Freaky Whistle in Guitar Amp

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Trout

Trout

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Joined 2004
poobah said:
Tubelab,

I am confused as hell. Presence is about boosting highs right? Which way does the pot wiper go (up or down) to increase presence?

By the way Gene... trash everything I said... How is the pot wired? Would an arrow meaning clockwise go up or down?

I gotta redraw this thing... not making sense to me here.
Click to expand...


Its Exactly like the schematic shows, Rotate pot clockwise (UP) and it reduces its value. The Wiper has the .1uf cap.
So basically, I think Tubelabs idea of adding a resistor between the cap and pot might be a quick easy fix, If I can figure out how to squeeze it in, I have hardly and distance between the pot and cap.
Edit:
The #1 lug of pot is not used.

An externally hosted image should be here but it was not working when we last tested it.


I always think backwards on these also, the first time I wired an amp last summer, ALL the controls increased counterclockwise :scratch:

Gene
 
Trout

Trout

Member
Joined 2004
poobah said:
Thanks Gene,

Yep... I got it backwards alright... a 470 - 1K with cap will do the trick.

Now I'm p1$$ed off though... couple days... when I have some time... I'll sit down and do all the math on this circuit... I clearly don't understand it.

:xeye: :xeye: :xeye: :xeye: :bawling: :bawling: :(
Click to expand...


hahaa Its got me pullin hairs too.

I still have as yet to figure out why 1 amp cured and the other didnt, Must be some kind of inductance differences in the 2 transformers, Or just the 5-10% parts tolerances ganging up on me.

1 thing I find interesting in these guitar amps in general is tube microphonics.

Put a nos set of mullard 12AX7's in 1 amp, They show no microphonics problems. Take the exact same tubes over to a different amp (different circuit entirely) And you cant even breath by them without getting noise.

I was playing with that EL84 SE this morning, If I move the opt primary lead 1/2 inch towards the center of the chassis, It SCREAMS with squeel/Whistle, I move it 1/2 inch towards the end of the chassis, and its one of the cleanest sounds I have ever heard in a practice amp.

I think it actually may need shielded primary leads!!! :idea:

I am about to add the resistor to the bandmaster presence pot, hopefully That will help.

Gene
 
Trout

Trout

Member
Joined 2004
Well,
I tried the resistor thing with no success.

As soon as I get the pot set to full presence/or no ohms, The whistle occurs. I also tried the bypass thing with a 220pf then a 470pf, Still the same results.

I suspect that I am looking in the wrong place for the wrong thing. I can also kill the whistle at the treble pot.

If I reduce the treble pot, It not only kills the whistle, But also has no effect on the overall tone. The upper 40% setting has no audible effect on end result output tone, But Does however effect the whistling. Just a 10% reduction kills it entirely.

So, To get the whistle the controls are set,
Master volume 100% Bass 100%, Treble 100%, Presence 100%, Either preamp gain above 20%

If I Use the amp as follows it does not whistle.
Master 80%, Any combo of the rest of controls 0-100% works fine. Or
Treble at 80% and any combo 0-100% works fine.
The treble control appears to interact alot.


Grrrrrr this is complicated:xeye:
Gene
 
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