Problem with 6L6 Monoblock

[sorenj07]

I finally finished one of my 6L6 monoblocks. Aluminum was tough to polish, but at last I was able to wire the things. The other one is still sitting on the bench, not quite done.

First, some photos. Then, the bad news





Here's my problem. The amp started up fine, fed by a discman into a decent speaker. Upon playing the music, though, this horrible distorted bass rumble started blaring. I shut down the amp and it "killed" the home-theater voltage regulator thing it was plugged into, resetting the TV, cable box, etc. Surprisingly, the fuse didn't blow.

The amp, ignoring the noise (which I suspect is motorboating), sounded pretty good otherwise. The cymbal hits on Miles Davis' Nefertiti album came through pretty decently.

Back at my bench, I put on some gloves, tipped the amp onto its side, and measured some voltages. Or tried to. Voltage on the first motor run cap, the 100uF main B+ reservoir, started at 450V and rapidly rose to above my crappy tester's 500V limit. I then stupidly blew up said DVM when I put the leads from one side of B+ AC to the otherwords, probably around 1000V. Damn.

I'd had the bias pots at around half way. Without being able to measure tube current, I turned the 20-turn 25K trimpots a few times in one direction, then a few times in another, without getting much difference in sound.

One last thing. Turning the amp on and off back in my workshop didn't blow any fuses. I left the CD player off, but still plugged in, and turned on the amp. It was quiet, just the faintest bit of hiss and B+ ripple noise. I tapped the side of the chassis firmly (as it was vertical). The rumbling hum noise started up again, building pretty quickly to a steady loudness. What the hell? I made sure that not too much stuff was dangling around, even super-gluing a stiff piece of wire to the top capacitor to keep it from vibrating.



My questions - if it is motorboating, is there anything I can do? I made sure not to mess up the wiring of the OPT, but those long runs of DC between those last 22.5uF motor run caps and the input stages might be a problem. My main issue though is that I've never had this problem of crazy noise, and don't know what could be causing it. Someone suggested that tapping the bias voltage off the B+ could lead to a feedback loop, could this be it?

[Sal Brisindi]

Soren,
Sorry I don't have an answer for you but great job on the amplifiers. The mono blocks look awesome! Where did you get the aluminum chassis and how did you punch the holes in it? Looks professional.

Regards,
Sal Brisindi

[the_manta]

Hi !
You need definitly some negative feedback there !
Here are two quickly scribbled schematics:



and



choose one or try both...

Hope it helps.
Regards, Simon

[danFrank]

I doubt that it's motorboating because you don't have any feedback loop; and you also say that it is quiet when your cheapie CD player is not hooked up to it.
Chances are that the output of the CD player has a DC component on it and your amp does not like that. Insert a coupling capacitor between the CD's output and your amps input. That will probably solve the problem. Otherwise use a different source that has a true line out (you will probably need a volume control then).
Hope this helps...
Good luck, Daniel

[danFrank]

One more thing... I didn't read your entire post the first time...
Try the tapping test again with the amp's input shorted to ground. See if you can get the hum thing started again with the input shorted to ground. If the hum starts up again, it's probably a bad / microphonic tube, or one with bad cathode to heater insulation.
If the hum doesn't start up with the input shorted, then it's most likely what I said in my original post.
Motorboating is a feedback problem, and by your schematic, it looks like the amp currently has no feedback.
Daniel

[anatech]

Hi sorenj07,
A quick look at your wiring and description of your fault tells me you are indeed suffering from motorboating. The culprit is the way you ran the grounds. Look at your schematic. The circuit ground should be near the local filter cap. Imagine each ground connection as a resistance and the problem should become clear to you.

You blew the fuse on your supply for your home theater. By chance was that a computer type UPS? If so, you can't use it for audio gear.

-Chris

[sorenj07]

Originally posted by Sal Brisindi
Soren,
Sorry I don't have an answer for you but great job on the amplifiers. The mono blocks look awesome! Where did you get the aluminum chassis and how did you punch the holes in it? Looks professional.

Regards,
Sal Brisindi

Thanks! I got the chassis made up for me at a place down in Dorchester, I forget the name but they use an industrial ultra-high-pressure water jet with sand in it to cut through even the thickest stuff. I gave a plan to the guy outlining just the basic stuff - all the bigger cuts that'd be hard to make on .1" material. After that, I just followed my plan and drilled the screwholes etc.

Originally posted by the_manta
Here are two quickly scribbled schematics

Hi, thanks a lot for the schematics. Why would adding feedback be so necessary though? I suspect that I have around the right values for capacitors that i need, but shouldn't I try getting the amp making decent music first before slapping some NFB in there? Also, what's the electrical difference between the two versions?

Originally posted by danFrank
Try the tapping test again with the amp's input shorted to ground. See if you can get the hum thing started again with the input shorted to ground. If the hum starts up again, it's probably a bad / microphonic tube, or one with bad cathode to heater insulation.
If the hum doesn't start up with the input shorted, then it's most likely what I said in my original post.
Motorboating is a feedback problem, and by your schematic, it looks like the amp currently has no feedback.
Daniel

Well, it doesn't hum until I tap it with the ground shorted. This makes sense because I'm using some old 6SN7's along with new - these 6SN7's date back to the very very first linestage I ever built and scrapped once I realized things like keeping heater wiring separate, how to build a good PSU, and how not to put PT and choke right next to each other in line Should I just try different 6SN7's? I recall tipping one of my 6L6WXT+'s over onto a table and it making a bit more rattling but I suspect that it's OK.

Originally posted by anatech
Hi sorenj07,
A quick look at your wiring and description of your fault tells me you are indeed suffering from motorboating. The culprit is the way you ran the grounds. Look at your schematic. The circuit ground should be near the local filter cap. Imagine each ground connection as a resistance and the problem should become clear to you.

You blew the fuse on your supply for your home theater. By chance was that a computer type UPS? If so, you can't use it for audio gear.
-Chris

I don't know whether it was a computer type, but I suspect so.

As for motorboating, I just recently read somewhere that it only is possible if there is a negative feedback connection - what is your take on this? The grounding job can probably use some cleaning up, but unfortunately I don't quite see the solution to this problem - would running a copper connector from the main B+ cap (closer to the PT) up to near the middle of the signal bar be better? could I try lifting signal ground with a 10 ohm 1/2w resistor?

[anatech]

Hi sorenj07,

As for motorboating, I just recently read somewhere that it only is possible if there is a negative feedback connection - what is your take on this?

Well, not a negative feedback connection. By definition, it's positive feedback (also when negative feedback goes wrong and becomes positive). But, current flowing through resistive connections (wire is not 0 ohms) creates a small voltage. Current increases making these bigger in amplitude. That is what I meant by looking at your layout. If you drew it and replaced the positive supply wires with small value resistors and the same for your ground traces, I think you may see the answer.

Don't assume wire resistance is negligible. "When is a ground, not a ground? in other words. It's a hard lession to learn sometimes.

Does that help?

Also, I should have said this earlier. Nice Job!

-Chris

Edit: I should have stated that you created a feedbak loop via the supply, which includes your ground.

[Sherman]

Motorboating can be caused by not decoupling the B+ between stages. I had a motorboating issue with a SE amp I built with no feedback loop at all. Decoupling the B+ to the input stage fixed it immediately.

[Johan Potgieter]

Sorenj07,

You do not have a high gain circuit, so motorboating is also rather funny to me. The usual weird first question, with respect, in detective "where were you at the time of the murder" style. You are sure of all the connections, etc.?

After that I would support Chris. From what I see (I did not enlarge) your earth (common) goes from your loudspeaker terminals picking up everything on the way to the input socket and then with a length of wire to the power supply caps. The first thing I would try is common of output and 6L6s directly to the reservoir caps' negative. All the other commons (drivers last) with a separate wire to the same reservoir caps' negative point. You could also earth the chassis there.

Forget about mechanically fixing caps etc. at this stage. There is neither your problem nor microphony there (urban legends exist thataway). Then also as others suggested a bad tube, though I doubt that; still. (Hope we are not all overlooking something silly.)

Good luck!