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Fender Bassman Ten, 1975
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N_Cook
Oct 15, 2015, 5:34:22 AM10/15/15
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Only 1 changed resistor inside in 40 years, pretty good for twice weekly
use over most of that time. Heaters glow but HT and bias supply stays up
and no extra current draw coming out of standby.
The 2.2K 7W W/W dropper for the minor HTs gone o/c.
First set went fine, switched to standby for break and then nothing
there for the second set. Is that resistor most stressed in standby?
No obvious overheating of the R or the whale hide or whatever "circuit
board" material.
Assuming no other problem , it just failed from old age/corrossion but
replace with same rating or go up to 11W?
No rubber insulation found , any health and safety considerations other
than the usual modern considerations? 2 issues with the Bulgin
plug+socket mounted on the cab but thats just a UK thing. I'll do the
usual remove the red voltage selctor knob on the rear and secure inside
and blank off the shaft of the switch, as too easy to drunkenly turn
that knob to "repair" a fault situation in UK 240V land.
use over most of that time. Heaters glow but HT and bias supply stays up
and no extra current draw coming out of standby.
The 2.2K 7W W/W dropper for the minor HTs gone o/c.
First set went fine, switched to standby for break and then nothing
there for the second set. Is that resistor most stressed in standby?
No obvious overheating of the R or the whale hide or whatever "circuit
board" material.
Assuming no other problem , it just failed from old age/corrossion but
replace with same rating or go up to 11W?
No rubber insulation found , any health and safety considerations other
than the usual modern considerations? 2 issues with the Bulgin
plug+socket mounted on the cab but thats just a UK thing. I'll do the
usual remove the red voltage selctor knob on the rear and secure inside
and blank off the shaft of the switch, as too easy to drunkenly turn
that knob to "repair" a fault situation in UK 240V land.
N_Cook
Oct 15, 2015, 8:08:28 AM10/15/15
to
Bad mains earth bond-point.
Decided to autopsy the Rockwood , presumably original, 2.2K 7W dropper,
the previously replaced one is elsewhere 1/2W .
Stopped going in further when it looked as though asbestos was part of
the construction. I'd not thought how fine the wire must be for 2.2K.
Looks as though the asbestos wraps around the core , for thermal
expansion give, allowing isolation of the turns at the same time, then
covered in fire-cement set in a moulded ceramic body. As brown colour at
one end I'll assume it was rust and failure was corrossion, the
resistance wire looked fine , what I could see of it but making contact
with it meant it locally broke.
Decided to autopsy the Rockwood , presumably original, 2.2K 7W dropper,
the previously replaced one is elsewhere 1/2W .
Stopped going in further when it looked as though asbestos was part of
the construction. I'd not thought how fine the wire must be for 2.2K.
Looks as though the asbestos wraps around the core , for thermal
expansion give, allowing isolation of the turns at the same time, then
covered in fire-cement set in a moulded ceramic body. As brown colour at
one end I'll assume it was rust and failure was corrossion, the
resistance wire looked fine , what I could see of it but making contact
with it meant it locally broke.
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