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Old 2nd January 2006, 04:24 PM   #1
Trout is offline Trout  United States
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Question Freaky Whistle in Guitar Amp

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.
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Old 2nd January 2006, 05:01 PM   #2
poobah is offline poobah  United States
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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.

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Old 2nd January 2006, 05:21 PM   #3
Trout is offline Trout  United States
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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
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Old 2nd January 2006, 06:06 PM   #4
poobah is offline poobah  United States
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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.

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Old 2nd January 2006, 10:52 PM   #5
anatech is offline anatech  Canada
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Gee poobah,
I was just about to ask for the model number too!

Gene,
Experiment with the 'scope. (ie: Play) First with your hand only. Did you get probe(s) with it?

I agree with poobah, that 25K needs to be reduced to the original 5K. Stick fixed resistors in to simulate the settings.

-Chris
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Old 2nd January 2006, 11:07 PM   #6
poobah is offline poobah  United States
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Hey Chris,

Mr. Trout is doing some fairly impressive work here considering he working with blindfolds on. We need to show him how easy a scope is. Especially a little 30 Meg B & K. You know for being... eh... eh... "junk"; There sure is alot of B & K stuff that just keeps going and going...
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Old 2nd January 2006, 11:09 PM   #7
Trout is offline Trout  United States
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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
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Old 2nd January 2006, 11:48 PM   #8
anatech is offline anatech  Canada
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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
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Old 2nd January 2006, 11:50 PM   #9
anatech is offline anatech  Canada
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Hi poobah,
Yup, B&K isn't my favorite. But they do keep going. As long as you don't depend on the reading too much, they tend to be fine. A 30 MHz 'scope isn't too bad at all for general use.

-Chris
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Old 3rd January 2006, 12:32 AM   #10
poobah is offline poobah  United States
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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?

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