Fender twin reverb - dog's breakfast

[N_Cook]

Low level bacon and eggs noise and low level whistling kettle noise. Goes if
V1 is pulled , no difference with change of valve V1. Noises disappear if no
input and normal volume turned to maximum.
Leaky DC decoupling cap between first and second triode ?

 

[N_Cook]

you can play through a vib ip with no pedal sw and its fine, so not a
V2.....onwards problem

 

[Phil Allison]

"Nutcase Kook"

Low level bacon and eggs noise and low level whistling kettle noise. Goes
if
V1 is pulled , no difference with change of valve V1.

** Assuming this is a 60s or 70s model ( there are about a dozen different
Fender amps all called " Twin Reverb " ) using the stiff grey cardboard
#material and hollow studs for the small components - the problem is due
to leakage across the material and between it and the second insulating
layer underneath.

First step, remove the self tapping screws holding the grey material and
slide a piece of similar shaped plastic between the layers. Laminating film
is ideal. Normally this works like a charm.

Step two, measure the DC voltage at the top of the volumes and tone pots -
should be no more than say 50mV. Track down any leaky film caps that may be
causing this and replace them.

Step three, heat the material with hot air to about 65 degrees C to simulate
normal running temps.

If DC voltages, squealing or crackling noises persist, you have a big
problem - the grey material may have to be replaced and the whole amp
rewired.

Other slightly less drastic measures may suggest themselves if you can
narrow down the leakage.

# The material is sometimes called "elephant hide " .



..... Phil

 

[N_Cook]

you can play through a vib ip with no pedal sw and its fine, so not a
V2.....onwards problem

What are the tubes designations for V1? It sounds like some sort of
cap problem? Have you tried shotgunning it and changing all the caps?

++++

I will get inside it today. Seems odd that varying the associated tone
controls has no effect on the whistle tone or level

 

[N_Cook]

That should give you a clue...



Ron

Not to me . The HV side corner of the board , not the nearby cap wires, is
extremely microphonic. Only mV DC lowside of the .1 and .047 uF caps .
Removing the fixing screw through the whale-hide or whatever junk that
passed for circuit board then, does seem to stop the noises. Some thin FRPB
will go between the 2 bits of whale-hide and an insulated collar under a
longer screw will go back through there. Consistent 2 to 3 volts on DVM
probe around the edge of the whale hide normal ?

 

[N_Cook]

The previous time I've come across this was with a Fender Concert and then I
put it down to someone previously soldering a component with the "active"
board in situ and had burnt part of the grounded whale-hide sheet
underneath. Placing some extra insulation cured that time also. So anyone
know precisely what the failure is . Presumably taking up humidity but what
next? salts in the material forming a conductive path in the core of the
material. I will try a megger on the material when I next get to the amp ,
no problem seen on external surfaces with a 30M DVM. And what is that stuff?
whale-hide? artificial leather ?

 

[N_Cook]

Under 30x magnification and some lighting did not look fibrous or patterned
in any way. Nothing observable with a Megger and a couple of needles pushed
into the edge.