Bad Distortion with each note on my LiverPull -- SOLVED!!

[dab]

Hi guys,

I have started getting a buzzy distortion behind each note or along with a chord. It starts with the pick attack (kind of a buzz behind the note). It quickly fades away leaving only the clean sound.

I have checked for microphonic tubes, loose hardware, changed out all of the tubes more than once with known good tubes. I turned the lights out to see if I could see any arcing inside the amp and didn't find any. I put a grid stopper resistor between the input jack and the grid on V1. I took the brite switch out of the circuit.

I have also chopsticked the amp for lead dress with no effect.

Any ideas on what to try next? I am considering replacing the tube sockets and also adding shielded wire to the volume pot. Could it be in the NFB circuit?


Thanks for your help!!

dave

[Firestorm]

I have started getting a buzzy distortion behind each note or along with a chord. It starts with the pick attack (kind of a buzz behind the note). It quickly fades away leaving only the clean sound.

Do you mean it definitely wasn't doing it before and now it is? Or do you mean that you've started to notice it? Two different directions in diagnosis.

[klingo]

just in case, is the speaker OK?... or is the amp doing the same with other speakers (guitars...etc...)

[dab]

Thanks for the replies.

I think maybe it was there a tiny bit but not enough to really notice. Now it is very obvious.

I have tried several speaker cabs with the amp and it does it with all of them.

I will be doing some mass parts replacements on Friday (tube sockets, rewire, some caps, etc.) so i was hoping that I could isolate the problem before that.

The thing that I am most curious about is why the distortion is there when I pick the note but fades out much more quickly than the good clean sound. Would that indicate some kind of mechanical issue?

Thanks again...Dave

[Stanz]

Not sure if you have posted all details of what you have tried. Basic troubleshooting process, assume everything is bad until proven otherwise, then narrow it down. I like to work either from both ends inward, or keep eliminating half of what is still suspect until you have it.

Have you eliminated the guitar and cords as suspect?

I had something similar with an AC15 I built which ended up being the OT wires not being twisted (one of my first builds) and being picked up somewhere by nearby wires, a high powered induction signal saturating a smaller signal further back in the chain. It produced a buzzy/fizzy sound that faded as the signal dropped below a certain level. It may not be exactly that, put some sort of induction into the signal chain.

[dab]

Used an audio probe. Signal is clean to the PI. Coming out of the PI it has a lot of gain boost and is distorted. I don't know if that is normal.

You can hear the signal but it is definitely louder and trashier sounding on both Pin 1 and Pin 6. It stays like that until it gets to the power tubes.

Coming out of the power tubes it is even more messed up. this may just be that the power tubes are amplifying the distortion that went into the.

So I am thinking it is in the PI. I have tried different tubes including a Telefunken smooth plate that I took out of my number 1 amp just to see if it was a bad tube. I can't see any arcing in the PI tube socket with the lights out. Don't see any trails on the socket. Don't see any shorted wires. I am going to resolder the PI and make sure its not a cold joint.

Could it be the bias on the PI? Am i driving the PI too hard maybe?

Thanks...Dave

[dab]

Okay I found something and I need you guys to help me understand (I am an electronics dufus).

the coupling cap between V2 and the PI (pin 1 of V2 to Pin 7 on the PI) showed DC on both sides of the cap. I switched it out with a new Sprague 715P series. Retested and things looked good (No DC showing on one side of the cap).

I went back and tested again today and I am again showing DC on both sides of the new cap.

Any ideas what is causing this?

Dave

[gearhead]

Not having DC on both sides is the odd situation. The coupling cap blocks DC, but you get it from the B+4 on one side and one PI triode grid on the other.

[dab]

ah yep. makes sense thanks

[dab]

I am still getting 4.4vdc for cathode voltage on V2. It just has a 10K resistor to ground with no cap (like the schematic shows). I am thinking I need to increase the resistor value to lower the cathode voltage. it should be bout 2.2vdc.

Or, should I be looking for something else that could be causing the cathode voltage to be too high? Plate voltage on V2 is 189vdc.

Dave

[keithrick]

Dave post some pics if you can. Some fresh eyes might be able to catch something you missed.

Good luck.

[gearhead]

Also, highly suggest posting a full rundown of your voltages, starting with wall voltage (measured at the switch, of course). Something like the one I did on this thread, halfway down: http://ampgarage.com/forum/viewtopic.php?t=4811

Gives you and us a much better understanding of the big picture. Plus, and a big one at that, if you make change(s), you can tell if something else is affected by the change(s).

[Firestorm]

I am still getting 4.4vdc for cathode voltage on V2. It just has a 10K resistor to ground with no cap (like the schematic shows). I am thinking I need to increase the resistor value to lower the cathode voltage. it should be bout 2.2vdc.

Or, should I be looking for something else that could be causing the cathode voltage to be too high? Plate voltage on V2 is 189vdc.

If you increase the cathode resistor, your cathode voltage will go up, not down. It looks to me like there's way too much current flowing in this tube. Your plate resistor on V2 is 100K, right? The B+4 node should be around 282V, so if your V2 plate voltage is 189, you've got something like 930 microamps flowing in that tube. That's got to be, like, four times what it should. Tube issue? Short?

EDIT: Well, maybe not that high -- I was looking at Express voltages. What's B+4 in the Liverpool?

[dab]

I am gonna recheck all the voltages and post them. Thanks for the feedback guys.

Dave

[dab]

OK here are the voltages.

V1 160 0 1.58 - - 137 0 1.2 -

V2 207 0 2.2 - - - - - -

V3 191 37 48 - - 191 36 48 -

V4 - 0 8.8 - - - 313 - 291

V5 - 0 8.8 - - - 311 - 291

V6 - 0 8.8 - - - 313 - 291

V7 - 0 8.8 - - - 315 - 294

I am a noob so if I say something really stupid you will know why. Look at V3. I have 37vdc on the Pin2 Grid and I have 36vdc on the Pin 7 Grid. that can't be right. Seems like I have a short right?

I have looked an prodded and checked and resoldered and can't make it go away. Any ideas?

Thanks...Dave