Yamaha T100C Ghost notes.

[Dave]

Hey fellas,


I'm working on a Yamaha T100C, its basically a Soldano SLO knock off. There is some ghosting going on that I have been trying to eliminate, but I'm having no luck.
I find that the ghosting first become visible on my scope at the output of the V2b which is a cold clipper. It is not present at the grid of V2b but it is present on down the chain after the coupling cap from V2b.
So, the problem, as I see it is happening inside the cold clipping stage. For that stage there is a 20uF filter, 100k plate load bypassed by .001 cap, and a 39k cathode resistor.


I'm posting a picture of my scope screen so you can see the two notes. At the particular frequency that I have my signal generator set to, the extra note is very clear on the scope screen. At other freqs, it oscillates wildly. I just need it gone.


I have changed power supply filter caps, preamp filters, chopsticked everything, changed the tubes, further isolated the two channels than that which is stock. Can't find it. I'd appreciate if someone would give me some ideas.


Oh, just as a side note, in case it has any value... When you turn the power off to the amplifier, the second note disappears while the amp is still running on the juice stored in the filter caps.

[sluckey]

How are you triggering the scope?

[Dave]

I'm not a scope expert... I just fiddle with it until it shows what I hear. But no matter how much fiddling I do, I don't see the second note before the V2b.


I just checked it and I have it set to autotrigger.


Dave

[sluckey]

Try this... Put a Tee adapter on the sig gen output jack. Connect one cable to the input of the amp. Connect the other cable to the EXT TRIG jack on the scope. (Alternately, if your sig gen has a trigger out, connect that to the scope EXT TRIG.) Set the scope to use external trigger. Connect the probe as usual. Do you still have a double trace?

The point of the above setup is to eliminate scope triggering as a problem. You are reliably triggering the scope on a known clean signal rather than allowing the scope to auto trigger on whatever is connected to the vertical input.

Not saying you don't really have a ghosting issue, but I have seen a very similar display when using internal triggering and the display would free run when changing frequencies.

[dwinstonwood]

Are you saying that the .001 cap is a plate load resistor bypass cap?

Just a wild shot in the dark, but I recently read in the Valve Wizard's preamp book that anode bypass caps should not connect to B+ (which is also "AC ground"), but to the same ground as the cathode resistor. Merlin warns against grounding it to B+ as it then couples any B+ noise to the anode. I don't know how your anode bypass is AC grounded, but I thought I'd throw this out there.

[Dave]

Sluckey, I'll have to do some rigging in order to do what you have suggested since I don't have a T-splitter. But I'll give it a go. In the mean time, I really don't think its necessary because the extra note that you see on the scope oscillates very slowly up and down in amplitude. You can totally hear it through the amp speaker. Another note, not the frequency generated by the generator, blooms and dissipates at about one cycle every 5 seconds. You can see it on the scope and you can hear it through the speaker. If you disconnect the sig gen, and use a guitar, its there too just much more annoying.


Dave

[Dave]

dwinstonwood, I lifted one leg of that cap to see if there would be any change and there was none.


Dave

[Dave]

Here's what it looks like when I pluck the high E-string on the guitar. Again I get a slowly oscillating, dissonant, extra note that is lower than the note I am playing. As it oscillates audibly, you can see it on the scope screen.


Dave

[Dave]

I did some poking around this morning and I found something that was wrong. The negative feedback resistor, according to the schematic, is supposed to be 39k. The one that is in the amp is a 3.9k. Not sure that this will solve the issue, but since I couldn't find any 39k resistors in my stuff, I ordered some. Anybody want to comment on whether it will make any difference or not?


Dave

[shooter]

remove the NFB and see if the ghost note goes away

[Dave]

Just did and it didn't change the ghost note at all.


Dave

[Dave]

Just found a .001 ceramic disk capacitor that makes it's own acoustic buzzing sound that changes frequencies as the signal generator freqs change. It is a capacitor that is connected to the high voltage connection of the reverb driver tube. I wonder if this acoustic vibration could somehow be forcing its way back upstream to be seen in the V2a? Anyway, I'll pull that out and replace it too. It won't shut up by touching it with a chop stick, but it does change its timbre quite a bit. Anyone else use real chopsticks in amps... Mine are from the buffet place down the road.

Update... That wasn't it.

Dave

[Dave]

Well, I have tried everything I could think to try. Changed the NFB resistor, changed tubes, filter caps, added a small snubber to the PI,further seperation of the channels, replaced some components that were questionable, reflowed some solder joints. I have managed to quiet the amp down a lot... tamed it down a little so that it doesn't right on the edge of blowing up all the time. Actually got it sounding pretty good if you ask me, but there are still ghost notes. They are very slight now and I probably only notice them because I am listening for them, but they are still there. I guess I'm going to call it good and button it back up and call it done. I don't use screaming overdrive very much anyway. Thanks guys...


Dave

[dwinstonwood]

Another wild shot in the dark...

This happens with every different guitar that you play through the amp?

[Dave]

Yeah... and the signal generator too.


Dave

[shooter]

did you ever swayp V2?

[Dave]

So, I had given up on this issue, but I still hadn't put the amp back together and I thought I'd try one more thing. On a long shot, I thought that maybe there was a ground loop or some lead dress issue that might be causing this, so I physically relocated the dropping resistors and filters for the V1 and V2. Although the ghosting did not go away, it dropped in intensity by maybe 75%. I'm feeling pretty good about that. Anyways, I thought you guys might glean some useful information from that.


Shooter... I replaced the V2 a half a dozen times.



Dave

[marshallguy]

So, I had given up on this issue, but I still hadn't put the amp back together and I thought I'd try one more thing. On a long shot, I thought that maybe there was a ground loop or some lead dress issue that might be causing this, so I physically relocated the dropping resistors and filters for the V1 and V2. Although the ghosting did not go away, it dropped in intensity by maybe 75%. I'm feeling pretty good about that. Anyways, I thought you guys might glean some useful information from that.

Shooter... I replaced the V2 a half a dozen times.

Dave

You could try putting a .1uf cap across the B+ filter cap for that node and see if it eliminates the  problem. Filter caps do provide AC ground but a good film cap of sufficient voltage rating helps clean up and garbage floating on the B+. Can you post a schematic of your build showing what tubes are on what nodes, etc?

fixed quote... sluckey