too long, in case there is potential to damage components (if they aren't already wrecked...).
So that's the Problem
. There are some things I've noticed
in trying to chase down the bug that might be helpful, or might just be red herrings. These symptoms seem consistent with a bad filter cap, but the DC voltages at the different taps seem to be "within spec" (my understanding is that these aren't often exactly as described in the schematic, and can vary depending on the amp's operation), that is, the voltage drop between the different terminals is pretty close to what it should be (within 20 volts of spec?). Same for the voltages at the pins on the tube sockets...not dead on, but mostly pretty close. I know there is some wiggle room but I'm not certain if any of these values have to be Dead on
Also, as I said before, the blue flashes in the power tubes. I don't want to say it is arcing, but that's probably what it is. When you first hit the standby switch the tubes are set awash with this faint blue plasma glow, and they get pretty hot rather quickly (I guess this is normal? KT-88s run hot?). I happen to have an old Heathkit IT-21 tube tester lying around so I plugged all the tubes in and they all checked out fine, which is good because I worried they were toast after the first time I powered on.
Also, and this might be nothing or it might be everything, the filter cap doesn't hold its charge very long. I shut it off and go to check the cap with my meter and it just drains RIGHT OUT. Like, within 10 seconds it is safe. Filter caps are supposed to be charged dangerously for up to an hour I thought? And before you say it's draining through my meter, it does the same thing even when I'm not reading it. 10 seconds after shutdown, no voltage. Could be a bad cap but could also mean a short somewhere?
There is a very real possibility that I have a cold solder somewhere, or just my crummy wiring is crap, and I'm fully expecting a less-than-ideal sound, but I have exhaustively checked and re-checked the current path and the schematic and everything seems in order, so unless one of those little modifications i mentioned is catastrophic, there's no reason the amp shouldn't
work, so I guess that leaves us with components. I'll try to boil this ramble down to a couple specific questions though since it seems like the problem could be anywhere
- Is it possible that the filter cap could have failed, even if the B+ voltages seem to be ok?
- Could the tube(s) have failed, even though it reads good in the tester?
- Has anyone had similar problems before that could point me in the right direction? (I couldn't find anything in searching this forum)
- where should I focus my troubleshooting efforts so I don't do expensive or time-consuming fixes only to find it was a cheap and simple problem (like how should I prioritize replacing components if necessary)?
I've had a lot of fun with this project and this forum has already helped me immensely...I know there's a great amp in there somewhere yearning to be let out...I suppose you could say we seem to be having some...complications in childbirth. Any help is appreciated, even if it is just to scold me for my carelessness!