Confused by this preamp tube behavior

[Mbira]

Don't know a better title, sorry. I have noticed on several ocasions when working on preamp tubes in a guitar amp, that there will be some sort of nasty noise (this can be a different noise from amp to amp) and that if I'm poking around with my volt meter, sometimes I'll touch a tube pin or a cap or something and then the noise just goes away-it's like the noise is discharging like static electricity or something. Then the noise may just suddenly come back or not and then go away the same way. This is definately not a cold solder joint thing. What the heck IS that?


:guinness:

 

[rafafredd]

look around for dirt on high impedance places in the cirduit.Maybe tube sockets??

 

[bcarso]

Also clean any contacts you can access, including those of the tube sockets.

What will happen sometimes is that corrosion etc. will cause a contact to get high resistance, and a strong signal will clear the corrosion and restore the connection for a while. You might induce this signal by probing around.

For years my late father had a JBL SA600 amp in our house that would start to hum, and he would cycle the power and attribute the problem to deteriorating electrolytic caps in the power supply. Turned out they were fine---it was the oxidizing and other -izing cadmium-plated RCA jacks and plugs of the phono inputs that were going south, and the transient of a power cycle would cut through and reestablish connection for a while.

 

[Mbira]

interesting...this makes sense to me. Thanks guys!

It's funny, because I've found that a little round file that I can fit inside a 1/4" jack has been one of my best tools just for that purpose. I got a great deal on a Gibson ES175 because the guy thought the electronics were all f'ed up. I filed a little bit in the 1/4" jack and all was well. That was 4 years ago and still going strong...

Joel

 

[Guest]

[quote author="Mbira"]
sometimes I'll touch a tube pin or a cap or something and then the noise just goes away.... This is definately not a cold solder joint thing. What the heck IS that?
[/quote]
May be some kind of HF oscilation. {like known
Kurz-Barkhausen in power tubes}
veryv high frequency,
ferite pearl in series with grid input can help,
maybe bridged by small resistor. Or resistor alone, but degrades noise
factor.

Here is nice thread about it some month ago.

xvlk

 

[NewYorkDave]

That's what I was thinking, that it could be HF oscillation that's being quenched when you apply your voltmeter probe. I've seen it before.

Look at it with a scope with an X10 probe. The low capacitance of the X10 probe is less likely to quench or otherwise damp the oscillation and thus you're more likely to "catch it in the act."

 

[Mbira]

Can that oscillation happen in an amp that was previously working without the problem? I always thought oscillation was mainly duy to layout or something that simply "won't work" in the circuit.

Thanks.
Joel

 

[CJ]

Thermal agitation noise, shot noise, induced grid noise, it's all good. :grin:

And random. Which is the part that drives you nuts. Sometimes a noisy tube will work in a different location. I have been putting up with exactly what your talkin about for years, never did find a sure way to fix it, short of replacing valves. You don't care what type of noise it is or where and why, you just want it to stop!
I thought it was connections also, but you replace the socket, and half an hour later, sun spots in you amplifier.
I do not think it's the connections, because the grid to cathode resisor is megohms, and anything will look like a good connection next to 1 meg grid resistor. You can try freeze spray of heat to see if the valve respponds.

RDH4, chapter 23, section 6.
7 pages just on tube noise.
Knock yourself up. I mean out.

I think I have a 40 doller EF804s that is going micro. :twisted:

Oh well. It's plug it in, wait six months, and if no noise, your good for 210 years.

 

[PRR]

> a little round file that I can fit inside a 1/4" jack

:shock: :shock: :shock: :shock:

Go to the gun store and get the kit to clean a .22 pistol.

If they ask nylon or brass, take both, but try to decline the magic fluid. You may be better off if they have an ala-carte rack where you can get just the brushes and the handy handle, without swabs, fluid, and case.

 

[Mbira]

PRR, that's a good idea about the brush. I'll check that out. but instead of a round brush...

...maybe an octagon shaped one :green: