presence squeal in express (was: low end oscillation)

[Roe]

after changing the lead dress to KF style my express has gotten much more stable and less noisy. however, theres one problem left: occasionally I'll get a massive bass boom, almost like a motorboat. Right now I only get this oscillation when the mass150 is turned in bypass mode. using the attenuator and removing the attenuation gradually does not lead to oscillation.

I had the same problem occasionally before I changed the lead dress in power supply, presence circuit and the heater wiring to KF style.

any ideas what can cause this?

the amp sounds great btw.

[Roe]

I got rid of the low end oscillation by working on the presence pot. however, I get oscillation in the highend when the presence is turned high. this problem only occurs without attenuation.

this is a ceriatone exp with pvc caps, one mustard 3n3 and a 100k instead of the 150k btw gainstage 2 and 3. also I've added a 15k grid stopper at the input of v1 and I've lowered the 220k bias splitter resistors connected to pin 5 on the el34s to 100k. before changing to the KF layout I had more gain but the amp was more unstable

[guitarsnguns04]

post some good pics of it....someone here will find it

[Roe]

here's a big pic:
http://folk.ntnu.no/roef/express2009
that's a green mustard type cap. I have hundreds of these.
the twisted wired to the post was an larry amps idea influenced by blackface fenders. seems to work great

[sliberty]

Do you have a bottom plate, or some shielding in the bottom of your head cab? That is what solved my high end ocsillations in a JTM45 build.

[Roe]

yes, the bottom plate helps a little on the noise but it does not take away the presence squeal. also, the presence knob works backwards on my express (clockwise is darker; counter-clockwise is brighter)

[sliberty]

Well the backwards presence is easy - rewire it the other way around. I can't tell you how many times I have wired pots backwards - very common.

[stink]

I had a similar problem in my build. The problem is the wire coming off V1 and going to the volume pot - IT MUST NOT TOUCH ANYTHING!!! Remove the solid wire & replace it with solid core, shielded coax and ground the end attached to the volume but leave the other side (the one attached to v1) alone or you will create a ground loop. Again, IT MUST NOT COME IN CONTACT WITH THE CHASSIS OR ANYTHING ELSE!!! This wire is like a great big microphone.

Secondly, insert a 33k resistor coming from the input jack to V1.

Worked for me...

-Steve
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LOST ADVICE

[Roe]

Steve,

thanks for your comments. Does the position of the wire on the pic look bad?

[swarty]

If you take that lead straight over the board to the tube you'll be able to lose about 1.5 inches or so.

[sliberty]

Secondly, insert a 33k resistor coming from the input jack to V1

This shouldn't be necessary, and some say it reduces the touch sensativity of the amp. Mine (and most others I have seen) do not have this resistor.

[Roe]

I seems to be a microfonic tube - probably the pi. not the first gainstage certainly

[Structo]

I'm having the same problem on an amp that I just built.
It's a mongrel 6V6 amp but if I max the volume and the presence I get the squeal too.

About the only thing I haven't tried is replacing the 6V6's since I don't have another pair on hand.

My chassis is very cramped and the circuit board is pretty much right on top the sockets, well I have 3/4" standoffs. So it may just be lead dress.

I'll be watching this to see if you find a fix.

[stink]

Steve,

thanks for your comments. Does the position of the wire on the pic look bad?

I can't tell by the picture but if the wire is laying on the chassis, it WILL pick up vibrations and start to oscillate. Let me guess, the oscillation occurs with the volume between 12 & 1 correct? If this is the case, use solid core coax and either take it over the board or bend it around so it touches nothing.

Try turning the amp up to 1 o'clock and tap on that wire, you'll see what I mean.

As far as the other post about the amp losing sensitivity by putting in a 33k resistor in-line with V1 - that's just BS! Virtually every other amp on the planet has a grid stopper going to V1.

I'll bet there isn't a single person here who could play two identically preps amps and tell which one has the grid stopper - it takes NOTHING away from feel or touch sensitivity.

Go to http://www.Aikenamps.com find the technical section and read about grid stoppers.

-Good luck,
-Steve
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DC MEDICAL MARIJUANA

[paulster]

I'm having the same problem on an amp that I just built.
It's a mongrel 6V6 amp but if I max the volume and the presence I get the squeal too.

The NFB wire going to the presence pot could well be the problem if it's too close to the power tube grid wires, especially running parallel to them.

It looks pretty close in the photos of the Roe's amp too.