N_Cook wrote:
Blown mains fuse and owner thinks he replaced the HT and mains ones after
initial failure.
Amp failed in tune 5 into a gig , playing loud but normal for this amp (does
"The Who" front label make it louder?) and fuse/s blew. Next day, he has a
1970 Hiwatt L 100W Mark 4 ? M41.. serial number anyway, and according to the
owner very rare, this one should be ok when it gets 4 EL34 valves again.
One of the 2005 EL34, the getter was white so he threw it out ,
unfortunately . I'm out of contact with him for a couple of days but looks
as though the other of one of the side pair was partially white and kept
this one same make and appearance as the others in the amp, now mixed in
with his box of untested valves. Anyway in the end he robbed 2 EL34 from the
working vintage one and still blew fuse/s.
All Rs look and measure ok in the PA section. Powered up with 70 percent
mains with no output valves, only prea ones , and voltages seem right for
that mains including bias voltage. Tx resistances seem normal.
This part white EL34 , 2/3 white , 1/3 mirror Cs getter, blown heater. The
vertical rod electrically attached to p1 , G3 , but physically on the other
side of the valve electrically connected by the top "halo", electrode is
discoloured to blue compared to the other valves. No obvious cracks , I
suppose deliberately breaking it will confirm was still functional getter.
But something coming off the rod reacting with the getter while hot and not
fully converted to a white form ?
Won't be testing them and his other "spares" until tomorrow.
One 8 pin black socket (marked Made in Italy , EF ?) is discoloured to dusty
grey appearance around pins 2,3 and 6, 6? no pin in that one ?.
Trying a 2.2mm drill shank (not the cutting end) it is close sliding fit to
the old amp socket pins but slack sliding fit for the 2005 one
Measured a few EL34 pin diameters, p1 and 8
2.22, 2.31 mm
2.26,2.30
2.18,2.33
2.26,2.30
EL24 from the 1970 amp
2,41,2.41 mm
2.40, 2.46
so suspect contacts in the sockets? and loss of bias voltage ?
Retaining springs for the new version , not considered necessary in the the
old one
If it's any thing like the EDM machines I used to work on years ago
that used 4-400Z tubes, the power supply caps get bad and when the amp
is under heavy load does not get sufficient power. THis also means the
bias circuit maybe lifting and causes the tubes to start cutting in DC
current and over heating.
It was common to see these tubes glowing a mild orange color when
they were fully working properly however, a bright orange or worst, near
white is a not so good.
You need to put a load on the amp, test signal, DMM and scope to
monitor the B+ and bias voltage under max load.
A lot of times the large caps may test ok for capacitance but have
leakage. This leakage, as small as it could be can add extra load on the
supply and thus the valves as you say (tube) circuit suffers at the
high end along with having ripple that may not be explainable.
ALso, high freq oscillation can cause unexplained over heating and can
be seen on the scope. THis usually is due to a small cap+R circuit that
has failed in the circuit to neutralize the effect.
Today we work on machines that employ a single oscillator tube that
pumps out at the plate around 275k watts. It's water cooled, the heater
is operated from a 12 volt AC 250 amp transformer and the simplest form
of thermal protection ever devised is a piece of nylon/plastic fishing
line tied to a post in the center of the tube assembly back to a wall
mounted whisker switch actuator arm. If the temps are high enough to
melt that line it simply breaks and the switch shuts things down.
That operation uses a hot cathode triode and the heater element is
made of compressed sheet metal which is also the cathode. THey make nice
lamps when their service is no longer useful
Jamie