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[Guest]

Hi All,

I have a Hafler T2 guitar preamp, and am experiencing an intermittent
noise generated when the amp is set at high gain. The only way I can
think of describing it is, "the buffeting of high winds" (think driving
down the highway with your window half open). The noise is under the
sound of the guitar, but it is there.

I have swapped out the 12ax7s and the problem has not improved. Could
the caps be causing this noise (opening/closing due to intermittent
failure at high power levels)? They are fairly old and probably need to
be replaced, but if this noise is not symptomatic of a cap problem then
I may not go through the trouble.

Thanks!

-brian

[Ian Iveson]

<flatpick@gmail.com> wrote

[Guest]

When I use a different preamp, the noise is not there so I am fairly
certain that it is not the guitar, cord or power amp causing this.

I've done some more experimenting, and have noticed that it also occurs
at moderate gain levels. It's really more of a scratchy noise. None of
the pots sound dirty, but that is how I would describe it. Like a dirty
pot getting turned whilst playing.

Also, the noise gets worse when playing notes of higher frequencies. It
is really bad at 15th fret D (2349 Hz) and decreases somewhat at higher
and lower frequencies. As the note decays, I hear this infernal,
intermittent, scratchy business under it. Once the guitar is quiet, the
noise is gone. I've also noticed that different makes of tube seem to
exacerbate the problem. Perhaps your theory about the bias capacitor
is correct.

[Bob]

On 22 Dec 2004 20:56:09 -0800, flatpick@gmail.com wrote:

[Ian Iveson]

A scratchy sound could be anything, nearly. Can't think why it
should be excited by particular frequencies either.

One common cause is power supply electrolytic caps. Another is loose
sockets or solder joints. Otherwise most components can cause
scratchy sounds.

I would check all valves for correct voltages and currents and check
amp is not oscillating.

Then I would poke about gently with a wooden stick to see if moving
wires or tapping passive components excites the problem.

Then I might check and/or replace vulnerable components...caps in
hot places or at high voltages, for example.

You have a problem that an experienced technician could probably
identify quite quickly if the amp were in front of him.

cheers, Ian

[Wbittle]

Hi,
These newer preamps suffer from many problems. First off. unplug
your guitar and make sure the input jack is a shorting type. This
insures that you are just hearing the preamp. Now turn it up and see if
the problem is there with nothing plugged into the input jack. If the
noise is not there, it is something other then the preamp. Ok, failing
this, you now have to start looking into the preamp. Try a new set of
tubes. See if the sound changes. By this I mean that you are not looking
for it to go totally away, but to change. If it does, you know that the
tubes are a contributing factor. The next thing on the list that causes
the white noise or wind rushing sound are faulty or poor quality plate
feed resistors. Most high gain stages must use high quality plate
resistors or the circuit will have a lot of this hiss. In extreme cases,
or in cases where the resistors are old, the amp will also make popping
sounds as well as the fluctuating hissing sounds. This sounds like a
newer preamp and it may also have transistor regulators as well as
switching circuits. These circuits can induce a lot of noise into the
preamp. This area can become a bit tricky to deal with. Another problem
could be the wiring. I know that certain guitar amplifiers make a lot of
this wind rush type noise because of the length, type and routing of
certain wires in the first stages of the preamps. Also, improper
shielding as well as the placement of components on a circuit board (if
used as opposed to point to point wiring) can contribute to this.
The final big question is this. Has the preamp always done this or
has this problem just started? I have worked with several tube guitar
preamps and most have a bit of hum and a lot of this wind rush sound
right out of the box when cranked way up. Most of these have a huge
amount of gain so as to give the guitar player everything from that
clean country sound to the most overdriven heavy metal sounds
imaginable. Also, most were made to drive a power amp and have a lot
more gain then what is needed to drive a regular guitar amp. So, if
plugged into something like a Fender Twin, they would sound quite noisy
since the Twin has a lot of gain in it's two preamps to begin with.
Bill B.

flatpick@gmail.com wrote:

[todd@NOSPamprepair.com]

10 to 1 it's a noisy plate resistor

flatpick@gmail.com wrote:

[Guest]

Thanks for all the help. I replaced the electrolytics today and there
was no improvement.

I went to the shop today and found a Hafler T3 preamp for $75, which I
purchased. I guess the T2 will get mothballed.

If somebody could clue me in to identifiying the plate resistor, I'll
try replacing that. I don't have any schematics handy. Is it hanging
off of some well-known pin?

-brian

[todd@NOSPamprepair.com]

Pins 1 & 6 are the plates (it's a dual triode). If you can tell that the
tubes are in series and they probably are, then pull the first stage
tube and see if your noise dissapears. If it does then the noise is
obviously in that part of the circuit. I would replace those plate
resist's and see. If no improvement, replace the cathode resistors as
well and see. (pins 3& 8). Without a schematic - you are sort of
grabbing at straws - for all we know there may be an opamp in there
somewhere or a discreet component gone noisy but plate and/or cathode
resitors gone noisy after some years is not uncommon. Good luck


flatpick@gmail.com wrote: