200S blowing fuses

[Isaac]

Finally got around to looking at my 200S. The fuse blows a few minutes after switching from standby to operate.

Fuse doesn't blow with no tubes. For reference, I have replaced the power supply filter caps with the SDS Labs circuit board (http://www.triodeel.com/dynaco.html) and the rectifier tube with diodes, also on the circuit board. I put one power tube back in. No problem. I removed that tube and put the other in. Fuse blew. Okay! Bad tube, right? Wrong. I put the first tube in the second socket, and the fuse blew again. So, something about that socket or the circuitry associated with that tube, but not the other one. Tricky. Looking at the schematic, I can't see anything obvious that might cause this.

[Isaac]

With no tubes, there were voltage discrepancies. On top of that, I made a dumb technician mistake. Not a bad one. My probe went through a hole in one of the socket terminals. No damage done, but when I did I noticed that the resistor connected to that terminal shorted out to the next terminal. Aha! That explained the voltage discrepancies. That's when I noticed the burnt resistor. Burnt bad. Couldn't tell the value. Back to the schematic.

Result was that the resistor wire was shorting from pin 5, the grid, to pin 4, the screen. That'll draw a bit of current, even through a 100K resistor. Which is what burned out. So, moved the resistor away from the neighboring terminal and replaced the resistor. Replaced the tubes, fired it up. No problem.

Until I started playing through it. Volume was low, and faded in and out. I remembered that I had encountered something similar in the past and that it had been the 6AN8 splitter/driver tube. Poked around until I found another. Replaced that, and voila! Amp is up and working fine.

I'll leave it on for a while to make sure, but everything looks good. I assume that I pushed the resistor over toward the other terminal when I put in the SDS circuit board. Another dumb technician mistake. But one that looks to be fixed.

Woohoo!

[Isaac]

Or not woohoo. Took the amp to rehearsal, turned it on, and the fuse blew.

Back to square one. No tubes in, no blown fuse. One power tube in, good. Both in, good (for an hour or so. More on that in a moment). Added in the 6AN8 splitter/driver tube. Noticed as I put it in that one of the power tubes was hotter than the other. didn't really think anything about it at the time. Noticed that one of the power tubes was glowing red where it shouldn't have been. Seems I've found the problem. I switch the tubes around, and the problem moves. So, bad tube, right? I substitute the power tubes, same result, one tube gets hot. WTF? I change out that tube, and the one I didn't switch out gets hot. This is getting confusing.

Glancing at the schematic, I notice that the bias is specced at -55V. I measured -47V. Too low. Could that cause the power tubes to be running too hot? Hot enough to blow the fuse? I had turned the amp off when I saw the tubes overheating, so I let the tubes cool down, then pulled them. I set the bias to -55V, then put the back tubes in and checked it. It had dropped a bit so I reset it to -55.

So now, all of the tubes are back in, no overheating, no blown fuses so far.

I'm asking anyone who might know: is the difference in bias voltage enough to cause the tubes to overheat and blow the fuse?

[Soundmasterg]

Likely what happened Isaac is that when the resistor was shorting, it caused the tube to draw too much current, which made that tube bad. Since the tubes work in a pair, if one fails it often causes the other to go bad too. Try a new set of tubes in it, and put some 1 ohm resistors between cathode and ground so you can measure the cathode current and see what the bias is directly instead of assuming it is ok because of the grid voltage being what an otherwise inaccurate schematic says it should be. On my old 200S I put tip pin jacks in with the 1 ohm resistors going to those on their way to ground. I also added a tip pin jack with a voltage divider on it to measure the reduced plate voltage. 550 volts would read as 5.5 volts. That way I could measure the bias completely and set it without having to open up the chassis.

Try the other stuff I mentioned in my email. If you can't get it figured out, you're welcome to stop by if you will be in the area. I'll get busy with school again in a couple weeks.

Greg

[Isaac]

Well, the bias may not be set perfectly, but I took the amp to rehearsal last night, and it came through with colors flying. Sounded sweet, too.

[Soundmasterg]

Well, the bias may not be set perfectly, but I took the amp to rehearsal last night, and it came through with colors flying. Sounded sweet, too.

Thats great that it worked Isaac! I'd keep an eye on those tubes though....and whenever you change them, I'd suggest to set the bias properly so you know what the current draw is.

Greg

[Isaac]

I intend to. Money is always tight, though.

[Soundmasterg]

I intend to. Money is always tight, though.

I hear ya...for me also. I've had to sell a lot of stuff to survive over the last couple years...Almost done with school though...hope that will lead to better jobs...

greg

[Isaac]

Good luck to you.

[Isaac]

Took the 200S and the Sentura II to Deep Woods rehearsal today.  Both sounded distinctly better than either the Fender Bassman or Sunn Concert Bass heads I've been using, and the 200S is better than the Sentura II. The Fender has some odd things with its frequency response. The bass instrument channel rolls off on the high end, and the normal channel seems to be lacking some bass. Turning up the bass control helps some, but I think it puts a hump in the response in the midbass. I also think it's distorted, long before it starts clipping. It just doesn't ever sound as clean as the Sunns.

Among the Sunns, the differences are more subtle. The Concert Bass might just sound different because it's solid state, but I'm not convinced that's it. Might be frequency response, might be distortion. I dunno. As I said, it's subtle.

I know that there are some frequency response differences between the Sentura and the 200S. The Sentura has a 6dB step down below a certain frequency ( I don't remember exactly which), which gives it more midrange presence. Good for guitar, but not necessarily for bass. The 200S has a 6dB/octave rolloff at 80 Hz, unless the Low Boost is in. In that case, the rolloff is at 40 Hz, IIRC. Beyond that, the Sentura seems to be lacking a bit on the top end. I shouldn't be able to hear that, as the JBL D140's in the 200S cabinet only go up to 2K, but something was different. the 200S just seemed more, I dunno, sparkly. The Sentura could probably use new tubes, but the 200S preamp and splitter tubes are ancient. Like almost 50 years old. Original tubes from another Sunn head.

Bottom line, I'm really glad to have the 200S back. It's the best sounding bass amp I have. Even if the tubes might be questionable.

[Soundmasterg]

Good luck to you.

Thanks Isaac, to you also!

Took the 200S and the Sentura II to Deep Woods rehearsal today.  Both sounded distinctly better than either the Fender Bassman or Sunn Concert Bass heads I've been using, and the 200S is better than the Sentura II. The Fender has some odd things with its frequency response. The bass instrument channel rolls off on the high end, and the normal channel seems to be lacking some bass. Turning up the bass control helps some, but I think it puts a hump in the response in the midbass. I also think it's distorted, long before it starts clipping. It just doesn't ever sound as clean as the Sunns.

Among the Sunns, the differences are more subtle. The Concert Bass might just sound different because it's solid state, but I'm not convinced that's it. Might be frequency response, might be distortion. I dunno. As I said, it's subtle.

I know that there are some frequency response differences between the Sentura and the 200S. The Sentura has a 6dB step down below a certain frequency ( I don't remember exactly which), which gives it more midrange presence. Good for guitar, but not necessarily for bass. The 200S has a 6dB/octave rolloff at 80 Hz, unless the Low Boost is in. In that case, the rolloff is at 40 Hz, IIRC. Beyond that, the Sentura seems to be lacking a bit on the top end. I shouldn't be able to hear that, as the JBL D140's in the 200S cabinet only go up to 2K, but something was different. the 200S just seemed more, I dunno, sparkly. The Sentura could probably use new tubes, but the 200S preamp and splitter tubes are ancient. Like almost 50 years old. Original tubes from another Sunn head.

Bottom line, I'm really glad to have the 200S back. It's the best sounding bass amp I have. Even if the tubes might be questionable.

I'm glad you apparently have it back working well. I ended up selling my Sonic 1/200S head to a guy in Canada a couple years ago and only have my 2000S head and cabinet these days...and the head needs a rebuild. The 200S is a great sound though....some day I might get another to get a lower volume sweet spot...but we'll see. Enjoy!

greg