The Italian sockets are maybe marked EE, anyway socket pins are now strong
sliding fit to 2.2mm drill shank.
Curvey Es like the E of the compliance CE controlle european mark, maker
name not found Googling
The 1970 amp is marked Sound City "one hundred" on the gold fascia panel.
Suspiciously small 63V 100uF bias cap replaced with 100V one in the 2005
amp.
O/p Tx tests good balanced with reverse feed AF sig gen.
Anyway with valve testing can be pretty sure of the "etiology" .
Both these amps are stored in the same unheated warehouse, the newer amp
looks far worse from internal corrosion, than the 1970 one.
Poor socket pins and corrossion on the valve pins lead to loss of bias. Then
owner robs a pair of valves from the good amp , but one of those valves has
a broken polarising nib. Tracing back to my markings on receipt this valve
was rotated one position out in the base, the one position that connects
anode to
cathode via the heater.
The old valve pins showed no corrossion but the recent JJ ones had bad grey
dust corrossion , leaving large grey deposits on cosmetic nail sanding
boards.
One valve tested on my Avo as no heater and took a lot of abraiding to the
pins to get
the heater current through. Another one tested as no gain until I repeated
the sanding.
The JJ valves had no date mark just an E in a cartouche. And marking
N5LN5
datecode for 2005 ?
I don't think the pin infil is PbF, but has the bright tin tinning of the
pin surface turned to tinpest? - those grey patches were certainly
insulating to 30M , DVM anyway.
As a puzzle for anyone with the inclination. If the valve with the broken
nib was placed in the socket with perhaps overheating marks in the bakelite
of pins 2,3 and 6 what orientation would lead to serious overheating at
those pins?
The heater chain on these amps uses "dummy" socket pin 6 as a resistor tag
from HT2 supply, EL34 are NC and usually NP for pin 6