Help with DIY 200S Build

[SUNNDAVE]

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!

Thanks,
    Dave

[Soundmasterg]

Hi Dave,

It sounds like there are a lot of possible things going on there that could be causing the problems. We need to see pictures and an exact schematic of what you have in order to troubleshoot adequately. Don't worry about how it looks and what not. You need to learn how to do this correctly and learning from your own build is the best way. So post up some good quality pics and let people help you learn. What I would suggest is to post this at the Hoffman forum and the main amp forum for Ampage. You will get a lot more traffic at those places and someone will likely be able to help you troubleshoot it. We helped a guy recently at the Hoffman bbs with a Vox AC100 build that had a lot of issues, and it took weeks but he finally got it sorted. Links are below.

http://el34world.com/Forum/index.php?board=13.0    Hoffman forum

http://music-electronics-forum.com/f10/  pick which forum you want the post in

It sounds like you have the wiring incorrect on the pots in your amp...the bangbang noise sounds like it might be motorboating, but it could be some other type of oscillation also. Those are usually due to layout or wire routing issues, but could be poor solder joints causing an incomplete circuit too.  You should also make sure you have a good grounding method as poor grounding can contribute to some of the problems you are experiencing. You said the voltages are close to the schematic on your amp, so likely you have correctly operating caps in there, and that part of the power supply checks out. The caps can drain quickly if they are wired such that a resistor or a load will drain the voltage down. The tubes are a load so with the tubes in there and the standby switch in play mode, most amps will drain down pretty quickly.

your troubleshooting efforts should be focused on localizing where the problems are and dealing with them one by one. With many amps you can pull the phase inverter tube and see if the problem still happens or not and that will tell you if the problem is before or after the tube, but in your case the problems seem to be volume and/or tone setting driven to some extent so that will be before the phase inverter. The bangbang issue could be after the phase inverter, in the power supply, or even before the phase inverter and could be caused by whatever wiring issues are happening in your amp, so it is hard to narrow down that way. Do you have a scope? A signal generator? This is where they become very handy as you can put a signal into the amp with the amp running into a resistor dummy load so you don't have to listen to loud noises as you troubleshoot, and you can follow the signal from beginning of amp to end of amp and localize the trouble pretty quickly. Without those it is hard to narrow down where the problems might be happening. Where are you located? Someone with the equipment or experience might be able to help you sort it out.

Anyway as I said, post this at some of these other forums as I mentioned instead of this Sunn specific one here that doesn't get much traffic comparatively speaking and you will likely get lots of help....but make sure to post pics and a schematic also because they will ask for them.

Greg

[Soundmasterg]

I forgot to mention to make sure the bias on the power tubes is correct. That is the most important thing when you bring a new amp up with power and have the tubes in it. The KT88 is a 42 watt dissipation tube, and 70% of that is 29.4 watts, and 60% of that is 25.2 watts, so your bias current should be in that range somewhere. In order to check the bias current you need a probe like the Weber Bias Rite, (Then you can measure with the Bias Rite meter) or put 1 ohm resistors between pin 8 and ground on each tube. If you do it this way then you can measure the DC voltage across the resistor with your DMM, and that will tell you the current through the resistor since the resistor is 1 ohm and the voltage equals the current at 1 ohm. Then you multiply that times the plate voltage to get the cathode current.

For example: say the plate voltage is 560 volts, and the reading on your meter is 45 volts. You take 560 volts times .045 mA which gives you 25.2 watts dissipation, so this would be the minimum 60% value which is usually good for fixed bias amps. You could turn the bias up hotter so it gets more into that 60%-70 % range if you want, though when you do this, the plate voltage will drop. So lets say you measure 555 volts on the B+ and 53 volts on the cathode, so do the math and you end up with 555 volts times .053 mA, which equals 29.415 watts. So anywhere in that range would be considered safe but bass amps tend to do well with colder bias, and colder biased tubes will also last longer, so if it sounds fine biased colder, then do it.

Greg

[SUNNDAVE]

Greg,

Thanks for the tip, I'll try those other forums as well.  I will try to get some pictures posted too.  I do have a scope and tone generator so i should be able to get that hooked up as well.  I'll try to getback with some pictures,  thanks fr the help!
    -Dave