Re: Twin hummmmmmmm
- From: "Trevor Ridney" <tridders@xxxxxxxxxxxxxxxxxxxxx>
- Date: Wed, 14 Mar 2007 08:54:09 GMT
<johncox@xxxxxxxxx> wrote in message
news:1173847165.085413.75360@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
My amp hums. I'm trying to see if anybody out there can help me
diagnose the problem. The setup is a Silverface Twin that was
blackfaced and then modded to a Climax (which some of you may know of)
that has a hot-rodded fender channel and a british channel. The
layout of the amp has remained much the same as the original
silverface layout. The power section is fueled by to KT66s. I've
gone through and grounded everything that I can to the same place to
no avail. The latest thing I noticed is that, when I wiggle the tap
from the PT that goes to the lamp and heaters, the hum is reduced.
But I also get a random popping sound. Is it possible that the wires
coming from this tap are shorting out to the chasis or something? (Has
anybody seen when spark plug wires wear out and arc between each
other?) I turned off the lights and watched it for a while, but I
didn't see any arcs. I'm pretty sure that this isn't a filtering
issue. It doesn't see to be a grounding issue. Can anybody suggest
where to look next? I'm really new to working on/modding amps, so be
gentle.
Hi Jon,
You may have discovered the problem already. On most Fender designs, the
6.3V heater supply also feeds the indicator lamp. You'll usually see a pair
of 100 Ohm resistors on the bulbholder going to earth. These are the 'faux'
centre tap for the heater windings. If either one or both of these has
developed a fault (and they often do), this will 'lift' the heater tap on
your transformer and give rise to a pronounced hum.
Replace these resistors and your hum should go away. 1/2 watt resistors
should get the job done.
Take *great care* not to cause any shorts or you may do serious damage to
your power tranny.
These resistors are usually the first thing to fry if your output valves
fail and arc over. The HT (B+) finds a way to earth, usually through these
resistors, by arcing from Pin 3 to pin 2 on the output valve socket(s).
That's why the resistors in question tend to go bad. Much cheaper, imo, than
having the PT go south <grin>. It could be that an output valve failure at
some time in the past has damaged the centre tap resistors.
You may like to check for arcing (carbon scoring) on the output valve
sockets whilst you're in there. The carbon 'track' offers a low resistance
path for your B+ and the problem will re-occur if this is the case.
Replacing the output valve sockets is the only cure, imo, if arcing damage
is present.
NB: You'll be around very high voltages. If you're not sure how to do this
kind of work, get thee to a tech. It ain't worth the cost of the repair to
risk meeting your ancestors prematurely.
Let me know how you get on
HTH,
Trev
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