thanks
motorboating question
Moderators: pompeiisneaks, Colossal
motorboating question
I've been working on a previous build of mine, just tweaking things trying to coax more mojo. It's a p/p 6v6 similar to a Plexi. Anyway, I changed the PI coupling caps from .022u to .047u, and I changed the feedback resistor on my presence control (standard .1u/5kL pot) from 100k to 27k. Now I'm getting a faint motorboating from 0-5 on the presence control. Once it's past about halfway up the motorboating stops and all sounds fine.(in fact, I like the sound of the .047's better than the .022's) What could be the cause of the motorboating -- the change in caps or the change in resistor?
thanks
thanks
-
Darkbluemurder
- Posts: 584
- Joined: Tue Jan 03, 2006 7:28 pm
Re: motorboating question
Motorboating is a low frequency oscillation. The fact that it occurred after you decreased the NFB resistor and only with the presence control set low means that you probably have the primary wires of your OT reversed. Therefore you have positive feedback instead of negative feedback. It just didn't appear with that big 100k feedback resistor. And it doesn't appear with high presence pot settings since a presence pot works by decopuling the feedback circuit.
Therefore try reversing the OT primary wires that go to the plates of the output tubes and see if that helps.
The coupling caps have nothing to do with it. You could use 0.1uF as almost any Fender, Mesa or Marshall bass amps use and be fine. I don't use anything else than 0.1uF but that is just my personal preference.
Good luck!
Stephan
Therefore try reversing the OT primary wires that go to the plates of the output tubes and see if that helps.
The coupling caps have nothing to do with it. You could use 0.1uF as almost any Fender, Mesa or Marshall bass amps use and be fine. I don't use anything else than 0.1uF but that is just my personal preference.
Good luck!
Stephan
-
skyboltone
- Posts: 2287
- Joined: Wed May 10, 2006 7:02 pm
- Location: Sparks, NV, where nowhere looks like home.
Re: motorboating question
Stephen, sometimes you just blow my mind. Good one!!Darkbluemurder wrote:Motorboating is a low frequency oscillation. The fact that it occurred after you decreased the NFB resistor and only with the presence control set low means that you probably have the primary wires of your OT reversed. Therefore you have positive feedback instead of negative feedback. It just didn't appear with that big 100k feedback resistor. And it doesn't appear with high presence pot settings since a presence pot works by decopuling the feedback circuit.
Therefore try reversing the OT primary wires that go to the plates of the output tubes and see if that helps.
The coupling caps have nothing to do with it. You could use 0.1uF as almost any Fender, Mesa or Marshall bass amps use and be fine. I don't use anything else than 0.1uF but that is just my personal preference.
Good luck!
Stephan
The Last of the World's Great Human Beings
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Seek immediate medical attention if you suddenly go either deaf or blind.
If you put the Federal Government in charge of the Sahara Desert, in five years time there would be a shortage of sand.
Re: motorboating question
Stephan may be right, it may be mis-wired for positive feedback.
Which even when it does not outright howl, will often sound strange. (Might be why you want to "coax more mojo".)
Stray tremolo is also a possibility.
My thinking is: you improved the bass response, you increased the NFB, you didn't improve the power supply filtering.... if the amp was marginal before (and many guitar amps are), then you increased the low-frequency forward gain enough to overcome the low-frequency loss from main B+ to driver, and made an oscillator.
> What could be the cause of the motorboating -- the change in caps or the change in resistor?
Perhaps the sum of two changes, not either one alone.
Once you wrap NFB around a system, "everything matters". And matters more when you increase NFB.
Do you really want 27K+5K? That looks like more NFB than most classic tube hi-fi amps.
The effect of the Presence cap is normally far above the motorboating range. It "shouldn't matter", but it does. Maybe the OT is wired backward.
My real question though is: how can you have "faint" motorboating? Most oscillators try to build-up to infinity, and go to maximum power quick. BIG thumps. "Faint" thumps is puzzling. Though maybe something inside the amp is thumping like mad, and not much gets past the OT and speaker.
> I changed the PI coupling caps from .022u to .047u
The grid resistors matter too. You find from 100K to 1Meg. Book-value for 6V6 in fixed-bias is 100K max (often cheated to 220K); for self-bias it is 470K max (1Meg usually gives no trouble). The resistor value is set by bias condition (and how reckless you can be), then you pick a coupling cap. For 100K grid resistors, you need 0.05 or 0.1 for really authoritative/deep bass; 470K couples bass pretty good with 0.02uFd.
Which even when it does not outright howl, will often sound strange. (Might be why you want to "coax more mojo".)
Stray tremolo is also a possibility.
My thinking is: you improved the bass response, you increased the NFB, you didn't improve the power supply filtering.... if the amp was marginal before (and many guitar amps are), then you increased the low-frequency forward gain enough to overcome the low-frequency loss from main B+ to driver, and made an oscillator.
> What could be the cause of the motorboating -- the change in caps or the change in resistor?
Perhaps the sum of two changes, not either one alone.
Once you wrap NFB around a system, "everything matters". And matters more when you increase NFB.
Do you really want 27K+5K? That looks like more NFB than most classic tube hi-fi amps.
The effect of the Presence cap is normally far above the motorboating range. It "shouldn't matter", but it does. Maybe the OT is wired backward.
My real question though is: how can you have "faint" motorboating? Most oscillators try to build-up to infinity, and go to maximum power quick. BIG thumps. "Faint" thumps is puzzling. Though maybe something inside the amp is thumping like mad, and not much gets past the OT and speaker.
> I changed the PI coupling caps from .022u to .047u
The grid resistors matter too. You find from 100K to 1Meg. Book-value for 6V6 in fixed-bias is 100K max (often cheated to 220K); for self-bias it is 470K max (1Meg usually gives no trouble). The resistor value is set by bias condition (and how reckless you can be), then you pick a coupling cap. For 100K grid resistors, you need 0.05 or 0.1 for really authoritative/deep bass; 470K couples bass pretty good with 0.02uFd.
Re: motorboating question
PRR,
I really like your comments! Please keep them coming!
Jelle
I really like your comments! Please keep them coming!
Jelle
Re: motorboating question
sorry, guys, I didn't mean to ask a question and duck out -- I've been out working with the wildfires going on here. There's some good comments/help/suggestions here and they're totally appreciated. I won't be able to take a closer look at this problem until maybe Sunday, but I'll jump back in and let you know what I've found.
thanks again!
thanks again!
Re: motorboating question
> didn't mean to ask a question and duck out
Forums are not IM or Chat. 2 days is not a "duck-out". Maybe you dial-in from the wilderness. Maybe you were busy.
> I've been out working with the wildfires going on here.
I just found out about that. Pretty nasty.
http://www.inciweb.org/incident/816/
Current Situation
Total Personnel - 960
Size - 44,001 acres
Growth Potential - Extreme
Terrain Difficulty - High
Containment Target Prediction at this time is 8 days.
Good luck!
> I'll jump back in and let you know what I've found
Do that. I am curious what's really happening there.
Oh: on another forum, three times now, people have discovered "odd subsonics" or "vibrato" which turned out to be..... a ceiling fan!!! The sound hits the blades, which have speed, so the reflection has a Doppler Effect. Sharp musical ears hear it. That may not agree with your "past about halfway up the motorboating stops" observation.... but something to keep in mind.
Forums are not IM or Chat. 2 days is not a "duck-out". Maybe you dial-in from the wilderness. Maybe you were busy.
> I've been out working with the wildfires going on here.
I just found out about that. Pretty nasty.
http://www.inciweb.org/incident/816/
Current Situation
Total Personnel - 960
Size - 44,001 acres
Growth Potential - Extreme
Terrain Difficulty - High
Containment Target Prediction at this time is 8 days.
Good luck!
> I'll jump back in and let you know what I've found
Do that. I am curious what's really happening there.
Oh: on another forum, three times now, people have discovered "odd subsonics" or "vibrato" which turned out to be..... a ceiling fan!!! The sound hits the blades, which have speed, so the reflection has a Doppler Effect. Sharp musical ears hear it. That may not agree with your "past about halfway up the motorboating stops" observation.... but something to keep in mind.
-
RockinLocken
- Posts: 28
- Joined: Sat Feb 11, 2006 4:04 am
- Location: El Cajon
Thanks Stephan!
Thanks Stephan. The swap OT wires to the output tubes worked on the 5F4 project I've been trouble shooting. The Coupe Deville is now rolling. I can now move on to the next one. Thank you Amp Garage!
Rocket or LP???
Rocket or LP???
You do not have the required permissions to view the files attached to this post.