Hiwatt AP CP103 ,2005 valve amp

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

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

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I went to test that part failed one yesterday and over a couple of days it had turned fully white, must have been a very tiny leak/self-stopped, handling was enough to make it open up a bit

Back working again, I'm still trying to find the maker of those Italian valve sockets if anyone has any ideas.