Did you try different power tubes?
My nephew has a 2x12 Hot Rod Deville that was making a buzzy noise on some lower string positions. new power tubes corrected the problem.
Name that buzz
Moderators: pompeiisneaks, Colossal
Re: Name that buzz
OK! Ignore everything I said about a parasitic oscillation in the PI. I got dirty with the scope last night and that's not actually happening. I think it was actually introduced by the probe. Sorry for the false alarm, I'm new to this oscilloscope thing but am getting so into it. Also, I have tried different tubes, thanks tho. It's funny how many times in the past new tubes fixed a problem in an amp that my neurotic past self thought was beyond repair.
Obviously PO could still be an issue (again, I don't know shit but I'm eager to learn), but I think I found something really interesting last night, and I'm hopeful that it's a good lead. I disconnected the NFB to see better what was happening in the power section and discovered that the bottom half of the waveform of a plucked guitar string had its peaks chopped off (no rounded edges or anything, like it was just chopped with hedge trimmers) when scoped at the line out of a dummy load while the top half of the waveform looked OK. The existence of this chopped portion of the wave directly corresponded to the nasty fizzy decay I originally posted in this thread. As the note decayed, the fizz sound disappeared as soon as the amplitude of the wave was lower than that threshold where it got chopped off. Eureka! Man, scopes are so cool. The threshold of the chop was always the same BTW.
So then I started probing around inside the amp. Everything in the pre-amp and PI looked okay - no chopped off waveforms. The grids of the power tubes looked fine too. Both plates of the power tubes, however, exhibited this exact chopping phenomena. I then looked at the screens and discovered that they showed this ugly weird bump only when the fizzy sound was occurring. It looked like a rounded off square wave with a severely unbalanced duty cycle. Like 75% or greater. It disappeared as soon as the fizz stopped. I tried to take pictures but it was already a 2-handed operation at this point and my GF wasn't there to assist. I'll try to enlist her tonight if you guys think she won't get pissed.
I've already tried new 470 ohm screen grid resistors without any change. I should also note that I tried big 100k grid stoppers on the power tubes and nothing changed. Now I'm thinking that this problem has to be something with the filter caps or power supply or a ground, but that's just me guessing. Or maybe I should increase the size of the screen grid resistors? Or increase the filtering in the PS? Thanks for any thoughts you guys might have. I feel like I'm witnessing something intrinsic to the operation of a tube amp and am trying to read as much as possible about what this might mean, but I'm just not putting it all together, yet.
Obviously PO could still be an issue (again, I don't know shit but I'm eager to learn), but I think I found something really interesting last night, and I'm hopeful that it's a good lead. I disconnected the NFB to see better what was happening in the power section and discovered that the bottom half of the waveform of a plucked guitar string had its peaks chopped off (no rounded edges or anything, like it was just chopped with hedge trimmers) when scoped at the line out of a dummy load while the top half of the waveform looked OK. The existence of this chopped portion of the wave directly corresponded to the nasty fizzy decay I originally posted in this thread. As the note decayed, the fizz sound disappeared as soon as the amplitude of the wave was lower than that threshold where it got chopped off. Eureka! Man, scopes are so cool. The threshold of the chop was always the same BTW.
So then I started probing around inside the amp. Everything in the pre-amp and PI looked okay - no chopped off waveforms. The grids of the power tubes looked fine too. Both plates of the power tubes, however, exhibited this exact chopping phenomena. I then looked at the screens and discovered that they showed this ugly weird bump only when the fizzy sound was occurring. It looked like a rounded off square wave with a severely unbalanced duty cycle. Like 75% or greater. It disappeared as soon as the fizz stopped. I tried to take pictures but it was already a 2-handed operation at this point and my GF wasn't there to assist. I'll try to enlist her tonight if you guys think she won't get pissed.
I've already tried new 470 ohm screen grid resistors without any change. I should also note that I tried big 100k grid stoppers on the power tubes and nothing changed. Now I'm thinking that this problem has to be something with the filter caps or power supply or a ground, but that's just me guessing. Or maybe I should increase the size of the screen grid resistors? Or increase the filtering in the PS? Thanks for any thoughts you guys might have. I feel like I'm witnessing something intrinsic to the operation of a tube amp and am trying to read as much as possible about what this might mean, but I'm just not putting it all together, yet.
Re: Name that buzz
Hey I think I figured it out last night. I had previously resoldered all my grounds - or so I thought. Turns out I never touched the ground to the chassis of the cap in the bias supply. Got out he big iron, reflowed it, and presto - the waveform looks OK. Much more balanced. Oh, and it sounds pretty good too 
If anyone's got some insight into why that ground could screw things up the way it did for me I'm all ears. I'm guessing that it was throwing off my bias voltage but only when the amp was cooking and thus throwing the tubes into cutoff. I never noticed the bias change on the bench.
I'm gonna take it down to the studio and record with it tonight, I'm hoping the gremlins don't come back. Thanks for all the suggestions and help everyone. I know it's insane to get emotionally involved with inanimate objects ("he treats objects like women" anyone?) but I am so effin psyched that I may have figured this out. Who's next? /promptly surfs craigslist looking for another rescue....
If anyone's got some insight into why that ground could screw things up the way it did for me I'm all ears. I'm guessing that it was throwing off my bias voltage but only when the amp was cooking and thus throwing the tubes into cutoff. I never noticed the bias change on the bench.I'm gonna take it down to the studio and record with it tonight, I'm hoping the gremlins don't come back. Thanks for all the suggestions and help everyone. I know it's insane to get emotionally involved with inanimate objects ("he treats objects like women" anyone?) but I am so effin psyched that I may have figured this out. Who's next? /promptly surfs craigslist looking for another rescue....
-
Stevem
- Posts: 4576
- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Name that buzz
Yes, now you know how crappy a cold biased amp can sound my friend!
This is why it always pays to have a one ohm 1% 2 watt resistor in series with the output tubes cathode so that you can check there idle current directly with a dc voltmeter.
I must admit your issue would have been the last thing I would have checked for not having the afore mentioned resistors in the amp! it's good you stuck with it?
This is why it always pays to have a one ohm 1% 2 watt resistor in series with the output tubes cathode so that you can check there idle current directly with a dc voltmeter.
I must admit your issue would have been the last thing I would have checked for not having the afore mentioned resistors in the amp! it's good you stuck with it?
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Name that buzz
I did a little recording of the buzz/fizz sound I mentioned earlier in the thread and was a little bummed at how simple the fix was. Didn't even get to power up the scope. It was only happening on the low strings and after I swapped the pre tube and nothing changed, it turned out to be just a bad EH 6V6.
Here's the clip I recorded.
Here's the clip I recorded.
You do not have the required permissions to view the files attached to this post.
-
eniam rognab
- Posts: 763
- Joined: Wed May 15, 2013 4:06 am
Re: Name that buzz
sounds like a crate!Blackburn wrote:I did a little recording of the buzz/fizz sound I mentioned earlier in the thread and was a little bummed at how simple the fix was. Didn't even get to power up the scope. It was only happening on the low strings and after I swapped the pre tube and nothing changed, it turned out to be just a bad EH 6V6.
Here's the clip I recorded.
Re: Name that buzz
You sound like a crate!eniam rognab wrote:sounds like a crate!Blackburn wrote:I did a little recording of the buzz/fizz sound I mentioned earlier in the thread and was a little bummed at how simple the fix was. Didn't even get to power up the scope. It was only happening on the low strings and after I swapped the pre tube and nothing changed, it turned out to be just a bad EH 6V6.
Here's the clip I recorded.
