The past week I have been putting the finishing touches on a couple modded guitar amps I just built.
After playing both of the fairly heavily, I have found a nasty whistle In a certain volume/treble/presence setting.
When I have the volume 1 or 2 at maximum setting, Master set at 30%,,Increase treble to max, Then Raise the presence control to about 90% or more I get a high frequency whistle. Not overwhelming in volume, But bad enough to address.
Both amps do this.
New Revision
The frequency is about the same as an old time police whistle.
Any thoughts or insight may be very helpful.
Gene
Oh, another bit of info,
If I put a 12AU7 in the phase inverter location, I am able to get rid of the whistle, But Volume is reduced significantly which is not a good thing.
After playing both of the fairly heavily, I have found a nasty whistle In a certain volume/treble/presence setting.
When I have the volume 1 or 2 at maximum setting, Master set at 30%,,Increase treble to max, Then Raise the presence control to about 90% or more I get a high frequency whistle. Not overwhelming in volume, But bad enough to address.
Both amps do this.
New Revision
The frequency is about the same as an old time police whistle.
Any thoughts or insight may be very helpful.
Gene
Oh, another bit of info,
If I put a 12AU7 in the phase inverter location, I am able to get rid of the whistle, But Volume is reduced significantly which is not a good thing.
Hi Gene,
This is a band-aid, but guitar amps ahve so much HF gain in their preamps that this stuff is typical.
I cant see a value for you presence pot... whatever the value is. take about 10-20% of that and place a resistor in series with the 0.1 presence cap... this will just limit the affect the presence pot can exert.
Otherwise, look for an output wire (plate) or component to close to to an input (grid) wire...
You got a scope yet bud??? If so, check and make sure you don't have some high frequency oscillation happening... you could be hearing a sub harmonic of a much larger oscillation above the range of hearing.
This is a band-aid, but guitar amps ahve so much HF gain in their preamps that this stuff is typical.
I cant see a value for you presence pot... whatever the value is. take about 10-20% of that and place a resistor in series with the 0.1 presence cap... this will just limit the affect the presence pot can exert.
Otherwise, look for an output wire (plate) or component to close to to an input (grid) wire...
You got a scope yet bud??? If so, check and make sure you don't have some high frequency oscillation happening... you could be hearing a sub harmonic of a much larger oscillation above the range of hearing.
The Presence pot is a 25K,
Interestingly, Its effect is only noticeable at the 70-100% settings, anything lower than that shows no audible difference.
The original schematic (bandmaster) showed a 5K pot there which might explain the lack of linearity.
I ran a set of Mullards in the prestages, 2 12AX7, & 1 12AU7 in the phase inverter and the problem is not showing up, or lets say, Not in human hearing range.
Another thing I noticed is the frequency can be altered by proximity to the second 12AX7. If you move your finger near or between the tubes it changes slightly.
I added tube shields but it seemed to make the problem worse.
Now, this whistle is not very loud, If the amp is pushing say 20W,
its about like a 1/4w by comparison.
Nope, Still no scope, or lets say one that I know works. I have an old old BK but no manual nor the ability to use it. It turns on, has lots of knobs and things LOL
I been watching Fleabay for a manual but no luck yet.
Gene
Interestingly, Its effect is only noticeable at the 70-100% settings, anything lower than that shows no audible difference.
The original schematic (bandmaster) showed a 5K pot there which might explain the lack of linearity.
I ran a set of Mullards in the prestages, 2 12AX7, & 1 12AU7 in the phase inverter and the problem is not showing up, or lets say, Not in human hearing range.
Another thing I noticed is the frequency can be altered by proximity to the second 12AX7. If you move your finger near or between the tubes it changes slightly.
I added tube shields but it seemed to make the problem worse.
Now, this whistle is not very loud, If the amp is pushing say 20W,
its about like a 1/4w by comparison.
Nope, Still no scope, or lets say one that I know works. I have an old old BK but no manual nor the ability to use it. It turns on, has lots of knobs and things LOL
I been watching Fleabay for a manual but no luck yet.
Gene
OK,
That pot value is your problem... you are probably sending your total gain up to high.
If your finger can change the sound it is definately a capacitive feedback type oscillation probably from components under the chassis. Keep in mind this problem will cure itself with the right pot probably.
Take the pot out of the circuit (temporarily) and just put a 4.7K... this will test the theory.
Gene!!! manual!!! we don't need no stinking manual!
Take a sweet photo of the front of your scope and we'll teach you how to use it! It ain't so hard. Or tell us the model number of the scope and someone will come up with manual.
That pot value is your problem... you are probably sending your total gain up to high.
If your finger can change the sound it is definately a capacitive feedback type oscillation probably from components under the chassis. Keep in mind this problem will cure itself with the right pot probably.
Take the pot out of the circuit (temporarily) and just put a 4.7K... this will test the theory.
Gene!!! manual!!! we don't need no stinking manual!
Take a sweet photo of the front of your scope and we'll teach you how to use it! It ain't so hard. Or tell us the model number of the scope and someone will come up with manual.
Oh My,
I have a question about bypass caps.
Traditionally, its common prcatice in guitar amp that the bypass cap is increased to get my overdrive type effect.
After playing this morning with the splitting of the cathodes and the first pre-stage, ( I was trying to get 1 input cleaner & 1 input normal or slightly more overdriven) Anyway, I decided to increase the bypass cap from 33uf/50V to 47uf/50V. Not a huge change but very interesting results.
#1-I clearly get more overdrive type sound.
#2- I am now able to run the 3rd 12AX7 in the phase inverter
#3- The Third 12AX7 increased the overdrive sound another say 20%
#4 The Whistle is gone!
Can possible low quality bypass caps be the source of this symptom? Given my history of this problem in other amps, Could It actually be just a 33 cent part ?
I have a champ here we could never cure to date and have also had trouble with a couple other amps I built squeeling or whistleing. Could it be?
Maybe wishful thinking here but its very possibly the commonality of the symptom and the use of those caps?
I had already switched brands of signal caps and everything else.
The Scope.
I will dust her off later tonite and get a picture ect. Its pretty old, but does power up and make squiggly lines in the tube! LOL
I do have a signal generater w/manual, An Eico model 324.
Gene
I have a question about bypass caps.
Traditionally, its common prcatice in guitar amp that the bypass cap is increased to get my overdrive type effect.
After playing this morning with the splitting of the cathodes and the first pre-stage, ( I was trying to get 1 input cleaner & 1 input normal or slightly more overdriven) Anyway, I decided to increase the bypass cap from 33uf/50V to 47uf/50V. Not a huge change but very interesting results.
#1-I clearly get more overdrive type sound.
#2- I am now able to run the 3rd 12AX7 in the phase inverter
#3- The Third 12AX7 increased the overdrive sound another say 20%
#4 The Whistle is gone!
Can possible low quality bypass caps be the source of this symptom? Given my history of this problem in other amps, Could It actually be just a 33 cent part ?
I have a champ here we could never cure to date and have also had trouble with a couple other amps I built squeeling or whistleing. Could it be?
Maybe wishful thinking here but its very possibly the commonality of the symptom and the use of those caps?
I had already switched brands of signal caps and everything else.
The Scope.
I will dust her off later tonite and get a picture ect. Its pretty old, but does power up and make squiggly lines in the tube! LOL
I do have a signal generater w/manual, An Eico model 324.
Gene
Hi Gene,
I agree with poobah that you are doing impressive work - blind! Once you get used to using an oscilloscope you will wonder how you got along without one.
There are some good tutorials on the net on 'scope use. Try to google them up. You can get a long way just by "playing" around. One thing to watch is the volt / div setting working on tube gear. The initial carging of the AC coupling cap can destroy the input circuit. Start high, switch to a lower setting, then high again. Touching ground will discarge the input cap with the same result as connecting to B+. So be aware of that. I already killed one and it cost $$$ to repair.
To answer your question about parts, yes. It's amazing how a single small part can really ball up the works. Your original cap may be defective in some way. There does not seem to be anything wrong with the type or make. One never knows.
-Chris
I agree with poobah that you are doing impressive work - blind! Once you get used to using an oscilloscope you will wonder how you got along without one.
There are some good tutorials on the net on 'scope use. Try to google them up. You can get a long way just by "playing" around. One thing to watch is the volt / div setting working on tube gear. The initial carging of the AC coupling cap can destroy the input circuit. Start high, switch to a lower setting, then high again. Touching ground will discarge the input cap with the same result as connecting to B+. So be aware of that. I already killed one and it cost $$$ to repair.
To answer your question about parts, yes. It's amazing how a single small part can really ball up the works. Your original cap may be defective in some way. There does not seem to be anything wrong with the type or make. One never knows.
-Chris
Hey Trout,
Tubelab ought to stroll along pretty soon... I think when you split your cathodes, you lowered you gain and therefore your oscillation. Or just moving a few things around underneath might have fixed things. Increasing the cathode cap increases your gain... at least at higher frequncies, so it not likely to be a majic cure.
Have patience, guitar amps have so d$mn much gain in as few tubes as possible that you're just beggin for oscillation. I knew an ole Fender engineer; told me you could move some resistors a 1/4" inch and make an amp sqeal or shutup... REALLY.
Are your caps new (ish). The reason I ask is because electrolytics go bad with age... especially if they're are not in a circuit being used For example, you wouldn't want to use (or buy) a new ten year old cap. Oh and by the way... you ARE measuring all those carbon comps before you stick 'em right?
Tubelab ought to stroll along pretty soon... I think when you split your cathodes, you lowered you gain and therefore your oscillation. Or just moving a few things around underneath might have fixed things. Increasing the cathode cap increases your gain... at least at higher frequncies, so it not likely to be a majic cure.Have patience, guitar amps have so d$mn much gain in as few tubes as possible that you're just beggin for oscillation. I knew an ole Fender engineer; told me you could move some resistors a 1/4" inch and make an amp sqeal or shutup... REALLY.
Are your caps new (ish). The reason I ask is because electrolytics go bad with age... especially if they're are not in a circuit being used For example, you wouldn't want to use (or buy) a new ten year old cap. Oh and by the way... you ARE measuring all those carbon comps before you stick 'em right?
Haha, 2 steps forward, 1 step back here.
The Bypass caps were purchased from parts express 3-4 weeks ago, So I assume they are fairly new.
My Carbon comps are checked as far as correct value with my ohm meter and anything worse than 5% has been disposed of.
The whistle returned, or possibly never left I must have missed the magic combo of treble/presence/bass.
It is not as bad, but can still occur especially during certain low notes like a low G. it actually makes a rattling type whistle mixed with that note.
Also occurs with no notes being played.
I am going to do the pot bypass thing next.
The Amp is not near the speaker and chassis vibration has no apparent effect.
Here a couple pictures of the amp, This is amp 2 of 2 identical units.
Amp
Bottom
Sound Clip
The amp sounds fairly nice, Its just this annoying noise! lol
Gene
The Bypass caps were purchased from parts express 3-4 weeks ago, So I assume they are fairly new.
My Carbon comps are checked as far as correct value with my ohm meter and anything worse than 5% has been disposed of.
The whistle returned, or possibly never left I must have missed the magic combo of treble/presence/bass.
It is not as bad, but can still occur especially during certain low notes like a low G. it actually makes a rattling type whistle mixed with that note.
Also occurs with no notes being played.
I am going to do the pot bypass thing next.
The Amp is not near the speaker and chassis vibration has no apparent effect.
Here a couple pictures of the amp, This is amp 2 of 2 identical units.
Amp
Bottom
Sound Clip
The amp sounds fairly nice, Its just this annoying noise! lol
Gene
poobah said:
You hit it on the head, I replaced the incorrect pot with a temporary 4700 ohm resistor, Tested, Re-tested, Re-tested with the strongest tubes in my fleet.
NO Whistle.
I need to buy some more pots, I never have anything as low as that around here. I would have never guess it was that.
THANKS!!!!!!!!!!!!!!!!!!!!!!!!!
While on the subject of pots, 1 of my previous build calls for 1 meg volume pots, I find it to be very sensitive at the lower setting, Very non linear.
I have ben buyingThese
The Volume jump from 0-1 on the scale of 0-10 is about 60% of the total volume. This is ultra sensitive.
Would a log taper provide better control?
If so, do you know of a fast low cost source?
Gene
Trout said:
A log pot would definitely give a more linear volume control if the amp was designed that way.
Check Antique Electronic Supply for pots of almost any value for gutar amps in both log and linear.
https://secure.tubesandmore.com/
Trout,
That's awesome... and it sounds awesome too... and you can play... right on!
OK, POT SCHOOL:
1. Whenever a pot is used with just 2 connections OR the wiper is connected to one end of the resistance element (still just 2 connections coming away from the pot); it is being used as a RHEOSTAT... big word for variable resistor... you can sub a resistor with any value between zero and the pot's value to approximate the value and the effect of the pot. For example a 2.5K resistor would sound the same as a 5K pot at 50%. IN ALL CASES, a rheostat must have the correct value to avoid changing the circuit (perhaps for the worst). In this particular case, you could could have just used the pot from the 0% to 20% settings and you would seen the proper performance (that would have been 0 - 5K ohms).
2. Whenever a pot is used with all 3 connections; it is being used a POTENTIOMETER. In this situation, it is SOMETIMES possible to use a diiferent value and still have a good result. It all depends on the circuit.
Question: before you run out buy pots... do you know about LINEAR versus LOG/AUDIO TAPER pots?
3. If you notice that at one end of the "dial" the pot seems to "do nothing" and at the other end of the dial the pot seems way "too touchy"; those are places where you likely want to use a AUDIO TAPER pot. These pots are built with the resistor "scrunched up" at one end to make it behave "evenly" as you turn the dial. Now, there is probably some half-4ss rule for which kind to use where... but it depends on the circuit. Most volume controls should be LOG/TAPER thoough. Tone controls are a whole 'nother deal. Post your schems andTubelab or someone could tell you before you spend the bucks...
Cool to hear you and your amp dude...ROCK ON,
[edit, wow we where posting at the same time! no harm done]
That's awesome... and it sounds awesome too... and you can play... right on!
OK, POT SCHOOL:
1. Whenever a pot is used with just 2 connections OR the wiper is connected to one end of the resistance element (still just 2 connections coming away from the pot); it is being used as a RHEOSTAT... big word for variable resistor... you can sub a resistor with any value between zero and the pot's value to approximate the value and the effect of the pot. For example a 2.5K resistor would sound the same as a 5K pot at 50%. IN ALL CASES, a rheostat must have the correct value to avoid changing the circuit (perhaps for the worst). In this particular case, you could could have just used the pot from the 0% to 20% settings and you would seen the proper performance (that would have been 0 - 5K ohms).
2. Whenever a pot is used with all 3 connections; it is being used a POTENTIOMETER. In this situation, it is SOMETIMES possible to use a diiferent value and still have a good result. It all depends on the circuit.
Question: before you run out buy pots... do you know about LINEAR versus LOG/AUDIO TAPER pots?
3. If you notice that at one end of the "dial" the pot seems to "do nothing" and at the other end of the dial the pot seems way "too touchy"; those are places where you likely want to use a AUDIO TAPER pot. These pots are built with the resistor "scrunched up" at one end to make it behave "evenly" as you turn the dial. Now, there is probably some half-4ss rule for which kind to use where... but it depends on the circuit. Most volume controls should be LOG/TAPER thoough. Tone controls are a whole 'nother deal. Post your schems and
Tubelab or someone could tell you before you spend the bucks...Cool to hear you and your amp dude...ROCK ON,
[edit, wow we where posting at the same time! no harm done]
Hey Trout,
Here's some links to DIGI-KEY and some good pots... these people are a joy to work with... mega corp for sure, but nice people, super prices, and stock on the shelf (millions), and they are not snotty about selling ONE.
Sorry but GOOD pots cost a bit more.
http://dkc3.digikey.com/PDF/T061/1283.pdf
http://dkc3.digikey.com/PDF/T061/1282.pdf
Later -
Here's some links to DIGI-KEY and some good pots... these people are a joy to work with... mega corp for sure, but nice people, super prices, and stock on the shelf (millions), and they are not snotty about selling ONE.
Sorry but GOOD pots cost a bit more.
http://dkc3.digikey.com/PDF/T061/1283.pdf
http://dkc3.digikey.com/PDF/T061/1282.pdf
Later -
Nice work indeed Mr. Trout.
Guitar amps can be a real handfull, having power supply, output stage and high gain inputs in close proximity. I found sticking probes in the amp and finding out the most sensitive areas for this was a great help as to diagnose with out tools where my major problems were located.
Aspen Pittmans tube amp handbook is very usefull for this kind of work.
Good luck. I know the feeling of making an amplifier that I could play myself, and it's brilliant!
Guitar amps can be a real handfull, having power supply, output stage and high gain inputs in close proximity. I found sticking probes in the amp and finding out the most sensitive areas for this was a great help as to diagnose with out tools where my major problems were located.
Aspen Pittmans tube amp handbook is very usefull for this kind of work.
Good luck. I know the feeling of making an amplifier that I could play myself, and it's brilliant!
The Amp that has a REALLY touchy volume control is a cloned Fender Deluxe 5E3 I modded a bit back.
I swapped out a cap to get a Bright & Normal channel. Otherwise its basically the Standard Schematic
The both volume controls are ultra sensitive at 0-1, then gets very flat response after about the 3 posistion. Especially so on the bright channel.
I use that amp alot, Its got a very clean sound and works well for recording in a small studio/computer room. I can get plenty of tone without blowing down the walls and strapping pillows to my ears
This EL34 modded bandmaster based amp we just got fixed up is deadly!!
I manage to get a good overdriven sound and only have to push the closed back speaker cab facing out the door into the family room to sit in the comp room while I play it.
Of Course recording while the wifes home is nearly impossible lol
I Thank You All for the Help!!
Gene
I swapped out a cap to get a Bright & Normal channel. Otherwise its basically the Standard Schematic
The both volume controls are ultra sensitive at 0-1, then gets very flat response after about the 3 posistion. Especially so on the bright channel.
I use that amp alot, Its got a very clean sound and works well for recording in a small studio/computer room. I can get plenty of tone without blowing down the walls and strapping pillows to my ears
This EL34 modded bandmaster based amp we just got fixed up is deadly!!
I manage to get a good overdriven sound and only have to push the closed back speaker cab facing out the door into the family room to sit in the comp room while I play it.
Of Course recording while the wifes home is nearly impossible lol
I Thank You All for the Help!!
Gene
Hey, You guys think sorting this one out was fun, I got a real stumper for ya.
Here You go, Somethings hateing me on this one.
SE EL84 Champ thingy
Maybe I should make a new thread.
This ones plumeting into a distorted cut-off at max volume.
Plus It smoked at least 1 EL84, ( old austrian Tube of questionable quality and age).
Gene
Here You go, Somethings hateing me on this one.
SE EL84 Champ thingy
Maybe I should make a new thread.
This ones plumeting into a distorted cut-off at max volume.
Plus It smoked at least 1 EL84, ( old austrian Tube of questionable quality and age).
Gene
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